poly1305.rs

// Import hacspec and all needed definitions.
use hacspec_lib::*;

// WARNING:
// This spec does not provide secret independence, and treats all keys as public.
// Consequently, it should only be used as a FORMAL SPEC, NOT as a reference implementation.

// Type definitions for use in poly1305.
bytes!(PolyKey, 32);

const BLOCKSIZE: usize = 16;

// These are type aliases for convenience
bytes!(PolyBlock, 16);

// These are actual types; fixed-length arrays.
bytes!(Poly1305Tag, 16);

// A byte sequence of length <= BLOCKSIZE
pub type SubBlock = ByteSeq;

// A length <= BLOCKSIZE
pub type BlockIndex = usize;

// This defines the field for modulo 2^130-5.
// In particular `FieldElement` and `FieldCanvas` are defined.
// The `FieldCanvas` is an integer type with 131-bit (to hold 2*(2^130-5)).
// The `FieldElement` is a natural integer modulo 2^130-5.

public_nat_mod!(
    type_name: FieldElement,
    type_of_canvas: FieldCanvas,
    bit_size_of_field: 131, // This amounts to 17 bytes
    modulo_value: "03fffffffffffffffffffffffffffffffb"
);

// Internal Poly1305 State
pub type PolyState = (FieldElement, FieldElement, PolyKey); //(accumulator,r,key)

pub fn poly1305_encode_r(b: &PolyBlock) -> FieldElement {
    let mut n = U128_from_le_bytes(U128Word::from_seq(b));
    n = n & U128(0x0fff_fffc_0fff_fffc_0fff_fffc_0fff_ffffu128);
    FieldElement::from_secret_literal(n)
}

pub fn poly1305_encode_block(b: &PolyBlock) -> FieldElement {
    let n = U128_from_le_bytes(U128Word::from_seq(b));
    let f = FieldElement::from_secret_literal(n);
    f + FieldElement::pow2(128)
}

// In Poly1305 as used in this spec, pad_len is always the length of b, i.e. there is no padding
// In Chacha20Poly1305, pad_len is set to BLOCKSIZE
pub fn poly1305_encode_last(pad_len: BlockIndex, b: &SubBlock) -> FieldElement {
    let n = U128_from_le_bytes(U128Word::from_slice(b, 0, b.len()));
    let f = FieldElement::from_secret_literal(n);
    f + FieldElement::pow2(8 * pad_len)
}

pub fn poly1305_init(k: PolyKey) -> PolyState {
    let r = poly1305_encode_r(&PolyBlock::from_slice(&k, 0, 16));
    (FieldElement::ZERO(), r, k)
}

pub fn poly1305_update_block(b: &PolyBlock, st: PolyState) -> PolyState {
    let (acc, r, k) = st;
    ((poly1305_encode_block(b) + acc) * r, r, k)
}

pub fn poly1305_update_blocks(m: &ByteSeq, st: PolyState) -> PolyState {
    let mut st = st;
    let n_blocks = m.len() / BLOCKSIZE;
    for i in 0..n_blocks {
        let block = PolyBlock::from_seq(&m.get_exact_chunk(BLOCKSIZE, i));
        st = poly1305_update_block(&block, st);
    }
    st
}

pub fn poly1305_update_last(pad_len: usize, b: &SubBlock, st: PolyState) -> PolyState {
    let mut st = st;
    if b.len() != 0 {
        let (acc, r, k) = st;
        st = ((poly1305_encode_last(pad_len, b) + acc) * r, r, k);
    }
    st
}

pub fn poly1305_update(m: &ByteSeq, st: PolyState) -> PolyState {
    let st = poly1305_update_blocks(m, st);
    let last = m.get_remainder_chunk(BLOCKSIZE);
    poly1305_update_last(last.len(), &last, st)
}

pub fn poly1305_finish(st: PolyState) -> Poly1305Tag {
    let (acc, _, k) = st;
    let n = U128_from_le_bytes(U128Word::from_slice(&k, 16, 16));
    let aby = acc.to_byte_seq_le();
    // We can't use from_seq here because the accumulator is larger than 16 bytes.
    let a = U128_from_le_bytes(U128Word::from_slice(&aby, 0, 16));
    Poly1305Tag::from_seq(&U128_to_le_bytes(a + n))
}

pub fn poly1305(m: &ByteSeq, key: PolyKey) -> Poly1305Tag {
    let mut st = poly1305_init(key);
    st = poly1305_update(m, st);
    poly1305_finish(st)
}