// SPDX-License-Identifier: AGPL-3.0-or-later
//! This module contains types providing the cryptographic primitives necessary to implement
//! Blocktree.
//! 
//! The [openssl] create is used for all of these primitives,
//! none of them are directly implemented in this module.
//! Rather, the types here wrap the functionality provided by OpenSSL in a more convenient
//! interface, and they serve as an abstraction layer, so that the underlying crypto library can be
//! more easily replaced.
pub mod tpm;

pub mod merkle_stream;
pub mod x509;
pub use merkle_stream::MerkleStream;
pub mod secret_stream;
pub use secret_stream::SecretStream;
mod envelope;
pub mod file_cred_store;
pub use envelope::Envelope;
//mod sign_stream;
//pub use sign_stream::SignStream;

use crate::{
    btensure, bterr, fmt, io, BigArray, BlockError, BlockMeta, BlockPath, Deserialize, Epoch,
    Formatter, Hashable, Principal, Principaled, Result, Serialize, Writecap, WritecapBody,
};

use btserde::{self, from_vec, to_vec, write_to};
use foreign_types::ForeignType;
use log::error;
use openssl::{
    encrypt::{Decrypter as OsslDecrypter, Encrypter as OsslEncrypter},
    error::ErrorStack,
    hash::{hash, DigestBytes, Hasher, MessageDigest},
    nid::Nid,
    pkcs5::pbkdf2_hmac,
    pkey::{HasPrivate, HasPublic, PKey, PKeyRef},
    rand::rand_bytes,
    rsa::{Padding as OpensslPadding, Rsa as OsslRsa},
    sign::{Signer as OsslSigner, Verifier as OsslVerifier},
    symm::{decrypt as openssl_decrypt, encrypt as openssl_encrypt, Cipher, Crypter, Mode},
};
use serde::{
    de::{self, DeserializeOwned, Deserializer, SeqAccess, Visitor},
    ser::{Error as SerError, SerializeStruct, Serializer},
};
use std::{
    cell::RefCell,
    fmt::Display,
    io::{Read, Write},
    marker::PhantomData,
    ops::{Deref, DerefMut},
    sync::Arc,
};
use strum_macros::{Display, EnumDiscriminants, FromRepr};
use zeroize::{ZeroizeOnDrop, Zeroizing};

/// The encryption of some type `T`.
/// 
/// This type is just e wrapper around a [Vec] of [u8] which remembers the type it was serialized
/// and encrypted from.
#[derive(Debug, PartialEq, Eq, Serialize, Deserialize, Clone)]
pub struct Ciphertext<T> {
    data: Vec<u8>,
    phantom: PhantomData<T>,
}

impl<T> Ciphertext<T> {
    fn new(data: Vec<u8>) -> Ciphertext<T> {
        Ciphertext {
            data,
            phantom: PhantomData,
        }
    }
}

/// A signature over the serialization of a type `T`.
/// 
/// This struct allows the signature over a serialization of `T` to be stored together with a
/// signature over  it.
pub struct Signed<T> {
    _data: Vec<u8>,
    sig: Signature,
    phantom: PhantomData<T>,
}

impl<T> Signed<T> {
    fn new(data: Vec<u8>, sig: Signature) -> Signed<T> {
        Signed {
            _data: data,
            sig,
            phantom: PhantomData,
        }
    }
}

/// Errors that can occur during cryptographic operations.
#[derive(Debug)]
pub enum Error {
    NoReadCap,
    NoKeyAvailable,
    MissingPrivateKey,
    KeyVariantUnsupported,
    BlockNotEncrypted,
    InvalidHashFormat,
    InvalidSignature,
    IncorrectSize {
        expected: usize,
        actual: usize,
    },
    TooSmall {
        required: usize,
        actual: usize,
    },
    IndexOutOfBounds {
        index: usize,
        limit: usize,
    },
    IndivisibleSize {
        divisor: usize,
        actual: usize,
    },
    InvalidOffset {
        actual: usize,
        limit: usize,
    },
    HashCmpFailure,
    RootHashNotVerified,
    SignatureMismatch(Box<SignatureMismatch>),
    /// This variant is used to convey errors that originated in an underlying library.
    Library(Box<dyn ::std::error::Error + Send + Sync + 'static>),
    /// Occurs when an attempt is made to finish an [Op] that is already finished.
    OpAlreadyFinished,
    /// Indicates that a writecap was not issued to a particular [Principal].
    NotIssuedTo,
    /// Indicates that no root credentials were present in a [CredStore].
    NoRootCreds,
    /// Indicates that the wrong root password was given when attempting to access the root
    /// credentials.
    WrongRootPassword,
}

impl Error {
    fn signature_mismatch(expected: Principal, actual: Principal) -> Error {
        Error::SignatureMismatch(Box::new(SignatureMismatch { expected, actual }))
    }

    fn library<E: std::error::Error + Send + Sync + 'static>(err: E) -> Error {
        Error::Library(Box::new(err))
    }
}

impl Display for Error {
    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
        match self {
            Error::NoReadCap => write!(f, "no readcap"),
            Error::NoKeyAvailable => write!(f, "no key available"),
            Error::MissingPrivateKey => write!(f, "private key was missing"),
            Error::KeyVariantUnsupported => write!(f, "unsupported key variant"),
            Error::BlockNotEncrypted => write!(f, "block was not encrypted"),
            Error::InvalidHashFormat => write!(f, "invalid format"),
            Error::InvalidSignature => write!(f, "invalid signature"),
            Error::IncorrectSize { expected, actual } => {
                write!(f, "expected size {expected} but got {actual}")
            }
            Error::TooSmall { required, actual } => {
                write!(f, "expected at least {required} but got {actual}")
            }
            Error::IndexOutOfBounds { index, limit } => write!(
                f,
                "index {index} is out of bounds, it must be strictly less than {limit}",
            ),
            Error::IndivisibleSize { divisor, actual } => write!(
                f,
                "expected a size which is divisible by {divisor} but got {actual}",
            ),
            Error::InvalidOffset { actual, limit } => write!(
                f,
                "offset {actual} is out of bounds, it must be strictly less than {limit}",
            ),
            Error::HashCmpFailure => write!(f, "hash data are not equal"),
            Error::RootHashNotVerified => write!(f, "root hash is not verified"),
            Error::SignatureMismatch(mismatch) => {
                let actual = &mismatch.actual;
                let expected = &mismatch.expected;
                write!(
                    f,
                    "expected a signature from {expected} but found one from {actual}"
                )
            }
            Error::Library(err) => err.fmt(f),
            Error::OpAlreadyFinished => write!(f, "operation is already finished"),
            Error::NotIssuedTo => write!(f, "writecap was not issued to the given principal"),
            Error::NoRootCreds => write!(f, "root creds are not present"),
            Error::WrongRootPassword => write!(f, "incorrect root password"),
        }
    }
}

impl std::error::Error for Error {}

impl From<ErrorStack> for Error {
    fn from(error: ErrorStack) -> Error {
        Error::library(error)
    }
}

/// Contains information about why a signature verification failed.
#[derive(Debug)]
pub struct SignatureMismatch {
    /// The principal whose signature was actually present.
    pub actual: Principal,
    /// The principal whose signature was expected.
    pub expected: Principal,
}

/// Returns an array of the given length filled with cryptographically strong random data.
pub fn rand_array<const LEN: usize>() -> Result<[u8; LEN]> {
    let mut array = [0; LEN];
    rand_bytes(&mut array)?;
    Ok(array)
}

/// Returns a vector of the given length with with cryptographically strong random data.
pub fn rand_vec(len: usize) -> Result<Vec<u8>> {
    let mut vec = vec![0; len];
    rand_bytes(&mut vec)?;
    Ok(vec)
}

/// An ongoing Init-Update-Finish operation.
pub trait Op {
    /// Update this operation using the given data.
    fn update(&mut self, data: &[u8]) -> Result<()>;

    /// Finish this operation and write the result into the given buffer. If the given buffer is not
    /// large enough the implementation must return Error::IncorrectSize.
    fn finish_into(&mut self, buf: &mut [u8]) -> Result<usize>;
}

impl<T: ?Sized + Op, P: DerefMut<Target = T>> Op for P {
    fn update(&mut self, data: &[u8]) -> Result<()> {
        self.deref_mut().update(data)
    }

    fn finish_into(&mut self, buf: &mut [u8]) -> Result<usize> {
        self.deref_mut().finish_into(buf)
    }
}

/// An ongoing hash operation.
pub trait HashOp: Op {
    /// The specific hash type which is returned by the finish method.
    type Hash: Hash;

    /// Returns the kind of hash this operation is computing.
    fn kind(&self) -> HashKind;

    /// Finish this operation and return a hash type containing the result.
    fn finish(&mut self) -> Result<Self::Hash>;
}

/// A hash operation which uses OpenSSL.
pub struct OsslHashOp<H> {
    hasher: Hasher,
    phantom: PhantomData<H>,
    kind: HashKind,
}

impl<H> OsslHashOp<H> {
    fn new(arg: HashKind) -> Result<Self> {
        let hasher = Hasher::new(arg.into())?;
        let phantom = PhantomData;
        Ok(OsslHashOp {
            hasher,
            phantom,
            kind: arg,
        })
    }
}

impl<H> Op for OsslHashOp<H> {
    fn update(&mut self, data: &[u8]) -> Result<()> {
        Ok(self.hasher.update(data)?)
    }

    fn finish_into(&mut self, buf: &mut [u8]) -> Result<usize> {
        if buf.len() < self.kind.len() {
            return Err(bterr!(Error::IncorrectSize {
                expected: self.kind.len(),
                actual: buf.len(),
            }));
        }
        let digest = self.hasher.finish()?;
        let slice = digest.as_ref();
        buf.copy_from_slice(slice);
        Ok(slice.len())
    }
}

impl<H: Hash + From<DigestBytes>> HashOp for OsslHashOp<H> {
    type Hash = H;

    fn kind(&self) -> HashKind {
        self.kind
    }

    fn finish(&mut self) -> Result<Self::Hash> {
        let digest = self.hasher.finish()?;
        Ok(H::from(digest))
    }
}

/// A wrapper which updates a [HashOp] when data is read or written.
pub struct HashStream<T, Op: HashOp> {
    inner: T,
    op: Op,
    update_failed: bool,
}

impl<T, Op: HashOp> HashStream<T, Op> {
    /// Create a new `HashWrap`.
    pub fn new(inner: T, op: Op) -> HashStream<T, Op> {
        HashStream {
            inner,
            op,
            update_failed: false,
        }
    }

    /// Finish this hash operation and write the result into the given buffer. The number of bytes
    /// written is returned.
    pub fn finish_into(&mut self, buf: &mut [u8]) -> Result<usize> {
        if self.update_failed {
            return Err(bterr!(
                "HashStream::finish_into can't produce result due to HashOp update failure",
            ));
        }
        self.op.finish_into(buf)
    }

    /// Finish this hash operation and return the resulting hash.
    pub fn finish(&mut self) -> Result<Op::Hash> {
        if self.update_failed {
            return Err(bterr!(
                "HashStream::finish can't produce result due to HashOp update failure",
            ));
        }
        self.op.finish()
    }
}

impl<T: Read, Op: HashOp> Read for HashStream<T, Op> {
    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
        if self.update_failed {
            return Err(bterr!(
                "HashStream::read can't continue due to previous HashOp update failure",
            )
            .into());
        }
        let read = self.inner.read(buf)?;
        if read > 0 {
            if let Err(err) = self.op.update(&buf[..read]) {
                self.update_failed = true;
                error!("HashWrap::read failed to update HashOp: {}", err);
            }
        }
        Ok(read)
    }
}

impl<T: Write, Op: HashOp> Write for HashStream<T, Op> {
    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
        self.op.update(buf)?;
        self.inner.write(buf)
    }

    fn flush(&mut self) -> io::Result<()> {
        self.inner.flush()
    }
}

/// A cryptographic hash.
pub trait Hash: AsRef<[u8]> + AsMut<[u8]> + Sized {
    /// The hash operation associated with this `Hash`.
    type Op: HashOp;
    /// The type of the argument required by `new`.
    type Arg;

