// Copyright 2015 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-based Extract-and-Expand Key Derivation Function.
//!
//! HKDF is specified in [RFC 5869].
//!
//! In most situations, it is best to use `extract_and_expand` to do both the
//! HKDF-Extract and HKDF-Expand as one atomic operation. It is only necessary
//! to use the separate `expand` and `extract` functions if a single derived
//! `PRK` (defined in RFC 5869) is used more than once.
//!
//! Salts have type `hmac::SigningKey` instead of `&[u8]` because they are
//! frequently used for multiple HKDF operations, and it is more efficient to
//! construct the `SigningKey` once and reuse it. Given a digest algorithm
//! `digest_alg` and a salt `salt: &[u8]`, the `SigningKey` should be
//! constructed as `hmac::SigningKey::new(digest_alg, salt)`.
//!
//! [RFC 5869]: https://tools.ietf.org/html/rfc5869


use hmac;

/// Fills `out` with the output of the HKDF Extract-and-Expand operation for
/// the given inputs.
///
/// `extract_and_expand` is exactly equivalent to:
///
/// ```
/// # use ring::{hkdf, hmac};
/// # fn foo(salt: &hmac::SigningKey, secret: &[u8], info: &[u8],
/// #        out: &mut [u8]) {
/// let prk = hkdf::extract(salt, secret);
/// hkdf::expand(&prk, info, out)
/// # }
/// ```
///
/// See the documentation for `extract` and `expand` for details.
///
/// # Panics
///
/// `extract_and_expand` panics if `expand` panics.
pub fn extract_and_expand(salt: &hmac::SigningKey, secret: &[u8],
                          info: &[u8], out: &mut [u8]) {
    let prk = extract(salt, secret);
    expand(&prk, info, out)
}

/// The HKDF-Extract operation.
///
/// | Parameter                 | RFC 5869 Term
/// |---------------------------|--------------
/// | `salt.digest_algorithm()` | Hash
/// | `secret`                  | IKM (Input Keying Material)
/// | [return value]            | PRK
pub fn extract(salt: &hmac::SigningKey, secret: &[u8]) -> hmac::SigningKey {
    // The spec says that if no salt is provided then a key of
    // `digest_alg.output_len` bytes of zeros is used. But, HMAC keys are
    // already zero-padded to the block length, which is larger than the output
    // length of the extract step (the length of the digest). Consequently, the
    // `SigningKey` constructor will automatically do the right thing for a
    // zero-length string.
    let prk = hmac::sign(salt, secret);
    hmac::SigningKey::new(salt.digest_algorithm(), prk.as_ref())
}

/// Fills `out` with the output of the HKDF-Expand operation for the given
/// inputs.
///
/// `prk` should be the return value of an earlier call to `extract`.
///
/// | Parameter  | RFC 5869 Term
/// |------------|--------------
/// | prk        | PRK
/// | info       | info
/// | out        | OKM (Output Keying Material)
/// | out.len()  | L (Length of output keying material in bytes)
///
/// # Panics
///
/// `expand` panics if the requested output length is larger than 255 times the
/// size of the digest algorithm, i.e. if
/// `out.len() > 255 * salt.digest_algorithm().output_len`. This is the limit
/// imposed by the HKDF specification, and is necessary to prevent overflow of
/// the 8-bit iteration counter in the expansion step.
pub fn expand(prk: &hmac::SigningKey, info: &[u8], out: &mut [u8]) {
    let digest_alg = prk.digest_algorithm();
    assert!(out.len() <= 255 * digest_alg.output_len);
    assert!(digest_alg.block_len >= digest_alg.output_len);

    let mut ctx = hmac::SigningContext::with_key(prk);

    let mut n = 1u8;
    let mut pos = 0;
    loop {
        ctx.update(info);
        ctx.update(&[n]);

        let t = ctx.sign();

        // Append `t` to the output.
        let to_copy = if out.len() - pos < digest_alg.output_len {
            out.len() - pos
        } else {
            digest_alg.output_len
        };
        let t_bytes = t.as_ref();
        for i in 0..to_copy {
            out[pos + i] = t_bytes[i];
        }
        if to_copy < digest_alg.output_len {
            break;
        }
        pos += digest_alg.output_len;

        ctx = hmac::SigningContext::with_key(prk);
        ctx.update(t_bytes);
        n += 1;
    }
}
