vendor/ring-0.16.20/src/hmac.rs - toolchain/rustc - Git at Google

 // Copyright 2015-2016 Brian Smith.
 //
 // Permission to use, copy, modify, and/or distribute this software for any
 // purpose with or without fee is hereby granted, provided that the above
 // copyright notice and this permission notice appear in all copies.
 //
 // THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHORS DISCLAIM ALL WARRANTIES
 // WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 // MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY
 // SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 // WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
 // OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
 // CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.

 //! HMAC is specified in [RFC 2104].
 //!
 //! After a `Key` is constructed, it can be used for multiple signing or
 //! verification operations. Separating the construction of the key from the
 //! rest of the HMAC operation allows the per-key precomputation to be done
 //! only once, instead of it being done in every HMAC operation.
 //!
 //! Frequently all the data to be signed in a message is available in a single
 //! contiguous piece. In that case, the module-level `sign` function can be
 //! used. Otherwise, if the input is in multiple parts, `Context` should be
 //! used.
 //!
 //! # Examples:
 //!
 //! ## Signing a value and verifying it wasn't tampered with
 //!
 //! ```
 //! use ring::{hmac, rand};
 //!
 //! let rng = rand::SystemRandom::new();
 //! let key = hmac::Key::generate(hmac::HMAC_SHA256, &rng)?;
 //!
 //! let msg = "hello, world";
 //!
 //! let tag = hmac::sign(&key, msg.as_bytes());
 //!
 //! // [We give access to the message to an untrusted party, and they give it
 //! // back to us. We need to verify they didn't tamper with it.]
 //!
 //! hmac::verify(&key, msg.as_bytes(), tag.as_ref())?;
 //!
 //! # Ok::<(), ring::error::Unspecified>(())
 //! ```
 //!
 //! ## Using the one-shot API:
 //!
 //! ```
 //! use ring::{digest, hmac, rand};
 //! use ring::rand::SecureRandom;
 //!
 //! let msg = "hello, world";
 //!
 //! // The sender generates a secure key value and signs the message with it.
 //! // Note that in a real protocol, a key agreement protocol would be used to
 //! // derive `key_value`.
 //! let rng = rand::SystemRandom::new();
 //! let key_value: [u8; digest::SHA256_OUTPUT_LEN] = rand::generate(&rng)?.expose();
 //!
 //! let s_key = hmac::Key::new(hmac::HMAC_SHA256, key_value.as_ref());
 //! let tag = hmac::sign(&s_key, msg.as_bytes());
 //!
 //! // The receiver (somehow!) knows the key value, and uses it to verify the
 //! // integrity of the message.
 //! let v_key = hmac::Key::new(hmac::HMAC_SHA256, key_value.as_ref());
 //! hmac::verify(&v_key, msg.as_bytes(), tag.as_ref())?;
 //!
 //! # Ok::<(), ring::error::Unspecified>(())
 //! ```
 //!
 //! ## Using the multi-part API:
 //! ```
 //! use ring::{digest, hmac, rand};
 //! use ring::rand::SecureRandom;
 //!
 //! let parts = ["hello", ", ", "world"];
 //!
 //! // The sender generates a secure key value and signs the message with it.
 //! // Note that in a real protocol, a key agreement protocol would be used to
 //! // derive `key_value`.
 //! let rng = rand::SystemRandom::new();
 //! let mut key_value: [u8; digest::SHA384_OUTPUT_LEN] = rand::generate(&rng)?.expose();
 //!
 //! let s_key = hmac::Key::new(hmac::HMAC_SHA384, key_value.as_ref());
 //! let mut s_ctx = hmac::Context::with_key(&s_key);
 //! for part in &parts {
 //!     s_ctx.update(part.as_bytes());
 //! }
 //! let tag = s_ctx.sign();
 //!
 //! // The receiver (somehow!) knows the key value, and uses it to verify the
 //! // integrity of the message.
 //! let v_key = hmac::Key::new(hmac::HMAC_SHA384, key_value.as_ref());
 //! let mut msg = Vec::<u8>::new();
 //! for part in &parts {
 //!     msg.extend(part.as_bytes());
 //! }
 //! hmac::verify(&v_key, &msg.as_ref(), tag.as_ref())?;
 //!
 //! # Ok::<(), ring::error::Unspecified>(())
 //! ```
 //!
 //! [RFC 2104]: https://tools.ietf.org/html/rfc2104
 //! [code for `ring::pbkdf2`]:
 //!     https://github.com/briansmith/ring/blob/main/src/pbkdf2.rs
 //! [code for `ring::hkdf`]:
 //!     https://github.com/briansmith/ring/blob/main/src/hkdf.rs

 use crate::{constant_time, digest, error, hkdf, rand};