    /// Returns a new `Hash` instance.
    fn new(arg: Self::Arg) -> Self;
    /// Returns the `HashKind` of self.
    fn kind(&self) -> HashKind;
    /// Starts a new hash operation.
    fn start_op(&self) -> Result<Self::Op>;
}

/// Trait for hash types which can be created with no arguments.
pub trait DefaultHash: Hash {
    fn default() -> Self;
}

impl<A: Default, T: Hash<Arg = A>> DefaultHash for T {
    fn default() -> Self {
        Self::new(A::default())
    }
}

/// Represents the SHA2-256 hash algorithm.
#[derive(Debug, PartialEq, Eq, PartialOrd, Ord, Serialize, Deserialize, Hashable, Clone)]
pub struct Sha2_256([u8; Self::LEN]);

impl Sha2_256 {
    pub const KIND: HashKind = HashKind::Sha2_256;
    pub const LEN: usize = Self::KIND.len();
}

impl AsRef<[u8]> for Sha2_256 {
    fn as_ref(&self) -> &[u8] {
        self.0.as_slice()
    }
}

impl AsMut<[u8]> for Sha2_256 {
    fn as_mut(&mut self) -> &mut [u8] {
        self.0.as_mut_slice()
    }
}

impl From<DigestBytes> for Sha2_256 {
    fn from(value: DigestBytes) -> Self {
        let mut hash = Sha2_256::new(());
        // TODO: It would be great if there was a way to avoid this copy.
        hash.as_mut().copy_from_slice(value.as_ref());
        hash
    }
}

impl From<[u8; Self::LEN]> for Sha2_256 {
    fn from(value: [u8; Self::LEN]) -> Self {
        Sha2_256(value)
    }
}

impl From<Sha2_256> for [u8; Sha2_256::LEN] {
    fn from(value: Sha2_256) -> Self {
        value.0
    }
}

impl Hash for Sha2_256 {
    type Op = OsslHashOp<Sha2_256>;
    type Arg = ();

    fn new(_: Self::Arg) -> Self {
        Sha2_256([0u8; Self::KIND.len()])
    }

    fn kind(&self) -> HashKind {
        Self::KIND
    }

    fn start_op(&self) -> Result<Self::Op> {
        OsslHashOp::new(Self::KIND)
    }
}

/// Represents the SHA2-512 hash algorithm.
#[derive(Debug, PartialEq, Eq, PartialOrd, Ord, Serialize, Deserialize, Hashable, Clone)]
pub struct Sha2_512(#[serde(with = "BigArray")] [u8; Self::LEN]);

impl Sha2_512 {
    pub const KIND: HashKind = HashKind::Sha2_512;
    pub const LEN: usize = Self::KIND.len();
}

impl AsRef<[u8]> for Sha2_512 {
    fn as_ref(&self) -> &[u8] {
        self.0.as_slice()
    }
}

impl AsMut<[u8]> for Sha2_512 {
    fn as_mut(&mut self) -> &mut [u8] {
        self.0.as_mut_slice()
    }
}

impl From<DigestBytes> for Sha2_512 {
    fn from(value: DigestBytes) -> Self {
        let mut hash = Sha2_512::new(());
        hash.as_mut().copy_from_slice(value.as_ref());
        hash
    }
}

impl From<[u8; Self::LEN]> for Sha2_512 {
    fn from(value: [u8; Self::LEN]) -> Self {
        Self(value)
    }
}

impl From<Sha2_512> for [u8; Sha2_512::LEN] {
    fn from(value: Sha2_512) -> Self {
        value.0
    }
}

impl Hash for Sha2_512 {
    type Op = OsslHashOp<Sha2_512>;
    type Arg = ();

    fn new(_: Self::Arg) -> Self {
        Sha2_512([0u8; Self::LEN])
    }

    fn kind(&self) -> HashKind {
        Self::KIND
    }

    fn start_op(&self) -> Result<Self::Op> {
        OsslHashOp::new(Self::KIND)
    }
}

/// One of several concrete hash types.
#[derive(
    Debug,
    PartialEq,
    Eq,
    Serialize,
    Deserialize,
    Hashable,
    Clone,
    EnumDiscriminants,
    PartialOrd,
    Ord,
)]
#[strum_discriminants(derive(FromRepr, Display, Serialize, Deserialize))]
#[strum_discriminants(name(HashKind))]
pub enum VarHash {
    Sha2_256(Sha2_256),
    Sha2_512(Sha2_512),
}

impl VarHash {
    /// The character that's used to separate a hash type from its value in its string
    /// representation.
    const HASH_SEP: char = '!';

    pub fn kind(&self) -> HashKind {
        self.into()
    }

    pub fn as_slice(&self) -> &[u8] {
        self.as_ref()
    }

    pub fn as_mut_slice(&mut self) -> &mut [u8] {
        self.as_mut()
    }
}

impl From<HashKind> for VarHash {
    fn from(kind: HashKind) -> VarHash {
        match kind {
            HashKind::Sha2_256 => VarHash::Sha2_256(Sha2_256::default()),
            HashKind::Sha2_512 => VarHash::Sha2_512(Sha2_512::default()),
        }
    }
}

impl AsRef<[u8]> for VarHash {
    fn as_ref(&self) -> &[u8] {
        match self {
            VarHash::Sha2_256(arr) => arr.as_ref(),
            VarHash::Sha2_512(arr) => arr.as_ref(),
        }
    }
}

impl AsMut<[u8]> for VarHash {
    fn as_mut(&mut self) -> &mut [u8] {
        match self {
            VarHash::Sha2_256(arr) => arr.as_mut(),
            VarHash::Sha2_512(arr) => arr.as_mut(),
        }
    }
}

impl TryFrom<MessageDigest> for VarHash {
    type Error = crate::Error;

    fn try_from(value: MessageDigest) -> Result<Self> {
        let kind: HashKind = value.try_into()?;
        Ok(kind.into())
    }
}

impl Hash for VarHash {
    type Op = VarHashOp;
    type Arg = HashKind;

    fn new(arg: Self::Arg) -> Self {
        arg.into()
    }

    fn kind(&self) -> HashKind {
        self.kind()
    }

    fn start_op(&self) -> Result<Self::Op> {
        VarHashOp::new(self.kind())
    }
}

impl TryFrom<&str> for VarHash {
    type Error = crate::Error;

    fn try_from(string: &str) -> Result<VarHash> {
        let mut split: Vec<&str> = string.split(Self::HASH_SEP).collect();
        if split.len() != 2 {
            return Err(bterr!(Error::InvalidHashFormat));
        };
        let second = split.pop().ok_or(Error::InvalidHashFormat)?;
        let first = split
            .pop()
            .ok_or(Error::InvalidHashFormat)?
            .parse::<usize>()
            .map_err(|_| Error::InvalidHashFormat)?;
        let mut hash = VarHash::from(HashKind::from_repr(first).ok_or(Error::InvalidHashFormat)?);
        base64_url::decode_to_slice(second, hash.as_mut()).map_err(|_| Error::InvalidHashFormat)?;
        Ok(hash)
    }
}

impl Display for VarHash {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        let hash_kind: HashKind = self.into();
        let hash_data = base64_url::encode(self.as_ref());
        write!(f, "{}{}{hash_data}", hash_kind as u32, VarHash::HASH_SEP)
    }
}

#[allow(clippy::derivable_impls)]
impl Default for HashKind {
    fn default() -> HashKind {
        HashKind::Sha2_256
    }
}

impl Default for VarHash {
    fn default() -> Self {
        HashKind::default().into()
    }
}

impl HashKind {
    #[allow(clippy::len_without_is_empty)]
    pub const fn len(self) -> usize {
        match self {
            HashKind::Sha2_256 => 32,
            HashKind::Sha2_512 => 64,
        }
    }

    pub fn digest<'a, I: Iterator<Item = &'a [u8]>>(self, dest: &mut [u8], parts: I) -> Result<()> {
        btensure!(
            dest.len() >= self.len(),
            Error::TooSmall {
                required: self.len(),
                actual: dest.len(),
            }
        );
        let mut hasher = Hasher::new(self.into())?;
        for part in parts {
            hasher.update(part)?;
        }
        let hash = hasher.finish()?;
        dest[..self.len()].copy_from_slice(&hash);
        Ok(())
    }
}

/// An implementation of [std::hash::Hasher] which allows cryptographic hash algorithms to be used.
pub struct BtHasher {
    hasher: RefCell<Hasher>,
}

impl BtHasher {
    pub fn new(kind: HashKind) -> Result<Self> {
        btensure!(
            kind.len() >= 8,
            bterr!("only digests which produce at least 8 bytes are supported")
        );
        let hasher = RefCell::new(Hasher::new(kind.into())?);
        Ok(Self { hasher })
    }
}

impl std::hash::Hasher for BtHasher {
    fn write(&mut self, bytes: &[u8]) {
        let hasher = self.hasher.get_mut();
        hasher.update(bytes).unwrap();
    }

    fn finish(&self) -> u64 {
        let mut hasher = self.hasher.borrow_mut();
        let hash = hasher.finish().unwrap();
        let mut buf = [0u8; 8];
        buf.copy_from_slice(&hash[..8]);
        u64::from_le_bytes(buf)
    }

    fn write_u8(&mut self, i: u8) {
        self.write(&[i])
    }

    fn write_u16(&mut self, i: u16) {
        self.write(&i.to_le_bytes())
    }

    fn write_u32(&mut self, i: u32) {
        self.write(&i.to_le_bytes())
    }

    fn write_u64(&mut self, i: u64) {
        self.write(&i.to_le_bytes())
    }

    fn write_u128(&mut self, i: u128) {
        self.write(&i.to_le_bytes())
    }

    fn write_usize(&mut self, i: usize) {
        self.write(&i.to_le_bytes())
    }

    fn write_i8(&mut self, i: i8) {
        self.write_u8(i as u8)
    }

    fn write_i16(&mut self, i: i16) {
        self.write_u16(i as u16)
    }

    fn write_i32(&mut self, i: i32) {
        self.write_u32(i as u32)
    }

    fn write_i64(&mut self, i: i64) {
        self.write_u64(i as u64)
    }

    fn write_i128(&mut self, i: i128) {
        self.write_u128(i as u128)
    }

    fn write_isize(&mut self, i: isize) {
        self.write_usize(i as usize)
    }
}

impl TryFrom<MessageDigest> for HashKind {
    type Error = crate::Error;

    fn try_from(value: MessageDigest) -> Result<Self> {
        let nid = value.type_();
        if Nid::SHA256 == nid {
            Ok(HashKind::Sha2_256)
        } else if Nid::SHA512 == nid {
            Ok(HashKind::Sha2_512)
        } else {
            Err(bterr!("Unsupported MessageDigest with NID: {:?}", nid))
        }
    }
}

impl From<HashKind> for MessageDigest {
    fn from(kind: HashKind) -> Self {
        match kind {
            HashKind::Sha2_256 => MessageDigest::sha256(),
            HashKind::Sha2_512 => MessageDigest::sha512(),
        }
    }
}

/// A [HashOp] which produces a [VarHash].
pub struct VarHashOp {
    kind: HashKind,
    hasher: Hasher,
}

impl VarHashOp {
    pub fn new(arg: HashKind) -> Result<Self> {
        let hasher = Hasher::new(arg.into())?;
        Ok(VarHashOp { kind: arg, hasher })
    }
}

impl Op for VarHashOp {
    fn update(&mut self, data: &[u8]) -> Result<()> {
        Ok(self.hasher.update(data)?)
    }

    fn finish_into(&mut self, buf: &mut [u8]) -> Result<usize> {
        btensure!(
            buf.len() >= self.kind.len(),
            bterr!(Error::IncorrectSize {
                expected: self.kind.len(),
                actual: buf.len(),
            })
        );
        let digest = self.hasher.finish()?;
        let slice = digest.as_ref();
        buf.copy_from_slice(slice);
        Ok(slice.len())
    }
}

impl HashOp for VarHashOp {
    type Hash = VarHash;

    fn kind(&self) -> HashKind {
        self.kind
    }

    fn finish(&mut self) -> Result<Self::Hash> {
        let digest = self.hasher.finish()?;
        let mut hash: VarHash = self.kind.into();
        hash.as_mut().copy_from_slice(digest.as_ref());
        Ok(hash)
    }
}

