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

//! ConcatKDF from NIST 800-56ar for ECDH-ES / ECDH-1PU

use core::{fmt::Debug, marker::PhantomData};

use digest::Digest;

use crate::generic_array::{typenum::Unsigned, GenericArray};

use crate::{buffer::WriteBuffer, error::Error};

/// A struct providing the key derivation for a particular hash function
#[derive(Clone, Copy, Debug)]
pub struct ConcatKDF<H>(PhantomData<H>);

/// Parameters for the key derivation
#[derive(Clone, Copy, Debug, Default, PartialEq, Eq)]
pub struct ConcatKDFParams<'p> {
    /// The algorithm name
    pub alg: &'p [u8],
    /// Sender identifier (PartyUInfo)
    pub apu: &'p [u8],
    /// Recipient identifier (PartyVInfo)
    pub apv: &'p [u8],
    /// SuppPubInfo as defined by the application
    pub pub_info: &'p [u8],
    /// SuppPrivInfo as defined by the application
    pub prv_info: &'p [u8],
}

impl<H> ConcatKDF<H>
where
    H: Digest,
{
    /// Perform the key derivation and write the result to the provided buffer
    pub fn derive_key(
        message: &[u8],
        params: ConcatKDFParams<'_>,
        mut output: &mut [u8],
    ) -> Result<(), Error> {
        let output_len = output.len();
        if output_len > H::OutputSize::USIZE * (u32::MAX as usize) - 1 {
            return Err(err_msg!(Usage, "Exceeded max output size for concat KDF"));
        }
        let mut hasher = ConcatKDFHash::<H>::new();
        let mut remain = output_len;
        while remain > 0 {
            hasher.start_pass();
            hasher.hash_message(message);
            hasher.hash_params(params);
            let hashed = hasher.finish_pass();
            let cp_size = hashed.len().min(remain);
            &output[..cp_size].copy_from_slice(&hashed[..cp_size]);
            output = &mut output[cp_size..];
            remain -= cp_size;
        }
        Ok(())
    }
}

/// Core hashing implementation of the multi-pass key derivation
#[derive(Debug)]
pub struct ConcatKDFHash<H: Digest> {
    hasher: H,
    counter: u32,
}

impl<H: Digest> ConcatKDFHash<H> {
    /// Create a new instance
    pub fn new() -> Self {
        Self {
            hasher: H::new(),
            counter: 1,
        }
    }

    /// Start a new pass of the key derivation
    pub fn start_pass(&mut self) {
        self.hasher.update(self.counter.to_be_bytes());
        self.counter += 1;
    }

    /// Hash input to the key derivation
    pub fn hash_message(&mut self, data: &[u8]) {
        self.hasher.update(data);
    }

    /// Hash the parameters of the key derivation
    pub fn hash_params(&mut self, params: ConcatKDFParams<'_>) {
        let hash = &mut self.hasher;
        hash.update((params.alg.len() as u32).to_be_bytes());
        hash.update(params.alg);
        hash.update((params.apu.len() as u32).to_be_bytes());
        hash.update(params.apu);
        hash.update((params.apv.len() as u32).to_be_bytes());
        hash.update(params.apv);
        hash.update(params.pub_info);
        hash.update(params.prv_info);
    }

    /// Complete this pass of the key derivation, returning the result
    pub fn finish_pass(&mut self) -> GenericArray<u8, H::OutputSize> {
        self.hasher.finalize_reset()
    }
}

impl<D: Debug + Digest> WriteBuffer for ConcatKDFHash<D> {
    fn buffer_write(&mut self, data: &[u8]) -> Result<(), Error> {
        self.hasher.update(data);
        Ok(())
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use sha2::Sha256;

    #[test]
    // testing with ConcatKDF - single pass via ConcatKDFHash is tested elsewhere
    fn expected_1pu_output() {
        let z = hex!(
            "9e56d91d817135d372834283bf84269cfb316ea3da806a48f6daa7798cfe90c4
            e3ca3474384c9f62b30bfd4c688b3e7d4110a1b4badc3cc54ef7b81241efd50d"
        );
        let mut output = [0u8; 32];
        ConcatKDF::<Sha256>::derive_key(
            &z,
            ConcatKDFParams {
                alg: b"A256GCM",
                apu: b"Alice",
                apv: b"Bob",
                pub_info: &(256u32).to_be_bytes(),
                prv_info: &[],
            },
            &mut output,
        )
        .unwrap();
        assert_eq!(
            output,
            hex!("6caf13723d14850ad4b42cd6dde935bffd2fff00a9ba70de05c203a5e1722ca7")
        );
    }
}