 /// An HMAC algorithm.
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
 pub struct Algorithm(&'static digest::Algorithm);

 impl Algorithm {
     /// The digest algorithm this HMAC algorithm is based on.
     #[inline]
     pub fn digest_algorithm(&self) -> &'static digest::Algorithm {
         self.0
     }
 }

 /// HMAC using SHA-1. Obsolete.
 pub static HMAC_SHA1_FOR_LEGACY_USE_ONLY: Algorithm = Algorithm(&digest::SHA1_FOR_LEGACY_USE_ONLY);

 /// HMAC using SHA-256.
 pub static HMAC_SHA256: Algorithm = Algorithm(&digest::SHA256);

 /// HMAC using SHA-384.
 pub static HMAC_SHA384: Algorithm = Algorithm(&digest::SHA384);

 /// HMAC using SHA-512.
 pub static HMAC_SHA512: Algorithm = Algorithm(&digest::SHA512);

 /// A deprecated alias for `Tag`.
 #[deprecated(note = "`Signature` was renamed to `Tag`. This alias will be removed soon.")]
 pub type Signature = Tag;

 /// An HMAC tag.
 ///
 /// For a given tag `t`, use `t.as_ref()` to get the tag value as a byte slice.
 #[derive(Clone, Copy, Debug)]
 pub struct Tag(digest::Digest);

 impl AsRef<[u8]> for Tag {
     #[inline]
     fn as_ref(&self) -> &[u8] {
         self.0.as_ref()
     }
 }

 /// A key to use for HMAC signing.
 #[derive(Clone)]
 pub struct Key {
     inner: digest::BlockContext,
     outer: digest::BlockContext,
 }

 /// `hmac::SigningKey` was renamed to `hmac::Key`.
 #[deprecated(note = "Renamed to `hmac::Key`.")]
 pub type SigningKey = Key;

 /// `hmac::VerificationKey` was merged into `hmac::Key`.
 #[deprecated(
     note = "The distinction between verification & signing keys was removed. Use `hmac::Key`."
 )]
 pub type VerificationKey = Key;

 impl core::fmt::Debug for Key {
     fn fmt(&self, f: &mut core::fmt::Formatter) -> Result<(), core::fmt::Error> {
         f.debug_struct("Key")
             .field("algorithm", self.algorithm().digest_algorithm())
             .finish()
     }
 }

 impl Key {
     /// Generate an HMAC signing key using the given digest algorithm with a
     /// random value generated from `rng`.
     ///
     /// The key will be `digest_alg.output_len` bytes long, based on the
     /// recommendation in [RFC 2104 Section 3].
     ///
     /// [RFC 2104 Section 3]: https://tools.ietf.org/html/rfc2104#section-3
     pub fn generate(
         algorithm: Algorithm,
         rng: &dyn rand::SecureRandom,
     ) -> Result<Self, error::Unspecified> {
         Self::construct(algorithm, |buf| rng.fill(buf))
     }

     fn construct<F>(algorithm: Algorithm, fill: F) -> Result<Self, error::Unspecified>
     where
         F: FnOnce(&mut [u8]) -> Result<(), error::Unspecified>,
     {
         let mut key_bytes = [0; digest::MAX_OUTPUT_LEN];
         let key_bytes = &mut key_bytes[..algorithm.0.output_len];
         fill(key_bytes)?;
         Ok(Self::new(algorithm, key_bytes))
     }