/// A cryptographic signature.
#[derive(Debug, PartialEq, Eq, Serialize, Deserialize, Clone, Default)]
pub struct Signature {
    kind: Sign,
    data: Vec<u8>,
}

impl Signature {
    pub fn new(kind: Sign, data: Vec<u8>) -> Self {
        Self { kind, data }
    }

    pub fn empty(kind: Sign) -> Signature {
        let data = vec![0; kind.key_len() as usize];
        Signature { kind, data }
    }

    pub fn copy_from(kind: Sign, from: &[u8]) -> Signature {
        let mut data = vec![0; kind.key_len() as usize];
        data.as_mut_slice().copy_from_slice(from);
        Signature { kind, data }
    }

    pub fn as_slice(&self) -> &[u8] {
        self.data.as_slice()
    }

    pub fn as_mut_slice(&mut self) -> &mut [u8] {
        self.data.as_mut_slice()
    }

    pub fn scheme(&self) -> Sign {
        self.kind
    }

    pub fn take_data(self) -> Vec<u8> {
        self.data
    }
}

impl AsRef<[u8]> for Signature {
    fn as_ref(&self) -> &[u8] {
        self.as_slice()
    }
}

impl AsMut<[u8]> for Signature {
    fn as_mut(&mut self) -> &mut [u8] {
        self.as_mut_slice()
    }
}

#[derive(Serialize, Deserialize)]
pub struct TaggedCiphertext<T, U> {
    aad: U,
    ciphertext: Ciphertext<T>,
    tag: Vec<u8>,
}

#[derive(EnumDiscriminants, ZeroizeOnDrop, Serialize, Deserialize, PartialEq, Eq)]
#[strum_discriminants(name(AeadKeyKind))]
#[strum_discriminants(derive(Serialize, Deserialize))]
pub enum AeadKey {
    AesGcm256 {
        key: [u8; AeadKeyKind::AesGcm256.key_len()],
        iv: [u8; AeadKeyKind::AesGcm256.iv_len()],
    },
}

impl AeadKeyKind {
    const fn key_len(self) -> usize {
        match self {
            AeadKeyKind::AesGcm256 => 32,
        }
    }

    const fn iv_len(self) -> usize {
        match self {
            AeadKeyKind::AesGcm256 => 16,
        }
    }
}

#[derive(Serialize, Deserialize)]
/// Parameters used to derive cryptographic keys from passwords.
pub struct DerivationParams {
    iter: usize,
    hash: HashKind,
    kind: AeadKeyKind,
    salt: Vec<u8>,
    iv: Vec<u8>,
}

impl DerivationParams {
    const PBKDF2_ITER: usize = 1000000;
    const PBKDF2_HASH: HashKind = HashKind::Sha2_256;
    /// The [AeadKeyKind] of the key derived from this struct.
    pub const EXPORT_KEY_KIND: AeadKeyKind = AeadKeyKind::AesGcm256;

    /// Creates a new struct containing the default value of all parameters.
    pub fn new() -> Result<DerivationParams> {
        const_assert!(
            DerivationParams::PBKDF2_HASH.len() == DerivationParams::EXPORT_KEY_KIND.key_len()
        );
        Ok(DerivationParams {
            iter: Self::PBKDF2_ITER,
            hash: Self::PBKDF2_HASH,
            kind: Self::EXPORT_KEY_KIND,
            salt: rand_vec(Self::PBKDF2_HASH.len())?,
            iv: rand_vec(Self::EXPORT_KEY_KIND.iv_len())?,
        })
    }

    /// Returns an HMAC of the given password based on the parameters in this struct.
    pub fn hmac(&self, password: &str) -> Result<Zeroizing<[u8; Self::EXPORT_KEY_KIND.key_len()]>> {
        let mut key = Zeroizing::new([0u8; Self::EXPORT_KEY_KIND.key_len()]);
        pbkdf2_hmac(
            password.as_bytes(),
            self.salt.as_slice(),
            self.iter,
            self.hash.into(),
            key.as_mut_slice(),
        )?;
        Ok(key)
    }

    /// Derives an [AeadKey] from the given password using the parameters in this struct.
    fn derive_key(&self, password: &str) -> Result<AeadKey> {
        let key = self.hmac(password)?;
        AeadKey::copy_components(self.kind, key.as_slice(), &self.iv)
    }
}

fn array_from<const N: usize>(slice: &[u8]) -> Result<[u8; N]> {
    let slice_len = slice.len();
    btensure!(
        N == slice_len,
        Error::IncorrectSize {
            actual: slice_len,
            expected: N,
        }
    );
    let mut array = [0u8; N];
    array.copy_from_slice(slice);
    Ok(array)
}

impl AeadKey {
    pub fn new(kind: AeadKeyKind) -> Result<AeadKey> {
        match kind {
            AeadKeyKind::AesGcm256 => Ok(AeadKey::AesGcm256 {
                key: rand_array()?,
                iv: rand_array()?,
            }),
        }
    }

    fn copy_components(kind: AeadKeyKind, key_buf: &[u8], iv_buf: &[u8]) -> Result<AeadKey> {
        match kind {
            AeadKeyKind::AesGcm256 => Ok(AeadKey::AesGcm256 {
                key: array_from(key_buf)?,
                iv: array_from(iv_buf)?,
            }),
        }
    }

    fn encrypt<T: Serialize + DeserializeOwned, U: Serialize + DeserializeOwned>(
        &self,
        aad: U,
        plaintext: &T,
    ) -> Result<TaggedCiphertext<T, U>> {
        let (cipher, key, iv, mut tag) = match self {
            AeadKey::AesGcm256 { key, iv } => (
                Cipher::aes_256_gcm(),
                key.as_slice(),
                iv.as_slice(),
                vec![0u8; 16],
            ),
        };

        let aad_data = to_vec(&aad)?;
        let plaintext_buf = to_vec(&plaintext)?;
        let mut ciphertext = vec![0u8; plaintext_buf.len() + cipher.block_size()];
        let mut crypter = Crypter::new(cipher, Mode::Encrypt, key, Some(iv))?;
        crypter.aad_update(&aad_data)?;
        let mut count = crypter.update(&plaintext_buf, &mut ciphertext)?;
        count += crypter.finalize(&mut ciphertext[count..])?;
        ciphertext.truncate(count);
        crypter.get_tag(&mut tag)?;

        Ok(TaggedCiphertext {
            aad,
            ciphertext: Ciphertext::new(ciphertext),
            tag,
        })
    }

    fn decrypt<T: Serialize + DeserializeOwned, U: Serialize + DeserializeOwned>(
        &self,
        tagged: &TaggedCiphertext<T, U>,
    ) -> Result<T> {
        let ciphertext = &tagged.ciphertext.data;
        let (cipher, key, iv) = match self {
            AeadKey::AesGcm256 { key, iv } => {
                (Cipher::aes_256_gcm(), key.as_slice(), iv.as_slice())
            }
        };

        let mut plaintext = vec![0u8; ciphertext.len() + cipher.block_size()];
        let mut crypter = Crypter::new(cipher, Mode::Decrypt, key, Some(iv))?;
        crypter.set_tag(&tagged.tag)?;
        let aad_buf = to_vec(&tagged.aad)?;
        crypter.aad_update(&aad_buf)?;
        let mut count = crypter.update(ciphertext, &mut plaintext)?;
        count += crypter.finalize(&mut plaintext[count..])?;
        plaintext.truncate(count);

        Ok(from_vec(&plaintext)?)
    }
}

#[derive(Debug, PartialEq, Eq, Serialize, Deserialize, Clone, EnumDiscriminants, ZeroizeOnDrop)]
#[strum_discriminants(name(SymKeyKind))]
pub enum SymKey {
    /// A key for the AES 256 cipher in Cipher Block Chaining mode. Note that this includes the
    /// initialization vector, so that a value of this variant contains all the information needed
    /// to fully initialize a cipher context.
    Aes256Cbc { key: [u8; 32], iv: [u8; 16] },
    /// A key for the AES 256 cipher in counter mode.
    Aes256Ctr { key: [u8; 32], iv: [u8; 16] },
}

struct SymParams<'a> {
    cipher: Cipher,
    key: &'a [u8],
    iv: Option<&'a [u8]>,
}

impl SymKey {
    pub(crate) fn generate(kind: SymKeyKind) -> Result<SymKey> {
        match kind {
            SymKeyKind::Aes256Cbc => Ok(SymKey::Aes256Cbc {
                key: rand_array()?,
                iv: rand_array()?,
            }),
            SymKeyKind::Aes256Ctr => Ok(SymKey::Aes256Ctr {
                key: rand_array()?,
                iv: rand_array()?,
            }),
        }
    }

    fn params(&self) -> SymParams {
        let (cipher, key, iv) = match self {
            SymKey::Aes256Cbc { key, iv } => (Cipher::aes_256_cbc(), key, Some(iv.as_slice())),
            SymKey::Aes256Ctr { key, iv } => (Cipher::aes_256_ctr(), key, Some(iv.as_slice())),
        };
        SymParams { cipher, key, iv }
    }

    fn block_size(&self) -> usize {
        let SymParams { cipher, .. } = self.params();
        cipher.block_size()
    }

    // The number of bytes that the plaintext expands by when encrypted.
    fn expansion_sz(&self) -> usize {
        match self {
            SymKey::Aes256Cbc { .. } => 16,
            SymKey::Aes256Ctr { .. } => 0,
        }
    }

    pub fn key_slice(&self) -> &[u8] {
        let SymParams { key, .. } = self.params();
        key
    }

    pub fn iv_slice(&self) -> Option<&[u8]> {
        let SymParams { iv, .. } = self.params();
        iv
    }
}

impl Encrypter for SymKey {
    fn encrypt(&self, slice: &[u8]) -> Result<Vec<u8>> {
        let SymParams { cipher, key, iv } = self.params();
        Ok(openssl_encrypt(cipher, key, iv, slice)?)
    }
}

impl Decrypter for SymKey {
    fn decrypt(&self, slice: &[u8]) -> Result<Vec<u8>> {
        let SymParams { cipher, key, iv } = self.params();
        Ok(openssl_decrypt(cipher, key, iv, slice)?)
    }
}

#[allow(clippy::derivable_impls)]
impl Default for SymKeyKind {
    fn default() -> Self {
        SymKeyKind::Aes256Ctr
    }
}

#[repr(u32)]
#[derive(Debug, Display, Clone, Copy, Serialize, Deserialize, PartialEq, Eq)]
pub enum BitLen {
    Bits128 = 16,
    Bits256 = 32,
    Bits512 = 64,
    Bits2048 = 256,
    Bits3072 = 384,
    Bits4096 = 512,
}

impl BitLen {
    const fn bits(self) -> u32 {
        8 * self as u32
    }

    fn try_from_u32(value: u32) -> Result<Self> {
        match value {
            16 => Ok(Self::Bits128),
            32 => Ok(Self::Bits256),
            64 => Ok(Self::Bits512),
            256 => Ok(Self::Bits2048),
            384 => Ok(Self::Bits3072),
            512 => Ok(Self::Bits4096),
            _ => Err(bterr!("invalid KeyLen value: {value}")),
        }
    }
}

impl TryFrom<u32> for BitLen {
    type Error = crate::Error;
    fn try_from(value: u32) -> std::result::Result<Self, Self::Error> {
        Self::try_from_u32(value)
    }
}

