1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148

//! An implementation of HKDF, the [HMAC-based Extract-and-Expand Key Derivation Function][1].
//!
//! # Usage
//!
//! ```rust
//! # extern crate hex;
//! # extern crate hkdf;
//! # extern crate sha2;
//!
//! # use sha2::Sha256;
//! # use hkdf::Hkdf;
//!
//! # fn main() {
//! let ikm = hex::decode("0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b").unwrap();
//! let salt = hex::decode("000102030405060708090a0b0c").unwrap();
//! let info = hex::decode("f0f1f2f3f4f5f6f7f8f9").unwrap();
//!
//! let h = Hkdf::<Sha256>::new(Some(&salt[..]), &ikm);
//! let mut okm = [0u8; 42];
//! h.expand(&info, &mut okm).unwrap();
//! println!("OKM is {}", hex::encode(&okm[..]));
//! # }
//! ```
//!
//! [1]: https://tools.ietf.org/html/rfc5869
#![no_std]

extern crate digest;
extern crate hmac;
#[cfg(feature = "std")]
extern crate std;

use core::fmt;
use digest::generic_array::{self, ArrayLength, GenericArray};
use digest::{BlockInput, FixedOutput, Input, Reset};
use hmac::{Hmac, Mac};

/// Error that is returned when supplied pseudorandom key (PRK) is not long enough.
#[derive(Copy, Clone, Eq, PartialEq, Debug)]
pub struct InvalidPrkLength;

/// Structure for InvalidLength, used for output error handling.
#[derive(Copy, Clone, Eq, PartialEq, Debug)]
pub struct InvalidLength;

/// Structure representing the HKDF, capable of HKDF-Expand and HKDF-extract operations.
#[derive(Clone)]
pub struct Hkdf<D>
where
    D: Input + BlockInput + FixedOutput + Reset + Default + Clone,
    D::BlockSize: ArrayLength<u8>,
    D::OutputSize: ArrayLength<u8>,
{
    hmac: Hmac<D>,
}

impl<D> Hkdf<D>
where
    D: Input + BlockInput + FixedOutput + Reset + Default + Clone,
    D::BlockSize: ArrayLength<u8>,
    D::OutputSize: ArrayLength<u8>,
{
    /// Convenience method for [`extract`] when the generated pseudorandom
    /// key can be ignored and only HKDF-Expand operation is needed. This is
    /// the most common constructor.
    pub fn new(salt: Option<&[u8]>, ikm: &[u8]) -> Hkdf<D> {
        let (_, hkdf) = Hkdf::extract(salt, ikm);
        hkdf
    }

    /// Create `Hkdf` from an already cryptographically strong pseudorandom key
    /// as per section 3.3 from RFC5869.
    pub fn from_prk(prk: &[u8]) -> Result<Hkdf<D>, InvalidPrkLength> {
        use generic_array::typenum::Unsigned;

        // section 2.3 specifies that prk must be "at least HashLen octets"
        if prk.len() < D::OutputSize::to_usize() {
            return Err(InvalidPrkLength);
        }

        Ok(Hkdf {
            hmac: Hmac::new_varkey(prk).expect("HMAC can take a key of any size"),
        })
    }

    /// The RFC5869 HKDF-Extract operation returning both the generated
    /// pseudorandom key and `Hkdf` struct for expanding.
    pub fn extract(salt: Option<&[u8]>, ikm: &[u8]) -> (GenericArray<u8, D::OutputSize>, Hkdf<D>) {
        let mut hmac = match salt {
            Some(s) => Hmac::<D>::new_varkey(s).expect("HMAC can take a key of any size"),
            None => Hmac::<D>::new(&Default::default()),
        };

        hmac.input(ikm);

        let prk = hmac.result().code();
        let hkdf = Hkdf::from_prk(&prk).expect("PRK size is correct");
        (prk, hkdf)
    }

    /// The RFC5869 HKDF-Expand operation
    pub fn expand(&self, info: &[u8], okm: &mut [u8]) -> Result<(), InvalidLength> {
        use generic_array::typenum::Unsigned;

        let mut prev: Option<GenericArray<u8, <D as digest::FixedOutput>::OutputSize>> = None;

        let hmac_output_bytes = D::OutputSize::to_usize();
        if okm.len() > hmac_output_bytes * 255 {
            return Err(InvalidLength);
        }

        let mut hmac = self.hmac.clone();
        for (blocknum, okm_block) in okm.chunks_mut(hmac_output_bytes).enumerate() {
            let block_len = okm_block.len();

            if let Some(ref prev) = prev {
                hmac.input(prev)
            };
            hmac.input(info);
            hmac.input(&[blocknum as u8 + 1]);

            let output = hmac.result_reset().code();
            okm_block.copy_from_slice(&output[..block_len]);

            prev = Some(output);
        }

        Ok(())
    }
}

impl fmt::Display for InvalidPrkLength {
    fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
        f.write_str("invalid pseudorandom key length, too short")
    }
}

#[cfg(feature = "std")]
impl ::std::error::Error for InvalidPrkLength {}

impl fmt::Display for InvalidLength {
    fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
        f.write_str("invalid number of blocks, too large output")
    }
}

#[cfg(feature = "std")]
impl ::std::error::Error for InvalidLength {}