     /// Construct an HMAC signing key using the given digest algorithm and key
     /// value.
     ///
     /// `key_value` should be a value generated using a secure random number
     /// generator (e.g. the `key_value` output by
     /// `SealingKey::generate_serializable()`) or derived from a random key by
     /// a key derivation function (e.g. `ring::hkdf`). In particular,
     /// `key_value` shouldn't be a password.
     ///
     /// As specified in RFC 2104, if `key_value` is shorter than the digest
     /// algorithm's block length (as returned by `digest::Algorithm::block_len`,
     /// not the digest length returned by `digest::Algorithm::output_len`) then
     /// it will be padded with zeros. Similarly, if it is longer than the block
     /// length then it will be compressed using the digest algorithm.
     ///
     /// You should not use keys larger than the `digest_alg.block_len` because
     /// the truncation described above reduces their strength to only
     /// `digest_alg.output_len * 8` bits. Support for such keys is likely to be
     /// removed in a future version of *ring*.
     pub fn new(algorithm: Algorithm, key_value: &[u8]) -> Self {
         let digest_alg = algorithm.0;
         let mut key = Self {
             inner: digest::BlockContext::new(digest_alg),
             outer: digest::BlockContext::new(digest_alg),
         };

         let key_hash;
         let key_value = if key_value.len() <= digest_alg.block_len {
             key_value
         } else {
             key_hash = digest::digest(digest_alg, key_value);
             key_hash.as_ref()
         };

         const IPAD: u8 = 0x36;

         let mut padded_key = [IPAD; digest::MAX_BLOCK_LEN];
         let padded_key = &mut padded_key[..digest_alg.block_len];

         // If the key is shorter than one block then we're supposed to act like
         // it is padded with zero bytes up to the block length. `x ^ 0 == x` so
         // we can just leave the trailing bytes of `padded_key` untouched.
         for (padded_key, key_value) in padded_key.iter_mut().zip(key_value.iter()) {
             *padded_key ^= *key_value;
         }
         key.inner.update(&padded_key);

         const OPAD: u8 = 0x5C;

         // Remove the `IPAD` masking, leaving the unmasked padded key, then
         // mask with `OPAD`, all in one step.
         for b in padded_key.iter_mut() {
             *b ^= IPAD ^ OPAD;
         }
         key.outer.update(&padded_key);

         key
     }

     /// The digest algorithm for the key.
     #[inline]
     pub fn algorithm(&self) -> Algorithm {
         Algorithm(self.inner.algorithm)
     }
 }

 impl hkdf::KeyType for Algorithm {
     fn len(&self) -> usize {
         self.digest_algorithm().output_len
     }
 }

 impl From<hkdf::Okm<'_, Algorithm>> for Key {
     fn from(okm: hkdf::Okm<Algorithm>) -> Self {
         Key::construct(*okm.len(), |buf| okm.fill(buf)).unwrap()
     }
 }

 /// A context for multi-step (Init-Update-Finish) HMAC signing.
 ///
 /// Use `sign` for single-step HMAC signing.
 #[derive(Clone)]
 pub struct Context {
     inner: digest::Context,
     outer: digest::BlockContext,
 }

 /// `hmac::SigningContext` was renamed to `hmac::Context`.
 #[deprecated(note = "Renamed to `hmac::Context`.")]
 pub type SigningContext = Context;

 impl core::fmt::Debug for Context {
     fn fmt(&self, f: &mut core::fmt::Formatter) -> Result<(), core::fmt::Error> {
         f.debug_struct("Context")
             .field("algorithm", self.inner.algorithm())
             .finish()
     }
 }

 impl Context {
     /// Constructs a new HMAC signing context using the given digest algorithm
     /// and key.
     pub fn with_key(signing_key: &Key) -> Self {
         Self {
             inner: digest::Context::clone_from(&signing_key.inner),
             outer: signing_key.outer.clone(),
         }
     }

     /// Updates the HMAC with all the data in `data`. `update` may be called
     /// zero or more times until `finish` is called.
     pub fn update(&mut self, data: &[u8]) {
         self.inner.update(data);
     }

     /// Finalizes the HMAC calculation and returns the HMAC value. `sign`
     /// consumes the context so it cannot be (mis-)used after `sign` has been
     /// called.
     ///
     /// It is generally not safe to implement HMAC verification by comparing
     /// the return value of `sign` to a tag. Use `verify` for verification
     /// instead.
     pub fn sign(self) -> Tag {
         let algorithm = self.inner.algorithm();
         let mut pending = [0u8; digest::MAX_BLOCK_LEN];
         let pending = &mut pending[..algorithm.block_len];
         let num_pending = algorithm.output_len;
         pending[..num_pending].copy_from_slice(self.inner.finish().as_ref());
         Tag(self.outer.finish(pending, num_pending))
     }
 }