/// A Cryptographic Scheme. This is a common type for operations such as encrypting, decrypting,
/// signing and verifying.
pub trait Scheme:
    DeserializeOwned + Serialize + Copy + std::fmt::Debug + PartialEq + Into<Self::Kind>
{
    type Kind: Scheme;
    fn as_enum(self) -> SchemeKind;
    fn hash_kind(&self) -> HashKind;
    fn padding(&self) -> Option<OpensslPadding>;
    fn public_from_der(self, der: &[u8]) -> Result<PKey<Public>>;
    fn private_from_der(self, der: &[u8]) -> Result<PKey<Private>>;
    fn generate(self) -> Result<AsymKeyPair<Self::Kind>>;
    fn key_len(self) -> BitLen;

    fn message_digest(&self) -> MessageDigest {
        self.hash_kind().into()
    }
}

pub enum SchemeKind {
    Sign(Sign),
    Encrypt(Encrypt),
}

#[derive(Deserialize, Serialize, Clone, Debug, PartialEq, Eq, Copy)]
pub enum Encrypt {
    RsaEsOaep(RsaEsOaep),
}

impl Scheme for Encrypt {
    type Kind = Encrypt;

    fn as_enum(self) -> SchemeKind {
        SchemeKind::Encrypt(self)
    }

    fn hash_kind(&self) -> HashKind {
        match self {
            Encrypt::RsaEsOaep(inner) => inner.hash_kind(),
        }
    }

    fn padding(&self) -> Option<OpensslPadding> {
        match self {
            Encrypt::RsaEsOaep(inner) => inner.padding(),
        }
    }

    fn public_from_der(self, der: &[u8]) -> Result<PKey<Public>> {
        match self {
            Encrypt::RsaEsOaep(inner) => inner.public_from_der(der),
        }
    }

    fn private_from_der(self, der: &[u8]) -> Result<PKey<Private>> {
        match self {
            Encrypt::RsaEsOaep(inner) => inner.private_from_der(der),
        }
    }

    fn generate(self) -> Result<AsymKeyPair<Self::Kind>> {
        match self {
            Encrypt::RsaEsOaep(inner) => inner.generate(),
        }
    }

    fn key_len(self) -> BitLen {
        match self {
            Encrypt::RsaEsOaep(inner) => inner.key_len(),
        }
    }
}

impl Encrypt {
    pub const RSA_OAEP_2048_SHA_256: Encrypt = Encrypt::RsaEsOaep(RsaEsOaep {
        key_len: BitLen::Bits2048,
        hash_kind: HashKind::Sha2_256,
    });

    pub const RSA_OAEP_3072_SHA_256: Encrypt = Encrypt::RsaEsOaep(RsaEsOaep {
        key_len: BitLen::Bits3072,
        hash_kind: HashKind::Sha2_256,
    });
}

#[derive(Deserialize, Serialize, Clone, Debug, PartialEq, Eq, Copy)]
pub enum Sign {
    RsaSsaPss(RsaSsaPss),
}

impl Default for Sign {
    fn default() -> Self {
        Self::RSA_PSS_2048_SHA_256
    }
}

impl Scheme for Sign {
    type Kind = Sign;

    fn as_enum(self) -> SchemeKind {
        SchemeKind::Sign(self)
    }

    fn hash_kind(&self) -> HashKind {
        match self {
            Sign::RsaSsaPss(inner) => inner.hash_kind(),
        }
    }

    fn padding(&self) -> Option<OpensslPadding> {
        match self {
            Sign::RsaSsaPss(inner) => inner.padding(),
        }
    }

    fn public_from_der(self, der: &[u8]) -> Result<PKey<Public>> {
        match self {
            Sign::RsaSsaPss(inner) => inner.public_from_der(der),
        }
    }

    fn private_from_der(self, der: &[u8]) -> Result<PKey<Private>> {
        match self {
            Sign::RsaSsaPss(inner) => inner.private_from_der(der),
        }
    }

    fn generate(self) -> Result<AsymKeyPair<Self::Kind>> {
        match self {
            Sign::RsaSsaPss(inner) => inner.generate(),
        }
    }

    fn key_len(self) -> BitLen {
        self.key_len_const()
    }
}

impl Sign {
    pub const RSA_PSS_2048_SHA_256: Sign = Sign::RsaSsaPss(RsaSsaPss {
        key_bits: BitLen::Bits2048,
        hash_kind: HashKind::Sha2_256,
    });

    pub const RSA_PSS_3072_SHA_256: Sign = Sign::RsaSsaPss(RsaSsaPss {
        key_bits: BitLen::Bits3072,
        hash_kind: HashKind::Sha2_256,
    });

    const fn key_len_const(self) -> BitLen {
        match self {
            Sign::RsaSsaPss(inner) => inner.key_bits,
        }
    }
}

enum Rsa {}

impl Rsa {
    /// The default public exponent to use for generated RSA keys.
    const EXP: u32 = 65537; // 2**16 + 1

    fn generate<S: Scheme>(scheme: S) -> Result<AsymKeyPair<S>> {
        let key = OsslRsa::generate(scheme.key_len().bits())?;
        // TODO: Separating the keys this way seems inefficient. Investigate alternatives.
        let public_der = key.public_key_to_der()?;
        let private_der = key.private_key_to_der()?;
        let public = AsymKey::<Public, S>::new(scheme, &public_der)?;
        let private = AsymKey::<Private, S>::new(scheme, &private_der)?;
        Ok(AsymKeyPair { public, private })
    }
}

#[derive(Deserialize, Serialize, Clone, Debug, PartialEq, Eq, Copy)]
pub struct RsaEsOaep {
    key_len: BitLen,
    hash_kind: HashKind,
}

impl Scheme for RsaEsOaep {
    type Kind = Encrypt;

    fn as_enum(self) -> SchemeKind {
        SchemeKind::Encrypt(self.into())
    }

    fn hash_kind(&self) -> HashKind {
        self.hash_kind
    }

    fn padding(&self) -> Option<OpensslPadding> {
        Some(OpensslPadding::PKCS1_OAEP)
    }

    fn public_from_der(self, der: &[u8]) -> Result<PKey<Public>> {
        Ok(PKey::public_key_from_der(der)?.conv_pub())
    }

    fn private_from_der(self, der: &[u8]) -> Result<PKey<Private>> {
        Ok(PKey::private_key_from_der(der)?.conv_priv())
    }

    fn generate(self) -> Result<AsymKeyPair<Self::Kind>> {
        Rsa::generate(self.into())
    }

    fn key_len(self) -> BitLen {
        self.key_len
    }
}

impl From<RsaEsOaep> for Encrypt {
    fn from(scheme: RsaEsOaep) -> Self {
        Encrypt::RsaEsOaep(scheme)
    }
}

#[derive(Deserialize, Serialize, Clone, Debug, PartialEq, Eq, Copy)]
pub struct RsaSsaPss {
    key_bits: BitLen,
    hash_kind: HashKind,
}

impl Scheme for RsaSsaPss {
    type Kind = Sign;

    fn as_enum(self) -> SchemeKind {
        SchemeKind::Sign(self.into())
    }

    fn hash_kind(&self) -> HashKind {
        self.hash_kind
    }

    fn padding(&self) -> Option<OpensslPadding> {
        Some(OpensslPadding::PKCS1_PSS)
    }

    fn public_from_der(self, der: &[u8]) -> Result<PKey<Public>> {
        Ok(PKey::public_key_from_der(der)?.conv_pub())
    }

    fn private_from_der(self, der: &[u8]) -> Result<PKey<Private>> {
        Ok(PKey::private_key_from_der(der)?.conv_priv())
    }

    fn generate(self) -> Result<AsymKeyPair<Self::Kind>> {
        Rsa::generate(self.into())
    }

    fn key_len(self) -> BitLen {
        self.key_bits
    }
}

impl From<RsaSsaPss> for Sign {
    fn from(scheme: RsaSsaPss) -> Self {
        Sign::RsaSsaPss(scheme)
    }
}

/// Marker trait for the `Public` and `Private` key privacy types.
pub trait KeyPrivacy {}

/// Represents keys which can be shared freely.
#[derive(Clone, Debug)]
pub enum Public {}

impl KeyPrivacy for Public {}

unsafe impl HasPublic for Public {}

#[derive(Debug, Clone)]
/// Represents keys which must be kept confidential.
pub enum Private {}

impl KeyPrivacy for Private {}

unsafe impl HasPrivate for Private {}

trait PKeyExt<T> {
    /// Converts a PKey<T> to a PKey<U>. This hack allows for converting between openssl's
    /// Public and Private types and ours.
    fn conv_pkey<U>(self) -> PKey<U>;

    /// Convert from openssl's Public type to `crypto::Public`.
    fn conv_pub(self) -> PKey<Public>;

    /// Convert from openssl's Private type to `crypto::Private`.
    fn conv_priv(self) -> PKey<Private>;
}

impl<T> PKeyExt<T> for PKey<T> {
    fn conv_pkey<U>(self) -> PKey<U> {
        let ptr = self.as_ptr();
        let new_pkey = unsafe { PKey::from_ptr(ptr) };
        std::mem::forget(self);
        new_pkey
    }

    fn conv_pub(self) -> PKey<Public> {
        self.conv_pkey()
    }

    fn conv_priv(self) -> PKey<Private> {
        self.conv_pkey()
    }
}

/// Represents any kind of asymmetric key.
#[derive(Debug, Clone)]
pub struct AsymKey<P, S> {
    scheme: S,
    pkey: PKey<P>,
}

impl<P, S: Copy> AsymKey<P, S> {
    pub fn scheme(&self) -> S {
        self.scheme
    }
}

pub type AsymKeyPub<S> = AsymKey<Public, S>;

impl<S: Scheme> AsymKey<Public, S> {
    pub(crate) fn new(scheme: S, der: &[u8]) -> Result<AsymKey<Public, S>> {
        let pkey = scheme.public_from_der(der)?;
        Ok(AsymKey { scheme, pkey })
    }
}

impl<S: Scheme> AsymKey<Private, S> {
    pub(crate) fn new(scheme: S, der: &[u8]) -> Result<AsymKey<Private, S>> {
        let pkey = scheme.private_from_der(der)?;
        Ok(AsymKey { scheme, pkey })
    }

    pub fn to_der(&self) -> Result<Vec<u8>> {
        self.pkey.private_key_to_der().map_err(|err| err.into())
    }
}

macro_rules! impl_asym_key_serialize {
    ($privacy:ty, $ser_lambda:expr) => {
        impl<S: Scheme> Serialize for AsymKey<$privacy, S> {
            fn serialize<T: Serializer>(&self, s: T) -> std::result::Result<T::Ok, T::Error> {
                let mut struct_s = s.serialize_struct(stringify!(AsymKey), 2)?;
                struct_s.serialize_field("scheme", &self.scheme)?;
                let der = $ser_lambda(&self.pkey).map_err(T::Error::custom)?;
                struct_s.serialize_field("pkey", der.as_slice())?;
                struct_s.end()
            }
        }
    };
}

impl_asym_key_serialize!(Public, |pkey: &PKey<Public>| pkey.public_key_to_der());
impl_asym_key_serialize!(Private, |pkey: &PKey<Private>| pkey.private_key_to_der());

macro_rules! impl_asym_key_deserialize {
    ($privacy:ty) => {
        impl<'de, S: Scheme> Deserialize<'de> for AsymKey<$privacy, S> {
            fn deserialize<D: Deserializer<'de>>(d: D) -> std::result::Result<Self, D::Error> {
                const FIELDS: &[&str] = &["scheme", "pkey"];

                struct StructVisitor<S: Scheme>(PhantomData<S>);

                impl<'de, S: Scheme> Visitor<'de> for StructVisitor<S> {
                    type Value = AsymKey<$privacy, S>;

                    fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
                        formatter.write_fmt(format_args!("struct {}", stringify!(AsymKey)))
                    }

                    fn visit_seq<V: SeqAccess<'de>>(
                        self,
                        mut seq: V,
                    ) -> std::result::Result<Self::Value, V::Error> {
                        let scheme: S = seq
                            .next_element()?
                            .ok_or_else(|| de::Error::missing_field(FIELDS[0]))?;
                        let der: Vec<u8> = seq
                            .next_element()?
                            .ok_or_else(|| de::Error::missing_field(FIELDS[1]))?;
                        AsymKey::<$privacy, _>::new(scheme, der.as_slice())
                            .map_err(de::Error::custom)
                    }
                }

                d.deserialize_struct(stringify!(AsymKey), FIELDS, StructVisitor(PhantomData))
            }
        }
    };
}

impl_asym_key_deserialize!(Public);
impl_asym_key_deserialize!(Private);

impl<S: Scheme> PartialEq for AsymKey<Public, S> {
    fn eq(&self, other: &Self) -> bool {
        self.scheme == other.scheme && self.pkey.public_eq(&other.pkey)
    }
}

impl Principaled for AsymKey<Public, Sign> {
    fn principal_of_kind(&self, kind: HashKind) -> Principal {
        let der = self.pkey.public_key_to_der().unwrap();
        let bytes = hash(kind.into(), der.as_slice()).unwrap();
        let mut hash_buf = VarHash::from(kind);
        hash_buf.as_mut().copy_from_slice(&bytes);
        Principal(hash_buf)
    }
}

