tsslib 0.2.0

Easy-to-use threshold signature schemes (FROST, ML-DSA, DKLs23) — wire- and save-data-compatible with the Go tss-lib.
Documentation 
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

//! Threshold ML-DSA-44 key material.

use super::Error;
use super::params::{MAX_PARTIES, ThresholdParams44, sharing_pattern};
use purecrypto::mldsa::hazmat::{self, Poly};
use std::collections::HashMap;

const K: usize = 4; // ML_DSA_44.k
const L: usize = 4; // ML_DSA_44.l

/// One `(s1, s2)` secret share, identified by the honest-signer subset mask it
/// was drawn for. Holds both plain (`s1`, `s2`) and cached NTT (`s1h`, `s2h`)
/// representations.
#[derive(Clone)]
pub struct Share44 {
    pub s1: [Poly; L],
    pub s2: [Poly; K],
    pub s1h: [Poly; L],
    pub s2h: [Poly; K],
}

/// One party's threshold ML-DSA-44 key: public material (`rho`, `tr`, `t1`) plus
/// the shares whose subset mask includes this party's `id`.
#[derive(Clone)]
pub struct Key44 {
    /// This party's 0-based id.
    pub id: u8,
    /// Public matrix seed.
    pub rho: [u8; 32],
    /// `SHAKE256(packed pk)` (the FIPS 204 `tr`).
    pub tr: [u8; 64],
    /// Public `t1` vector (high bits of `t = A·s1 + s2`).
    pub t1: [Poly; K],
    /// Shares keyed by honest-signer mask.
    pub shares: HashMap<u8, Share44>,
}

impl Key44 {
    /// Expands the public matrix `A` (row-major, NTT domain) from `rho`.
    /// `A[i*L + j] = sample_ntt_poly(rho, j, i)` (FIPS 204 ExpandA).
    pub fn matrix(&self) -> Vec<Poly> {
        expand_matrix(&self.rho)
    }

    /// Checks the key is well-formed: it holds at least one share, and every
    /// share's mask includes this party's id.
    pub fn validate(&self) -> Result<(), Error> {
        if self.shares.is_empty() {
            return Err(Error::Validation("key has no shares".into()));
        }
        for &mask in self.shares.keys() {
            if mask & (1 << self.id) == 0 {
                return Err(Error::Validation("share mask excludes own id".into()));
            }
        }
        Ok(())
    }

    /// Reconstructs this party's NTT-domain `(s1, s2)` contribution to the
    /// aggregated secret for the signing set `act` (a bitmask of participating
    /// party ids), per the replicated-sharing pattern.
    pub fn recover_share(
        &self,
        act: u8,
        params: &ThresholdParams44,
    ) -> Result<([Poly; L], [Poly; K]), Error> {
        let mut s1h = [Poly::zero(); L];
        let mut s2h = [Poly::zero(); K];

        // t == n: each party holds exactly the full-signer mask's share.
        if params.t == params.n {
            let sh = self
                .shares
                .values()
                .next()
                .ok_or_else(|| Error::Validation("recover_share(t==n): no shares".into()))?;
            return Ok((sh.s1h, sh.s2h));
        }

        let pattern = sharing_pattern(params.t, params.n)
            .ok_or_else(|| Error::Validation("no sharing pattern for (t,n)".into()))?;

        // perm: ids in `act` (low→high) at [0..t), then the rest at [t..n).
        let mut perm = [0u8; MAX_PARTIES];
        let (mut i1, mut i2) = (0u8, params.t);
        let mut currenti: i32 = -1;
        for j in 0..params.n {
            if j == self.id {
                currenti = i1 as i32;
            }
            if act & (1 << j) != 0 {
                perm[i1 as usize] = j;
                i1 += 1;
            } else {
                perm[i2 as usize] = j;
                i2 += 1;
            }
        }
        if currenti < 0 || currenti >= params.t as i32 {
            return Err(Error::Validation(
                "this key is not in the signing set".into(),
            ));
        }
        let pattern_for_me = pattern[currenti as usize];

        for &u in pattern_for_me {
            // Translate abstract mask u (permuted positions) → real mask (ids).
            let mut u_real = 0u8;
            for i in 0..params.n {
                if u & (1 << i) != 0 {
                    u_real |= 1 << perm[i as usize];
                }
            }
            let share = self
                .shares
                .get(&u_real)
                .ok_or_else(|| Error::Validation("missing share in sharing pattern".into()))?;
            for j in 0..L {
                s1h[j] = s1h[j].add(&share.s1h[j]);
            }
            for j in 0..K {
                s2h[j] = s2h[j].add(&share.s2h[j]);
            }
        }
        Ok((s1h, s2h))
    }
}

/// Expands matrix `A` (row-major `K×L`, NTT domain) from `rho`.
pub fn expand_matrix(rho: &[u8; 32]) -> Vec<Poly> {
    let mut a = Vec::with_capacity(K * L);
    for i in 0..K {
        for j in 0..L {
            a.push(hazmat::sample_ntt_poly(rho, j as u8, i as u8));
        }
    }
    a
}