 /// Calculates the HMAC of `data` using the key `key` in one step.
 ///
 /// Use `Context` to calculate HMACs where the input is in multiple parts.
 ///
 /// It is generally not safe to implement HMAC verification by comparing the
 /// return value of `sign` to a tag. Use `verify` for verification instead.
 pub fn sign(key: &Key, data: &[u8]) -> Tag {
     let mut ctx = Context::with_key(key);
     ctx.update(data);
     ctx.sign()
 }

 /// Calculates the HMAC of `data` using the signing key `key`, and verifies
 /// whether the resultant value equals `tag`, in one step.
 ///
 /// This is logically equivalent to, but more efficient than, constructing a
 /// `Key` with the same value as `key` and then using `verify`.
 ///
 /// The verification will be done in constant time to prevent timing attacks.
 pub fn verify(key: &Key, data: &[u8], tag: &[u8]) -> Result<(), error::Unspecified> {
     constant_time::verify_slices_are_equal(sign(key, data).as_ref(), tag)
 }

 #[cfg(test)]
 mod tests {
     use crate::{hmac, rand};

     // Make sure that `Key::generate` and `verify_with_own_key` aren't
     // completely wacky.
     #[test]
     pub fn hmac_signing_key_coverage() {
         let rng = rand::SystemRandom::new();

         const HELLO_WORLD_GOOD: &[u8] = b"hello, world";
         const HELLO_WORLD_BAD: &[u8] = b"hello, worle";

         for algorithm in &[
             hmac::HMAC_SHA1_FOR_LEGACY_USE_ONLY,
             hmac::HMAC_SHA256,
             hmac::HMAC_SHA384,
             hmac::HMAC_SHA512,
         ] {
             let key = hmac::Key::generate(*algorithm, &rng).unwrap();
             let tag = hmac::sign(&key, HELLO_WORLD_GOOD);
             assert!(hmac::verify(&key, HELLO_WORLD_GOOD, tag.as_ref()).is_ok());
             assert!(hmac::verify(&key, HELLO_WORLD_BAD, tag.as_ref()).is_err())
         }
     }
 }
	// Copyright 2015-2016 Brian Smith.
	//
	// Permission to use, copy, modify, and/or distribute this software for any
	// purpose with or without fee is hereby granted, provided that the above
	// copyright notice and this permission notice appear in all copies.
	//
	// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHORS DISCLAIM ALL WARRANTIES
	// WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
	// MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY
	// SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
	// WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
	// OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
	// CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.