impl Encrypter for AsymKey<Public, Encrypt> {
    fn encrypt(&self, slice: &[u8]) -> Result<Vec<u8>> {
        let mut encrypter = OsslEncrypter::new(&self.pkey)?;
        if let Some(padding) = self.scheme.padding() {
            encrypter.set_rsa_padding(padding)?;
        }
        {
            let Encrypt::RsaEsOaep(inner) = self.scheme;
            encrypter.set_rsa_oaep_md(inner.message_digest())?;
        }
        let buffer_len = encrypter.encrypt_len(slice)?;
        let mut ciphertext = vec![0; buffer_len];
        let ciphertext_len = encrypter.encrypt(slice, &mut ciphertext)?;
        ciphertext.truncate(ciphertext_len);
        Ok(ciphertext)
    }
}

impl Decrypter for AsymKey<Private, Encrypt> {
    fn decrypt(&self, slice: &[u8]) -> Result<Vec<u8>> {
        let mut decrypter = OsslDecrypter::new(&self.pkey)?;
        if let Some(padding) = self.scheme.padding() {
            decrypter.set_rsa_padding(padding)?;
        }
        {
            let Encrypt::RsaEsOaep(inner) = self.scheme;
            decrypter.set_rsa_oaep_md(inner.message_digest())?;
        }
        let buffer_len = decrypter.decrypt_len(slice)?;
        let mut plaintext = vec![0; buffer_len];
        let plaintext_len = decrypter.decrypt(slice, &mut plaintext)?;
        plaintext.truncate(plaintext_len);
        Ok(plaintext)
    }
}

impl Signer for AsymKey<Private, Sign> {
    fn init_sign(&self) -> Result<Box<dyn '_ + SignOp>> {
        let op = OsslSignOp::new((self.scheme, self.pkey.as_ref()))?;
        Ok(Box::new(op))
    }

    fn sign(&self, parts: &mut dyn Iterator<Item = &[u8]>) -> Result<Signature> {
        let mut signer = OsslSigner::new(self.scheme.message_digest(), &self.pkey)?;
        if let Some(padding) = self.scheme.padding() {
            signer.set_rsa_padding(padding)?;
        }
        for part in parts {
            signer.update(part)?;
        }
        let mut signature = Signature::empty(self.scheme);
        signer.sign(signature.as_mut_slice())?;
        Ok(signature)
    }

    fn kind(&self) -> Sign {
        self.scheme
    }
}

impl Verifier for AsymKey<Public, Sign> {
    fn init_verify(&self) -> Result<Box<dyn '_ + VerifyOp>> {
        let op = OsslVerifyOp::init(self.scheme, self.pkey.as_ref())?;
        Ok(Box::new(op))
    }

    fn verify(&self, parts: &mut dyn Iterator<Item = &[u8]>, signature: &[u8]) -> Result<()> {
        let mut verifier = OsslVerifier::new(self.scheme.message_digest(), &self.pkey)?;
        if let Some(padding) = self.scheme.padding() {
            verifier.set_rsa_padding(padding)?;
        }
        for part in parts {
            verifier.update(part)?;
        }
        if verifier.verify(signature)? {
            Ok(())
        } else {
            Err(bterr!(Error::InvalidSignature))
        }
    }

    fn kind(&self) -> Sign {
        self.scheme
    }
}

#[derive(Clone, Serialize)]
pub struct AsymKeyPair<S: Scheme> {
    public: AsymKey<Public, S>,
    private: AsymKey<Private, S>,
}

impl<'de, S: Scheme> Deserialize<'de> for AsymKeyPair<S> {
    fn deserialize<D: Deserializer<'de>>(d: D) -> std::result::Result<Self, D::Error> {
        const FIELDS: &[&str] = &["public", "private"];

        struct StructVisitor<S: Scheme>(PhantomData<S>);

        impl<'de, S: Scheme> Visitor<'de> for StructVisitor<S> {
            type Value = AsymKeyPair<S>;

            fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
                formatter.write_fmt(format_args!("struct {}", stringify!(AsymKeyPair)))
            }

            fn visit_seq<V: SeqAccess<'de>>(
                self,
                mut seq: V,
            ) -> std::result::Result<Self::Value, V::Error> {
                let public: AsymKey<Public, S> = seq
                    .next_element()?
                    .ok_or_else(|| de::Error::missing_field(FIELDS[0]))?;
                let private: AsymKey<Private, S> = seq
                    .next_element()?
                    .ok_or_else(|| de::Error::missing_field(FIELDS[1]))?;
                Ok(AsymKeyPair { public, private })
            }
        }

        d.deserialize_struct(stringify!(AsymKey), FIELDS, StructVisitor(PhantomData))
    }
}

impl<S: Scheme> AsymKeyPair<S> {
    pub fn new(scheme: S, public_der: &[u8], private_der: &[u8]) -> Result<AsymKeyPair<S>> {
        let public = AsymKey::<Public, _>::new(scheme, public_der)?;
        let private = AsymKey::<Private, _>::new(scheme, private_der)?;
        Ok(AsymKeyPair { public, private })
    }

    pub fn public(&self) -> &AsymKey<Public, S> {
        &self.public
    }

    pub fn private(&self) -> &AsymKey<Private, S> {
        &self.private
    }
}

// Note that only signing keys are associated with a Principal.
impl Principaled for AsymKeyPair<Sign> {
    fn principal_of_kind(&self, kind: HashKind) -> Principal {
        self.public.principal_of_kind(kind)
    }
}

impl Encrypter for AsymKeyPair<Encrypt> {
    fn encrypt(&self, slice: &[u8]) -> Result<Vec<u8>> {
        self.public.encrypt(slice)
    }
}

impl Decrypter for AsymKeyPair<Encrypt> {
    fn decrypt(&self, slice: &[u8]) -> Result<Vec<u8>> {
        self.private.decrypt(slice)
    }
}

impl Signer for AsymKeyPair<Sign> {
    fn init_sign(&self) -> Result<Box<dyn '_ + SignOp>> {
        self.private.init_sign()
    }
    fn sign(&self, parts: &mut dyn Iterator<Item = &[u8]>) -> Result<Signature> {
        self.private.sign(parts)
    }
    fn kind(&self) -> Sign {
        self.private.kind()
    }
}

impl Verifier for AsymKeyPair<Sign> {
    fn init_verify(&self) -> Result<Box<dyn '_ + VerifyOp>> {
        self.public.init_verify()
    }

    fn verify(&self, parts: &mut dyn Iterator<Item = &[u8]>, signature: &[u8]) -> Result<()> {
        self.public.verify(parts, signature)
    }

    fn kind(&self) -> Sign {
        self.public.kind()
    }
}

#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct ConcretePub {
    pub sign: AsymKeyPub<Sign>,
    pub enc: AsymKeyPub<Encrypt>,
}

impl Principaled for ConcretePub {
    fn principal_of_kind(&self, kind: HashKind) -> Principal {
        self.sign.principal_of_kind(kind)
    }
}

impl Encrypter for ConcretePub {
    fn encrypt(&self, slice: &[u8]) -> Result<Vec<u8>> {
        self.enc.encrypt(slice)
    }
}

impl Verifier for ConcretePub {
    fn init_verify(&self) -> Result<Box<dyn '_ + VerifyOp>> {
        self.sign.init_verify()
    }

    fn verify(&self, parts: &mut dyn Iterator<Item = &[u8]>, signature: &[u8]) -> Result<()> {
        self.sign.verify(parts, signature)
    }

    fn kind(&self) -> Sign {
        self.sign.kind()
    }
}

impl CredsPub for ConcretePub {
    fn public_sign(&self) -> &AsymKey<Public, Sign> {
        &self.sign
    }

    fn concrete_pub(&self) -> ConcretePub {
        self.clone()
    }
}

impl PartialEq for ConcretePub {
    fn eq(&self, other: &Self) -> bool {
        self.principal() == other.principal()
    }
}

#[derive(Clone, Serialize, Deserialize)]
pub struct ConcreteCreds {
    sign: AsymKeyPair<Sign>,
    encrypt: AsymKeyPair<Encrypt>,
    writecap: Option<Writecap>,
}

impl ConcreteCreds {
    pub fn new(sign: AsymKeyPair<Sign>, encrypt: AsymKeyPair<Encrypt>) -> ConcreteCreds {
        ConcreteCreds {
            sign,
            encrypt,
            writecap: None,
        }
    }

    pub fn generate() -> Result<ConcreteCreds> {
        let encrypt = Encrypt::RSA_OAEP_3072_SHA_256.generate()?;
        let sign = Sign::RSA_PSS_3072_SHA_256.generate()?;
        Ok(ConcreteCreds {
            sign,
            encrypt,
            writecap: None,
        })
    }

    pub fn set_writecap(&mut self, writecap: Writecap) -> Result<()> {
        writecap.assert_issued_to(&self.principal())?;
        self.writecap = Some(writecap);
        Ok(())
    }

    pub fn sign_pair(&self) -> &AsymKeyPair<Sign> {
        &self.sign
    }

    pub fn encrypt_pair(&self) -> &AsymKeyPair<Encrypt> {
        &self.encrypt
    }
}

impl Verifier for ConcreteCreds {
    fn init_verify(&self) -> Result<Box<dyn '_ + VerifyOp>> {
        self.sign.init_verify()
    }

    fn verify(&self, parts: &mut dyn Iterator<Item = &[u8]>, signature: &[u8]) -> Result<()> {
        self.sign.verify(parts, signature)
    }

    fn kind(&self) -> Sign {
        Verifier::kind(&self.sign)
    }
}

impl Encrypter for ConcreteCreds {
    fn encrypt(&self, slice: &[u8]) -> Result<Vec<u8>> {
        self.encrypt.encrypt(slice)
    }
}

impl Principaled for ConcreteCreds {
    fn principal_of_kind(&self, kind: HashKind) -> Principal {
        self.sign.principal_of_kind(kind)
    }
}

impl CredsPub for ConcreteCreds {
    fn public_sign(&self) -> &AsymKey<Public, Sign> {
        &self.sign.public
    }

    fn concrete_pub(&self) -> ConcretePub {
        ConcretePub {
            sign: self.sign.public.clone(),
            enc: self.encrypt.public.clone(),
        }
    }
}

impl Signer for ConcreteCreds {
    fn init_sign(&self) -> Result<Box<dyn '_ + SignOp>> {
        self.sign.init_sign()
    }

    fn sign(&self, parts: &mut dyn Iterator<Item = &[u8]>) -> Result<Signature> {
        self.sign.sign(parts)
    }

    fn kind(&self) -> Sign {
        Signer::kind(&self.sign)
    }
}

impl Decrypter for ConcreteCreds {
    fn decrypt(&self, slice: &[u8]) -> Result<Vec<u8>> {
        self.encrypt.decrypt(slice)
    }
}

impl CredsPriv for ConcreteCreds {
    fn writecap(&self) -> Option<&Writecap> {
        self.writecap.as_ref()
    }
}

pub trait Encrypter {
    fn encrypt(&self, slice: &[u8]) -> Result<Vec<u8>>;
}

impl<T: ?Sized + Encrypter, P: Deref<Target = T>> Encrypter for P {
    fn encrypt(&self, slice: &[u8]) -> Result<Vec<u8>> {
        self.deref().encrypt(slice)
    }
}

pub trait EncrypterExt: Encrypter {
    /// Serializes the given value into a new vector, then encrypts it and returns the resulting
    /// ciphertext.
    fn ser_encrypt<T: Serialize>(&self, value: &T) -> Result<Ciphertext<T>> {
        let data = to_vec(value)?;
        let data = self.encrypt(&data)?;
        Ok(Ciphertext::new(data))
    }
}

impl<T: Encrypter + ?Sized> EncrypterExt for T {}

pub trait Decrypter {
    fn decrypt(&self, slice: &[u8]) -> Result<Vec<u8>>;
}

impl<T: ?Sized + Decrypter, P: Deref<Target = T>> Decrypter for P {
    fn decrypt(&self, slice: &[u8]) -> Result<Vec<u8>> {
        self.deref().decrypt(slice)
    }
}

pub trait DecrypterExt: Decrypter {
    fn ser_decrypt<T: DeserializeOwned>(&self, ct: &Ciphertext<T>) -> Result<T> {
        let pt = self.decrypt(ct.data.as_slice())?;
        Ok(from_vec(&pt)?)
    }
}

impl<T: Decrypter + ?Sized> DecrypterExt for T {}

/// Represents an ongoing signing operation.
pub trait SignOp: Op {
    /// Returns the signature scheme that this operation is using.
    fn scheme(&self) -> Sign;

    /// Finishes this signature operation and returns a new signature containing the result.
    fn finish(&mut self) -> Result<Signature> {
        let scheme = self.scheme();
        let mut sig = Signature::empty(scheme);
        self.finish_into(sig.as_mut())?;
        Ok(sig)
    }
}

impl<T: ?Sized + SignOp, P: DerefMut<Target = T>> SignOp for P {
    fn scheme(&self) -> Sign {
        self.deref().scheme()
    }
}

pub struct OsslSignOp<'a> {
    signer: OsslSigner<'a>,
    scheme: Sign,
}

impl<'a> OsslSignOp<'a> {
    pub fn new(arg: (Sign, &'a PKeyRef<Private>)) -> Result<Self> {
        let scheme = arg.0;
        let mut signer = OsslSigner::new(arg.0.message_digest(), arg.1)?;
        if let Some(padding) = scheme.padding() {
            signer.set_rsa_padding(padding)?;
        }
        Ok(OsslSignOp { signer, scheme })
    }
}

impl<'a> Op for OsslSignOp<'a> {
    fn update(&mut self, data: &[u8]) -> Result<()> {
        Ok(self.signer.update(data)?)
    }

    fn finish_into(&mut self, buf: &mut [u8]) -> Result<usize> {
        Ok(self.signer.sign(buf)?)
    }
}

impl<'a> SignOp for OsslSignOp<'a> {
    fn scheme(&self) -> Sign {
        self.scheme
    }
}

/// A struct which computes a signature over data as it is written to it.
pub struct SignWrite<T, Op> {
    inner: T,
    op: Op,
}

impl<T, Op: SignOp> SignWrite<T, Op> {
    pub fn new(inner: T, op: Op) -> Self {
        SignWrite { inner, op }
    }

    pub fn finish_into(mut self, buf: &mut [u8]) -> Result<(usize, T)> {
        Ok((self.op.finish_into(buf)?, self.inner))
    }

    pub fn finish(mut self) -> Result<(Signature, T)> {
        Ok((self.op.finish()?, self.inner))
    }
}

impl<T: Write, Op: SignOp> Write for SignWrite<T, Op> {
    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
        self.op.update(buf)?;
        self.inner.write(buf)
    }

    fn flush(&mut self) -> io::Result<()> {
        self.inner.flush()
    }
}

pub trait Signer {
    /// Starts a new signing operation and returns the struct representing it.
    fn init_sign(&self) -> Result<Box<dyn '_ + SignOp>>;

    /// Returns a signature over the given parts. It's critical that subsequent invocations
    /// of this method on the same instance return a [Signature] with `data` fields of the same
    /// length.
    fn sign(&self, parts: &mut dyn Iterator<Item = &[u8]>) -> Result<Signature>;

    fn sign_writecap(&self, writecap: &mut Writecap) -> Result<()> {
        let signed = self.ser_sign(&writecap.body)?;
        writecap.signature = signed.sig;
        Ok(())
    }

    fn kind(&self) -> Sign;
}

impl<T: ?Sized + Signer> Signer for &T {
    fn init_sign(&self) -> Result<Box<dyn '_ + SignOp>> {
        (*self).init_sign()
    }

    fn sign(&self, parts: &mut dyn Iterator<Item = &[u8]>) -> Result<Signature> {
        (*self).sign(parts)
    }

    fn kind(&self) -> Sign {
        (*self).kind()
    }
}

impl<T: ?Sized + Signer> Signer for Arc<T> {
    fn init_sign(&self) -> Result<Box<dyn '_ + SignOp>> {
        self.deref().init_sign()
    }

    fn sign(&self, parts: &mut dyn Iterator<Item = &[u8]>) -> Result<Signature> {
        self.deref().sign(parts)
    }

    fn kind(&self) -> Sign {
        self.deref().kind()
    }
}

pub trait SignerExt: Signer {
    fn ser_sign<T: Serialize>(&self, value: &T) -> Result<Signed<T>> {
        let data = to_vec(value)?;
        let sig = self.sign(&mut std::iter::once(data.as_slice()))?;
        Ok(Signed::new(data, sig))
    }

    fn ser_sign_into<T: Serialize>(&self, value: &T, buf: &mut Vec<u8>) -> Result<Signature> {
        write_to(value, &mut *buf)?;
        self.sign(&mut std::iter::once(buf.as_slice()))
    }
}

impl<T: ?Sized + Signer> SignerExt for T {}

pub trait VerifyOp {
    fn update(&mut self, data: &[u8]) -> Result<()>;

    fn finish(&mut self, sig: &[u8]) -> Result<()>;

    fn scheme(&self) -> Sign;
}

impl<T: ?Sized + VerifyOp, P: DerefMut<Target = T>> VerifyOp for P {
    fn update(&mut self, data: &[u8]) -> Result<()> {
        self.deref_mut().update(data)
    }

    fn finish(&mut self, sig: &[u8]) -> Result<()> {
        self.deref_mut().finish(sig)
    }

    fn scheme(&self) -> Sign {
        self.deref().scheme()
    }
}

pub struct OsslVerifyOp<'a> {
    verifier: OsslVerifier<'a>,
    scheme: Sign,
}

impl<'a> OsslVerifyOp<'a> {
    pub fn init(scheme: Sign, pkey: &'a PKeyRef<Public>) -> Result<Self> {
        let mut verifier = OsslVerifier::new(scheme.message_digest(), pkey)?;
        if let Some(padding) = scheme.padding() {
            verifier.set_rsa_padding(padding)?;
        }
        Ok(OsslVerifyOp { verifier, scheme })
    }
}

impl<'a> VerifyOp for OsslVerifyOp<'a> {
    fn update(&mut self, data: &[u8]) -> Result<()> {
        Ok(self.verifier.update(data)?)
    }

    fn finish(&mut self, sig: &[u8]) -> Result<()> {
        match self.verifier.verify(sig) {
            Ok(true) => Ok(()),
            Ok(false) => Err(bterr!(Error::InvalidSignature)),
            Err(err) => Err(err.into()),
        }
    }

    fn scheme(&self) -> Sign {
        self.scheme
    }
}

pub struct VerifyRead<T, Op> {
    inner: T,
    op: Op,
    update_failed: bool,
}

impl<T: Read, Op: VerifyOp> VerifyRead<T, Op> {
    pub fn new(inner: T, op: Op) -> Self {
        VerifyRead {
            inner,
            op,
            update_failed: false,
        }
    }

    pub fn finish(mut self, sig: &[u8]) -> std::result::Result<T, (T, crate::Error)> {
        if self.update_failed {
            return Err((
                self.inner,
                bterr!("VerifyRead::finish: update_failed was true"),
            ));
        }
        match self.op.finish(sig) {
            Ok(_) => Ok(self.inner),
            Err(err) => Err((self.inner, err)),
        }
    }
}

impl<T: Read, Op: VerifyOp> Read for VerifyRead<T, Op> {
    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
        if self.update_failed {
            return Err(bterr!("VerifyRead::read update previously failed").into());
        }
        let read = self.inner.read(buf)?;
        if read > 0 {
            if let Err(err) = self.op.update(&buf[..read]) {
                self.update_failed = true;
                error!("VerifyRead::read failed to update VerifyOp: {err}");
            }
        }
        Ok(read)
    }
}

pub trait Verifier {
    fn init_verify(&self) -> Result<Box<dyn '_ + VerifyOp>>;

    fn verify(&self, parts: &mut dyn Iterator<Item = &[u8]>, signature: &[u8]) -> Result<()>;

    fn kind(&self) -> Sign;
}

impl<V: ?Sized + Verifier> Verifier for &V {
    fn init_verify<'a>(&'a self) -> Result<Box<dyn 'a + VerifyOp>> {
        (*self).init_verify()
    }

    fn verify(&self, parts: &mut dyn Iterator<Item = &[u8]>, signature: &[u8]) -> Result<()> {
        (*self).verify(parts, signature)
    }

    fn kind(&self) -> Sign {
        (*self).kind()
    }
}

impl<V: ?Sized + Verifier> Verifier for Arc<V> {
    fn init_verify<'a>(&'a self) -> Result<Box<dyn 'a + VerifyOp>> {
        self.deref().init_verify()
    }

    fn verify(&self, parts: &mut dyn Iterator<Item = &[u8]>, signature: &[u8]) -> Result<()> {
        self.deref().verify(parts, signature)
    }

    fn kind(&self) -> Sign {
        self.deref().kind()
    }
}

pub trait VerifierExt: Verifier {
    fn ser_verify<T: Serialize>(&self, value: &T, signature: &[u8]) -> Result<()>;
}

impl<V: ?Sized + Verifier> VerifierExt for V {
    fn ser_verify<T: Serialize>(&self, value: &T, signature: &[u8]) -> Result<()> {
        let data = to_vec(value)?;
        self.verify(&mut std::iter::once(data.as_slice()), signature)
    }
}

/// Trait for types which can be used as public credentials.
pub trait CredsPub: Verifier + Encrypter + Principaled {
    /// Returns a reference to the public signing key which can be used to verify signatures.
    fn public_sign(&self) -> &AsymKey<Public, Sign>;

    fn concrete_pub(&self) -> ConcretePub;

    fn sign_kind(&self) -> Sign {
        Verifier::kind(self)
    }
}

impl<T: ?Sized + CredsPub> CredsPub for &T {
    fn public_sign(&self) -> &AsymKey<Public, Sign> {
        (*self).public_sign()
    }

    fn concrete_pub(&self) -> ConcretePub {
        (*self).concrete_pub()
    }
}

impl<T: ?Sized + CredsPub> CredsPub for Arc<T> {
    fn public_sign(&self) -> &AsymKey<Public, Sign> {
        self.deref().public_sign()
    }

    fn concrete_pub(&self) -> ConcretePub {
        self.deref().concrete_pub()
    }
}

/// Trait for types which contain private credentials.
pub trait CredsPriv: Decrypter + Signer {
    /// Returns a reference to the writecap associated with these credentials, if one has been
    /// issued.
    fn writecap(&self) -> Option<&Writecap>;

    fn sign_kind(&self) -> Sign {
        Signer::kind(self)
    }

    /// Returns the path these credentials are authorized to bind to according,
    /// as specified by their [Writecap]. If these creds haven't been issued a [Writecap], then
    /// an `Err` variant containing [BlockError::MissingWritecap] is returned.
    fn bind_path(&self) -> Result<BlockPath> {
        Ok(self
            .writecap()
            .ok_or(BlockError::MissingWritecap)?
            .bind_path())
    }
}

impl<T: ?Sized + CredsPriv> CredsPriv for &T {
    fn writecap(&self) -> Option<&Writecap> {
        (*self).writecap()
    }
}

impl<T: ?Sized + CredsPriv> CredsPriv for Arc<T> {
    fn writecap(&self) -> Option<&Writecap> {
        self.deref().writecap()
    }
}

/// Trait for types which contain both public and private credentials.
pub trait Creds: CredsPriv + CredsPub + Send + Sync {
    fn issue_writecap(
        &self,
        issued_to: Principal,
        path_components: &mut dyn Iterator<Item = &str>,
        expires: Epoch,
    ) -> Result<Writecap> {
        // The root principal is given by the path in our writecap, or if we don't have a writecap,
        // then we assume we are the root principal.
        let root_principal = self
            .writecap()
            .map(|e| e.root_principal())
            .unwrap_or_else(|| self.principal());
        let path = BlockPath::from_components(root_principal, path_components);
        let body = WritecapBody {
            issued_to,
            path,
            expires,
            signing_key: self.public_sign().to_owned(),
        };
        let signed = self.ser_sign(&body)?;
        Ok(Writecap {
            body,
            signature: signed.sig,
            next: self.writecap().map(|e| Box::new(e.to_owned())),
        })
    }

    fn pub_sign_kind(&self) -> Sign {
        CredsPub::sign_kind(self)
    }

    fn priv_sign_kind(&self) -> Sign {
        CredsPriv::sign_kind(self)
    }
}

impl<C: ?Sized + CredsPriv + CredsPub + Send + Sync> Creds for C {}

/// A trait for types which store credentials.
pub trait CredStore {
    /// The type of the credential handle returned by this store.
    type CredHandle: 'static + Creds;
    /// The type of the exported credentials returned by this store.
    type ExportedCreds: 'static + Serialize + DeserializeOwned;