	//! HMAC is specified in [RFC 2104].
	//!
	//! After a `Key` is constructed, it can be used for multiple signing or
	//! verification operations. Separating the construction of the key from the
	//! rest of the HMAC operation allows the per-key precomputation to be done
	//! only once, instead of it being done in every HMAC operation.
	//!
	//! Frequently all the data to be signed in a message is available in a single
	//! contiguous piece. In that case, the module-level `sign` function can be
	//! used. Otherwise, if the input is in multiple parts, `Context` should be
	//! used.
	//!
	//! # Examples:
	//!
	//! ## Signing a value and verifying it wasn't tampered with
	//!
	//! ```
	//! use ring::{hmac, rand};
	//!
	//! let rng = rand::SystemRandom::new();
	//! let key = hmac::Key::generate(hmac::HMAC_SHA256, &rng)?;
	//!
	//! let msg = "hello, world";
	//!
	//! let tag = hmac::sign(&key, msg.as_bytes());
	//!
	//! // [We give access to the message to an untrusted party, and they give it
	//! // back to us. We need to verify they didn't tamper with it.]
	//!
	//! hmac::verify(&key, msg.as_bytes(), tag.as_ref())?;
	//!
	//! # Ok::<(), ring::error::Unspecified>(())
	//! ```
	//!
	//! ## Using the one-shot API:
	//!
	//! ```
	//! use ring::{digest, hmac, rand};
	//! use ring::rand::SecureRandom;
	//!
	//! let msg = "hello, world";
	//!
	//! // The sender generates a secure key value and signs the message with it.
	//! // Note that in a real protocol, a key agreement protocol would be used to
	//! // derive `key_value`.
	//! let rng = rand::SystemRandom::new();
	//! let key_value: [u8; digest::SHA256_OUTPUT_LEN] = rand::generate(&rng)?.expose();
	//!
	//! let s_key = hmac::Key::new(hmac::HMAC_SHA256, key_value.as_ref());
	//! let tag = hmac::sign(&s_key, msg.as_bytes());
	//!
	//! // The receiver (somehow!) knows the key value, and uses it to verify the
	//! // integrity of the message.
	//! let v_key = hmac::Key::new(hmac::HMAC_SHA256, key_value.as_ref());
	//! hmac::verify(&v_key, msg.as_bytes(), tag.as_ref())?;
	//!
	//! # Ok::<(), ring::error::Unspecified>(())
	//! ```
	//!
	//! ## Using the multi-part API:
	//! ```
	//! use ring::{digest, hmac, rand};
	//! use ring::rand::SecureRandom;
	//!
	//! let parts = ["hello", ", ", "world"];
	//!
	//! // The sender generates a secure key value and signs the message with it.
	//! // Note that in a real protocol, a key agreement protocol would be used to
	//! // derive `key_value`.
	//! let rng = rand::SystemRandom::new();
	//! let mut key_value: [u8; digest::SHA384_OUTPUT_LEN] = rand::generate(&rng)?.expose();
	//!
	//! let s_key = hmac::Key::new(hmac::HMAC_SHA384, key_value.as_ref());
	//! let mut s_ctx = hmac::Context::with_key(&s_key);
	//! for part in &parts {
	//! s_ctx.update(part.as_bytes());
	//! }
	//! let tag = s_ctx.sign();
	//!
	//! // The receiver (somehow!) knows the key value, and uses it to verify the
	//! // integrity of the message.
	//! let v_key = hmac::Key::new(hmac::HMAC_SHA384, key_value.as_ref());
	//! let mut msg = Vec::<u8>::new();
	//! for part in &parts {
	//! msg.extend(part.as_bytes());
	//! }
	//! hmac::verify(&v_key, &msg.as_ref(), tag.as_ref())?;
	//!
	//! # Ok::<(), ring::error::Unspecified>(())
	//! ```
	//!
	//! [RFC 2104]: https://tools.ietf.org/html/rfc2104
	//! [code for `ring::pbkdf2`]:
	//! https://github.com/briansmith/ring/blob/main/src/pbkdf2.rs
	//! [code for `ring::hkdf`]:
	//! https://github.com/briansmith/ring/blob/main/src/hkdf.rs

	use crate::{constant_time, digest, error, hkdf, rand};

	/// An HMAC algorithm.
	#[derive(Clone, Copy, Debug, PartialEq, Eq)]
	pub struct Algorithm(&'static digest::Algorithm);

	impl Algorithm {
	/// The digest algorithm this HMAC algorithm is based on.
	#[inline]
	pub fn digest_algorithm(&self) -> &'static digest::Algorithm {
	self.0
	}
	}

	/// HMAC using SHA-1. Obsolete.
	pub static HMAC_SHA1_FOR_LEGACY_USE_ONLY: Algorithm = Algorithm(&digest::SHA1_FOR_LEGACY_USE_ONLY);

	/// HMAC using SHA-256.
	pub static HMAC_SHA256: Algorithm = Algorithm(&digest::SHA256);

	/// HMAC using SHA-384.
	pub static HMAC_SHA384: Algorithm = Algorithm(&digest::SHA384);

	/// HMAC using SHA-512.
	pub static HMAC_SHA512: Algorithm = Algorithm(&digest::SHA512);

	/// A deprecated alias for `Tag`.
	#[deprecated(note = "`Signature` was renamed to `Tag`. This alias will be removed soon.")]
	pub type Signature = Tag;

	/// An HMAC tag.
	///
	/// For a given tag `t`, use `t.as_ref()` to get the tag value as a byte slice.
	#[derive(Clone, Copy, Debug)]
	pub struct Tag(digest::Digest);

	impl AsRef<[u8]> for Tag {
	#[inline]
	fn as_ref(&self) -> &[u8] {
	self.0.as_ref()
	}
	}

	/// A key to use for HMAC signing.
	#[derive(Clone)]
	pub struct Key {
	inner: digest::BlockContext,
	outer: digest::BlockContext,
	}