    /// Returns the node credentials. If credentials haven't been generated, they are generated
    /// stored and returned.
    fn node_creds(&self) -> Result<Self::CredHandle>;
    /// Returns the root credentials. If no root credentials have been generated, or the provided
    /// password is incorrect, then an error is returned.
    fn root_creds(&self, password: &str) -> Result<Self::CredHandle>;
    /// Returns a public key which can be used to encrypt data intended only to be accessed by this
    /// node. The returned key can be given as the `new_parent` parameter to the
    /// [CredStore::export_root_creds] method.
    fn storage_key(&self) -> Result<AsymKeyPub<Encrypt>>;
    /// Exports the root credentials. These can be serialized and persisted external to the
    /// application and later loaded and deserialized and passed to the
    /// [CredStoreMut::import_root_creds] method.
    /// The `password` argument must match the value provided when the [CredStore::root_creds]
    /// method was called. The `new_parent` argument is the public key of the node that is to import
    /// the root key, which can be obtained using the [CredStoreMut::gen_root_creds] method on the
    /// importing node.
    fn export_root_creds(
        &self,
        root_creds: &Self::CredHandle,
        password: &str,
        new_parent: &AsymKeyPub<Encrypt>,
    ) -> Result<Self::ExportedCreds>;
}

impl<T: ?Sized + CredStore, P: Deref<Target = T>> CredStore for P {
    type CredHandle = T::CredHandle;
    type ExportedCreds = T::ExportedCreds;

    fn node_creds(&self) -> Result<Self::CredHandle> {
        self.deref().node_creds()
    }

    fn root_creds(&self, password: &str) -> Result<Self::CredHandle> {
        self.deref().root_creds(password)
    }

    fn storage_key(&self) -> Result<AsymKeyPub<Encrypt>> {
        self.deref().storage_key()
    }

    fn export_root_creds(
        &self,
        root_creds: &Self::CredHandle,
        password: &str,
        new_parent: &AsymKeyPub<Encrypt>,
    ) -> Result<Self::ExportedCreds> {
        self.deref()
            .export_root_creds(root_creds, password, new_parent)
    }
}

/// An extension of [CredStore] which exposes additional methods for mutating the credential store.
pub trait CredStoreMut: CredStore {
    /// Generates the root credentials and protects them using the given password. If the root
    /// credentials have already been generated then an error is returned.
    fn gen_root_creds(&self, password: &str) -> Result<Self::CredHandle>;
    /// Imports root credentials that were previously created with [CredStore::export_root_creds].
    /// The provided password must match the value that was given to that method.
    fn import_root_creds(
        &self,
        password: &str,
        exported: Self::ExportedCreds,
    ) -> Result<Self::CredHandle>;
    /// Assigns the given [Writecap] to the node credentials referred to by the given handle.
    ///  This method is responsible for committing the given [Writecap] to durable storage.
    fn assign_node_writecap(&self, handle: &mut Self::CredHandle, writecap: Writecap)
        -> Result<()>;
    /// Assigns `writecap` to the root credentials referred to by `handle`. This method
    /// is responsible for committing the given [Writecap] to durable storage.
    fn assign_root_writecap(&self, handle: &mut Self::CredHandle, writecap: Writecap)
        -> Result<()>;

    /// Generates new root credentials protected by `password` and issues them a self-signed
    /// [Writecap] which expires after `valid_for`. The newly generated root credentials are
    /// returned.
    fn provision_root(&self, password: &str, expires: Epoch) -> Result<Self::CredHandle> {
        let mut root_creds = self.gen_root_creds(password)?;
        let writecap =
            root_creds.issue_writecap(root_creds.principal(), &mut std::iter::empty(), expires)?;
        self.assign_root_writecap(&mut root_creds, writecap)?;
        Ok(root_creds)
    }
    /// Begin the provisioning process for a node by generating a new set of node credentials. The
    /// [Principal] of the newly generated credentials is returned. This [Principal] may then be
    /// transmitted to a root node which can use it to issue a [Writecap] to this node.
    fn provision_node_start(&self) -> Result<Principal> {
        let node_creds = self.node_creds()?;
        Ok(node_creds.principal())
    }
    /// Assigns the given [Writecap] to the node credentials and commits it to durable storage.
    /// A handle to the node credentials is returned.
    fn provision_node_finish(&self, writecap: Writecap) -> Result<Self::CredHandle> {
        let mut node_creds = self.node_creds()?;
        self.assign_node_writecap(&mut node_creds, writecap)?;
        Ok(node_creds)
    }
}

impl<T: ?Sized + CredStoreMut, P: Deref<Target = T>> CredStoreMut for P {
    fn gen_root_creds(&self, password: &str) -> Result<T::CredHandle> {
        self.deref().gen_root_creds(password)
    }

    fn import_root_creds(
        &self,
        password: &str,
        exported: Self::ExportedCreds,
    ) -> Result<Self::CredHandle> {
        self.deref().import_root_creds(password, exported)
    }

    fn assign_node_writecap(
        &self,
        handle: &mut Self::CredHandle,
        writecap: Writecap,
    ) -> Result<()> {
        self.deref().assign_node_writecap(handle, writecap)
    }

    fn assign_root_writecap(
        &self,
        handle: &mut Self::CredHandle,
        writecap: Writecap,
    ) -> Result<()> {
        self.deref().assign_root_writecap(handle, writecap)
    }
}

impl BlockMeta {
    /// Validates that this metadata struct contains a valid writecap, that this writecap is
    /// permitted to write to the path of this block and that the signature in this metadata struct
    /// is valid and matches the key the writecap was issued to.
    pub fn assert_valid(&self, path: &BlockPath) -> Result<()> {
        let body = &self.body;
        let writecap = body
            .writecap
            .as_ref()
            .ok_or(crate::BlockError::MissingWritecap)?;
        writecap.assert_valid_for(path)?;
        let signed_by = body.signing_key.principal();
        if writecap.body.issued_to != signed_by {
            return Err(bterr!(Error::signature_mismatch(
                writecap.body.issued_to.clone(),
                signed_by,
            )));
        }
        body.signing_key.ser_verify(&body, self.sig.as_slice())
    }
}

/// The types of errors which can occur when verifying a writecap chain is authorized to write to
/// a given path.
#[derive(Debug, PartialEq, Eq, Display)]
pub enum WritecapAuthzErr {
    /// The chain is not valid for use on the given path.
    UnauthorizedPath,
    /// At least one writecap in the chain is expired.
    Expired,
    /// The given writecaps do not actually form a chain.
    NotChained,
    /// The principal the root writecap was issued to does not own the given path.
    RootDoesNotOwnPath,
    /// An error occurred while serializing a writecap.
    Serde(String),
    /// The write cap chain was too long to be validated. The value contained in this error is
    /// the maximum allowed length.
    ChainTooLong(usize),
}

impl Writecap {
    /// Verifies that the given [Writecap] actually grants permission to write to the given
    /// [BlockPath].
    pub fn assert_valid_for(&self, path: &BlockPath) -> Result<()> {
        let mut writecap = self;
        const CHAIN_LEN_LIMIT: usize = 256;
        let mut prev: Option<&Writecap> = None;
        let mut sig_input_buf = Vec::new();
        let now = Epoch::now();
        for _ in 0..CHAIN_LEN_LIMIT {
            if !writecap.body.path.contains(path) {
                return Err(bterr!(WritecapAuthzErr::UnauthorizedPath));
            }
            if writecap.body.expires <= now {
                return Err(bterr!(WritecapAuthzErr::Expired));
            }
            if let Some(prev) = &prev {
                if prev
                    .body
                    .signing_key
                    .principal_of_kind(writecap.body.issued_to.kind())
                    != writecap.body.issued_to
                {
                    return Err(bterr!(WritecapAuthzErr::NotChained));
                }
            }
            sig_input_buf.clear();
            write_to(&writecap.body, &mut sig_input_buf)
                .map_err(|e| bterr!(WritecapAuthzErr::Serde(e.to_string())))?;
            writecap.body.signing_key.verify(
                &mut std::iter::once(sig_input_buf.as_slice()),
                writecap.signature.as_slice(),
            )?;
            match &writecap.next {
                Some(next) => {
                    prev = Some(writecap);
                    writecap = next;
                }
                None => {
                    // We're at the root key. As long as the signer of this writecap is the owner of
                    // the path, then the writecap is valid.
                    if writecap
                        .body
                        .signing_key
                        .principal_of_kind(path.root().kind())
                        == *path.root()
                    {
                        return Ok(());
                    } else {
                        return Err(bterr!(WritecapAuthzErr::RootDoesNotOwnPath));
                    }
                }
            }
        }
        Err(bterr!(WritecapAuthzErr::ChainTooLong(CHAIN_LEN_LIMIT)))
    }
}

#[cfg(test)]
mod tests {
    use std::{
        io::{Seek, SeekFrom},
        time::Duration,
    };

    use super::*;
    use crate::{
        crypto::secret_stream::SecretStream,
        test_helpers::{self, *},
        Sectored, TryCompose,
    };

    #[test]
    fn encrypt_decrypt_block() {
        const SECT_SZ: usize = 16;
        const SECT_CT: usize = 8;
        let mut block = make_block_with();
        write_fill(&mut block, SECT_SZ, SECT_CT);
        block.rewind().expect("rewind failed");
        read_check(block, SECT_SZ, SECT_CT);
    }

    #[test]
    fn rsa_sign_and_verify() -> Result<()> {
        let key = make_key_pair();
        let header = b"About: lyrics".as_slice();
        let message = b"Everything that feels so good is bad bad bad.".as_slice();
        let signature = key.sign(&mut [header, message].into_iter())?;
        key.verify(&mut [header, message].into_iter(), signature.as_slice())
    }

    #[test]
    fn hash_to_string() {
        let hash = make_principal().0;
        let string = hash.to_string();
        assert_eq!("0!dSip4J0kurN5VhVo_aTipM-ywOOWrqJuRRVQ7aa-bew", string)
    }

    #[test]
    fn hash_to_string_round_trip() -> Result<()> {
        let expected = make_principal().0;
        let string = expected.to_string();
        let actual = VarHash::try_from(string.as_str())?;
        assert_eq!(expected, actual);
        Ok(())
    }

    #[test]
    fn verify_writecap_valid() {
        let writecap = make_writecap(["apps", "verse"].into_iter());
        writecap
            .assert_valid_for(&writecap.body.path)
            .expect("failed to verify writecap");
    }

    #[test]
    fn verify_writecap_invalid_signature() -> Result<()> {
        let mut writecap = make_writecap(["apps", "verse"].into_iter());
        writecap.signature = Signature::empty(Sign::RSA_PSS_3072_SHA_256);
        let result = writecap.assert_valid_for(&writecap.body.path);
        if let Err(ref err) = result {
            if let Some(err) = err.downcast_ref::<Error>() {
                if let Error::InvalidSignature = err {
                    return Ok(());
                }
            }
        }
        Err(bterr!("unexpected result {:?}", result))
    }

    fn assert_authz_err<T: std::fmt::Debug>(
        expected: WritecapAuthzErr,
        result: Result<T>,
    ) -> Result<()> {
        if let Some(err) = result.as_ref().err() {
            if let Some(actual) = err.downcast_ref::<WritecapAuthzErr>() {
                if *actual == expected {
                    return Ok(());
                }
            }
        }
        Err(bterr!("unexpected result: {:?}", result))
    }

    #[test]
    fn verify_writecap_invalid_path_not_contained() -> Result<()> {
        let writecap = make_writecap(["apps", "verse"].into_iter());
        let mut path = writecap.body.path.clone();
        path.pop_component();
        // `path` is now a superpath of `writecap.path`, thus the writecap is not authorized to
        // write to it.
        let result = writecap.assert_valid_for(&path);
        assert_authz_err(WritecapAuthzErr::UnauthorizedPath, result)
    }

    #[test]
    fn verify_writecap_invalid_expired() -> Result<()> {
        let mut writecap = make_writecap(["apps", "verse"].into_iter());
        writecap.body.expires = Epoch::now() - Duration::from_secs(1);
        let result = writecap.assert_valid_for(&writecap.body.path);
        assert_authz_err(WritecapAuthzErr::Expired, result)
    }

    #[test]
    fn verify_writecap_invalid_not_chained() -> Result<()> {
        let (mut root_writecap, root_key) = make_self_signed_writecap();
        root_writecap.body.issued_to = Principal(VarHash::from(HashKind::Sha2_256));
        root_key.sign_writecap(&mut root_writecap)?;
        let node_principal = NODE_CREDS.principal();
        let writecap = make_writecap_trusted_by(
            root_writecap,
            &root_key,
            node_principal,
            ["apps", "contacts"].into_iter(),
        );
        let result = writecap.assert_valid_for(&writecap.body.path);
        assert_authz_err(WritecapAuthzErr::NotChained, result)
    }

    #[test]
    fn verify_writecap_invalid_root_doesnt_own_path() -> Result<()> {
        let (mut root_writecap, root_key) = make_self_signed_writecap();
        let owner = Principal(VarHash::from(HashKind::Sha2_256));
        root_writecap.body.path = make_path_with_root(owner, std::iter::empty());
        root_key.sign_writecap(&mut root_writecap)?;
        let node_principal = NODE_CREDS.principal();
        let writecap = make_writecap_trusted_by(
            root_writecap,
            &root_key,
            node_principal,
            ["apps", "contacts"].into_iter(),
        );
        let result = writecap.assert_valid_for(&writecap.body.path);
        assert_authz_err(WritecapAuthzErr::RootDoesNotOwnPath, result)
    }

    #[test]
    fn aeadkey_encrypt_decrypt_aes256gcm() {
        let key = AeadKey::new(AeadKeyKind::AesGcm256).expect("failed to create key");
        let aad = [1u8; 16];
        let expected = [2u8; 32];
        let tagged = key.encrypt(aad, &expected).expect("encrypt failed");
        let actual = key.decrypt(&tagged).expect("decrypt failed");
        assert_eq!(expected, actual.as_slice());
    }

    #[test]
    fn aeadkey_decrypt_fails_when_ct_modified() {
        let key = AeadKey::new(AeadKeyKind::AesGcm256).expect("failed to create key");
        let aad = [1u8; 16];
        let expected = [2u8; 32];
        let mut tagged = key.encrypt(aad, &expected).expect("encrypt failed");
        tagged.ciphertext.data[0] = tagged.ciphertext.data[0].wrapping_add(1);
        let result = key.decrypt(&tagged);
        assert!(result.is_err())
    }

    #[test]
    fn aeadkey_decrypt_fails_when_aad_modified() {
        let key = AeadKey::new(AeadKeyKind::AesGcm256).expect("failed to create key");
        let aad = [1u8; 16];
        let expected = [2u8; 32];
        let mut tagged = key.encrypt(aad, &expected).expect("encrypt failed");
        tagged.aad[0] = tagged.aad[0].wrapping_add(1);
        let result = key.decrypt(&tagged);
        assert!(result.is_err())
    }

    #[test]
    fn compose_merkle_and_secret_streams() {
        use merkle_stream::tests::make_merkle_stream_filled_with_zeros;
        const SECT_SZ: usize = 4096;
        const SECT_CT: usize = 16;
        let merkle = make_merkle_stream_filled_with_zeros(SECT_SZ, SECT_CT);
        let key = SymKey::generate(SymKeyKind::Aes256Cbc).expect("key generation failed");
        let mut secret = SecretStream::new(key)
            .try_compose(merkle)
            .expect("compose for secret failed");
        let secret_sect_sz = secret.sector_sz();
        write_fill(&mut secret, secret_sect_sz, SECT_CT);
        secret.rewind().expect("rewind failed");
        read_check(secret, secret_sect_sz, SECT_CT);
    }

    fn ossl_hash_op_same_as_digest_test_case<H: Hash + From<DigestBytes>>(kind: HashKind) {
        let parts = (0u8..32).map(|k| vec![k; kind.len()]).collect::<Vec<_>>();
        let expected = {
            let mut expected = vec![0u8; kind.len()];
            kind.digest(expected.as_mut(), parts.iter().map(|a| a.as_slice()))
                .unwrap();
            expected
        };
        let mut op = OsslHashOp::<H>::new(kind).unwrap();

        for part in parts.iter() {
            op.update(part.as_slice()).unwrap();
        }
        let actual = op.finish().unwrap();

        assert_eq!(expected.as_slice(), actual.as_ref());
    }

    /// Tests that the hash computed using an `OsslHashOp` is the same as the one returned by the
    /// `HashKind::digest` method.
    #[test]
    fn ossl_hash_op_same_as_digest() {
        ossl_hash_op_same_as_digest_test_case::<Sha2_256>(Sha2_256::KIND);
        ossl_hash_op_same_as_digest_test_case::<Sha2_512>(Sha2_512::KIND);
    }

    /// Tests that a `HashWrap` instance calculates the same hash as a call to the `digest` method.
    #[test]
    fn hash_stream_agrees_with_digest_method() {
        let cursor = BtCursor::new([0u8; 3 * 32]);
        let parts = (1u8..4).map(|k| [k; Sha2_512::LEN]).collect::<Vec<_>>();
        let expected = {
            let mut expected = Sha2_512::default();
            HashKind::Sha2_512
                .digest(expected.as_mut(), parts.iter().map(|a| a.as_slice()))
                .unwrap();
            expected
        };
        let op = OsslHashOp::<Sha2_512>::new(Sha2_512::KIND).unwrap();
        let mut wrap = HashStream::new(cursor, op);

        for part in parts.iter() {
            wrap.write(part.as_slice()).unwrap();
        }
        let actual = wrap.finish().unwrap();

        assert_eq!(expected, actual);
    }

    /// Tests that the `VarHash` computed by `VarHashOp` is the same as the one returned by the
    /// `digest` method.
    #[test]
    fn var_hash_op_agress_with_digest_method() {
        let parts = (32..64u8).map(|k| [k; Sha2_512::LEN]).collect::<Vec<_>>();
        let expected = {
            let mut expected = VarHash::from(HashKind::Sha2_512);
            HashKind::Sha2_512
                .digest(expected.as_mut(), parts.iter().map(|a| a.as_slice()))
                .unwrap();
            expected
        };
        let mut op = VarHashOp::new(HashKind::Sha2_512).unwrap();

        for part in parts.iter() {
            op.update(part.as_slice()).unwrap();
        }
        let actual = op.finish().unwrap();

        assert_eq!(expected, actual);
    }

    /// Tests that the signature produced by `OsslSignOp` can be verified.
    #[test]
    fn ossl_sign_op_sig_can_be_verified() {
        let keys = &test_helpers::NODE_CREDS;
        let part_values = (1..9u8).map(|k| [k; 32]).collect::<Vec<_>>();
        let get_parts = || part_values.iter().map(|a| a.as_slice());
        let mut sign_op = keys.init_sign().expect("init_sign failed");

        for part in get_parts() {
            sign_op.update(part).expect("update failed");
        }
        let sig = sign_op.finish().expect("finish failed");

        keys.verify(&mut get_parts(), sig.as_ref())
            .expect("verify failed");
    }

    /// Tests that the signature produced by a `SignWrite` can be verified.
    #[test]
    fn sign_write_sig_can_be_verified() {
        use crate::Decompose;
        const LEN: usize = 512;

        let cursor = BtCursor::new([0u8; LEN]);
        let keys = &test_helpers::NODE_CREDS;
        let sign_op = keys.sign.private.init_sign().expect("init_sign failed");
        let mut sign_write = SignWrite::new(cursor, sign_op);

        for part in (1..9u8).map(|k| [k; LEN / 8]) {
            sign_write.write(part.as_slice()).expect("write failed");
        }
        let (sig, cursor) = sign_write.finish().expect("finish failed");
        let array = cursor.into_inner();

        keys.verify(&mut std::iter::once(array.as_slice()), sig.as_ref())
            .expect("verify failed");
    }

    /// Tests that data signed using a `SignWrite` can later be verified using a `VerifyRead`.
    #[test]
    fn sign_write_then_verify_read() {
        const LEN: usize = 512;
        let cursor = BtCursor::new([0u8; LEN]);
        let keys = &test_helpers::NODE_CREDS;
        let sign_op = keys.sign.private.init_sign().expect("init_sign failed");
        let mut sign_write = SignWrite::new(cursor, sign_op);

        for part in (1..9u8).map(|k| [k; LEN / 8]) {
            sign_write.write(part.as_slice()).expect("write failed");
        }
        let (sig, mut cursor) = sign_write.finish().expect("finish failed");
        cursor.seek(SeekFrom::Start(0)).expect("seek failed");

        let verify_op = keys.sign.public.init_verify().expect("init_verify failed");
        let mut verify_read = VerifyRead::new(cursor, verify_op);
        let mut buf = Vec::with_capacity(LEN);
        verify_read
            .read_to_end(&mut buf)
            .expect("read_to_end failed");

        verify_read
            .finish(sig.as_ref())
            .expect("failed to verify signature");
    }

    /// Tests that validate the dependencies of this module.
    mod dependency_tests {
        use super::*;
        use openssl::{
            ec::{EcGroup, EcKey},
            nid::Nid,
        };

        /// This test validates that data encrypted with AES 256 CBC can later be decrypted.
        #[test]
        fn aes_256_cbc_roundtrip() {
            use super::*;
            let expected = b"We attack at the crack of noon!";
            let cipher = Cipher::aes_256_cbc();

            let key = BLOCK_KEY.key_slice();
            let iv = BLOCK_KEY.iv_slice();
            let ciphertext = openssl_encrypt(cipher, key, iv, expected).unwrap();
            let actual = openssl_decrypt(cipher, key, iv, ciphertext.as_slice()).unwrap();

            assert_eq!(expected, actual.as_slice());
        }

        /// Tests that the keys for the SECP256K1 curve are the expected sizes.
        #[test]
        fn secp256k1_key_lengths() {
            let group = EcGroup::from_curve_name(Nid::SECP256K1).unwrap();
            let key = EcKey::generate(&group).unwrap();
            let public = key.public_key_to_der().unwrap();
            let private = key.private_key_to_der().unwrap();
            let public_len = public.len();
            let private_len = private.len();
            assert_eq!(88, public_len);
            assert_eq!(118, private_len);
        }

        #[test]
        fn ed25519_key_lengths() {
            let key = PKey::generate_x25519().unwrap();
            let public = key.public_key_to_der().unwrap();
            let private = key.private_key_to_der().unwrap();
            let public_len = public.len();
            let private_len = private.len();
            assert_eq!(44, public_len);
            assert_eq!(48, private_len);
        }
    }

    mod obj_safety {
        use super::*;

        #[test]
        fn op_obj_safe() {
            assert_obj_safe!(Op);
        }

        #[test]
        fn sign_op_obj_safe() {
            assert_obj_safe!(SignOp);
        }

        #[test]
        fn verify_op_obj_safe() {
            assert_obj_safe!(VerifyOp);
        }

        #[test]
        fn encrypter_obj_safe() {
            assert_obj_safe!(Encrypter);
        }

        #[test]
        fn decrypter_obj_safe() {
            assert_obj_safe!(Decrypter);
        }

        #[test]
        fn verifier_obj_safe() {
            assert_obj_safe!(Verifier);
        }

        #[test]
        fn signer_obj_safe() {
            assert_obj_safe!(Signer);
        }

        #[test]
        fn creds_pub_obj_safe() {
            assert_obj_safe!(CredsPub);
        }

        #[test]
        fn creds_priv_obj_safe() {
            assert_obj_safe!(CredsPriv);
        }

        #[test]
        fn creds_obj_safe() {
            assert_obj_safe!(Creds);
        }
    }
}