	/// `hmac::SigningKey` was renamed to `hmac::Key`.
	#[deprecated(note = "Renamed to `hmac::Key`.")]
	pub type SigningKey = Key;

	/// `hmac::VerificationKey` was merged into `hmac::Key`.
	#[deprecated(
	note = "The distinction between verification & signing keys was removed. Use `hmac::Key`."
	)]
	pub type VerificationKey = Key;

	impl core::fmt::Debug for Key {
	fn fmt(&self, f: &mut core::fmt::Formatter) -> Result<(), core::fmt::Error> {
	f.debug_struct("Key")
	.field("algorithm", self.algorithm().digest_algorithm())
	.finish()
	}
	}

	impl Key {
	/// Generate an HMAC signing key using the given digest algorithm with a
	/// random value generated from `rng`.
	///
	/// The key will be `digest_alg.output_len` bytes long, based on the
	/// recommendation in [RFC 2104 Section 3].
	///
	/// [RFC 2104 Section 3]: https://tools.ietf.org/html/rfc2104#section-3
	pub fn generate(
	algorithm: Algorithm,
	rng: &dyn rand::SecureRandom,
	) -> Result<Self, error::Unspecified> {
	Self::construct(algorithm, \|buf\| rng.fill(buf))
	}

	fn construct<F>(algorithm: Algorithm, fill: F) -> Result<Self, error::Unspecified>
	where
	F: FnOnce(&mut [u8]) -> Result<(), error::Unspecified>,
	{
	let mut key_bytes = [0; digest::MAX_OUTPUT_LEN];
	let key_bytes = &mut key_bytes[..algorithm.0.output_len];
	fill(key_bytes)?;
	Ok(Self::new(algorithm, key_bytes))
	}

	/// Construct an HMAC signing key using the given digest algorithm and key
	/// value.
	///
	/// `key_value` should be a value generated using a secure random number
	/// generator (e.g. the `key_value` output by
	/// `SealingKey::generate_serializable()`) or derived from a random key by
	/// a key derivation function (e.g. `ring::hkdf`). In particular,
	/// `key_value` shouldn't be a password.
	///
	/// As specified in RFC 2104, if `key_value` is shorter than the digest
	/// algorithm's block length (as returned by `digest::Algorithm::block_len`,
	/// not the digest length returned by `digest::Algorithm::output_len`) then
	/// it will be padded with zeros. Similarly, if it is longer than the block
	/// length then it will be compressed using the digest algorithm.
	///
	/// You should not use keys larger than the `digest_alg.block_len` because
	/// the truncation described above reduces their strength to only
	/// `digest_alg.output_len * 8` bits. Support for such keys is likely to be
	/// removed in a future version of ring.
	pub fn new(algorithm: Algorithm, key_value: &[u8]) -> Self {
	let digest_alg = algorithm.0;
	let mut key = Self {
	inner: digest::BlockContext::new(digest_alg),
	outer: digest::BlockContext::new(digest_alg),
	};

	let key_hash;
	let key_value = if key_value.len() <= digest_alg.block_len {
	key_value
	} else {
	key_hash = digest::digest(digest_alg, key_value);
	key_hash.as_ref()
	};

	const IPAD: u8 = 0x36;

	let mut padded_key = [IPAD; digest::MAX_BLOCK_LEN];
	let padded_key = &mut padded_key[..digest_alg.block_len];

	// If the key is shorter than one block then we're supposed to act like
	// it is padded with zero bytes up to the block length. `x ^ 0 == x` so
	// we can just leave the trailing bytes of `padded_key` untouched.
	for (padded_key, key_value) in padded_key.iter_mut().zip(key_value.iter()) {
	padded_key ^= key_value;
	}
	key.inner.update(&padded_key);

	const OPAD: u8 = 0x5C;

	// Remove the `IPAD` masking, leaving the unmasked padded key, then
	// mask with `OPAD`, all in one step.
	for b in padded_key.iter_mut() {
	*b ^= IPAD ^ OPAD;
	}
	key.outer.update(&padded_key);

	key
	}

	/// The digest algorithm for the key.
	#[inline]
	pub fn algorithm(&self) -> Algorithm {
	Algorithm(self.inner.algorithm)
	}
	}

	impl hkdf::KeyType for Algorithm {
	fn len(&self) -> usize {
	self.digest_algorithm().output_len
	}
	}

	impl From<hkdf::Okm<'_, Algorithm>> for Key {
	fn from(okm: hkdf::Okm<Algorithm>) -> Self {
	Key::construct(*okm.len(), \|buf\| okm.fill(buf)).unwrap()
	}
	}

	/// A context for multi-step (Init-Update-Finish) HMAC signing.
	///
	/// Use `sign` for single-step HMAC signing.
	#[derive(Clone)]
	pub struct Context {
	inner: digest::Context,
	outer: digest::BlockContext,
	}

	/// `hmac::SigningContext` was renamed to `hmac::Context`.
	#[deprecated(note = "Renamed to `hmac::Context`.")]
	pub type SigningContext = Context;

	impl core::fmt::Debug for Context {
	fn fmt(&self, f: &mut core::fmt::Formatter) -> Result<(), core::fmt::Error> {
	f.debug_struct("Context")
	.field("algorithm", self.inner.algorithm())
	.finish()
	}
	}

	impl Context {
	/// Constructs a new HMAC signing context using the given digest algorithm
	/// and key.
	pub fn with_key(signing_key: &Key) -> Self {
	Self {
	inner: digest::Context::clone_from(&signing_key.inner),
	outer: signing_key.outer.clone(),
	}
	}

	/// Updates the HMAC with all the data in `data`. `update` may be called
	/// zero or more times until `finish` is called.
	pub fn update(&mut self, data: &[u8]) {
	self.inner.update(data);
	}

	/// Finalizes the HMAC calculation and returns the HMAC value. `sign`
	/// consumes the context so it cannot be (mis-)used after `sign` has been
	/// called.
	///
	/// It is generally not safe to implement HMAC verification by comparing
	/// the return value of `sign` to a tag. Use `verify` for verification
	/// instead.
	pub fn sign(self) -> Tag {
	let algorithm = self.inner.algorithm();
	let mut pending = [0u8; digest::MAX_BLOCK_LEN];
	let pending = &mut pending[..algorithm.block_len];
	let num_pending = algorithm.output_len;
	pending[..num_pending].copy_from_slice(self.inner.finish().as_ref());
	Tag(self.outer.finish(pending, num_pending))
	}
	}

	/// Calculates the HMAC of `data` using the key `key` in one step.
	///
	/// Use `Context` to calculate HMACs where the input is in multiple parts.
	///
	/// It is generally not safe to implement HMAC verification by comparing the
	/// return value of `sign` to a tag. Use `verify` for verification instead.
	pub fn sign(key: &Key, data: &[u8]) -> Tag {
	let mut ctx = Context::with_key(key);
	ctx.update(data);
	ctx.sign()
	}

	/// Calculates the HMAC of `data` using the signing key `key`, and verifies
	/// whether the resultant value equals `tag`, in one step.
	///
	/// This is logically equivalent to, but more efficient than, constructing a
	/// `Key` with the same value as `key` and then using `verify`.
	///
	/// The verification will be done in constant time to prevent timing attacks.
	pub fn verify(key: &Key, data: &[u8], tag: &[u8]) -> Result<(), error::Unspecified> {
	constant_time::verify_slices_are_equal(sign(key, data).as_ref(), tag)
	}

	#[cfg(test)]
	mod tests {
	use crate::{hmac, rand};

	// Make sure that `Key::generate` and `verify_with_own_key` aren't
	// completely wacky.
	#[test]
	pub fn hmac_signing_key_coverage() {
	let rng = rand::SystemRandom::new();

	const HELLO_WORLD_GOOD: &[u8] = b"hello, world";
	const HELLO_WORLD_BAD: &[u8] = b"hello, worle";

	for algorithm in &[
	hmac::HMAC_SHA1_FOR_LEGACY_USE_ONLY,
	hmac::HMAC_SHA256,
	hmac::HMAC_SHA384,
	hmac::HMAC_SHA512,
	] {
	let key = hmac::Key::generate(*algorithm, &rng).unwrap();
	let tag = hmac::sign(&key, HELLO_WORLD_GOOD);
	assert!(hmac::verify(&key, HELLO_WORLD_GOOD, tag.as_ref()).is_ok());
	assert!(hmac::verify(&key, HELLO_WORLD_BAD, tag.as_ref()).is_err())
	}
	}
	}