puressh 0.2.4

A pure-Rust SSH (Secure Shell) protocol library, in the spirit of libssh, built on purecrypto.
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
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233

//! `ssh-mldsa44-ed25519@openssh.com` — OpenSSH's post-quantum composite
//! signature (OpenSSH 10.3+): ML-DSA-44 (FIPS 204) and Ed25519 over the same
//! message, both of which must verify, following the IETF LAMPS composite
//! signatures (`COMPSIG-MLDSA44-Ed25519-SHA512`).
//!
//! ```text
//! public key:  string "ssh-mldsa44-ed25519@openssh.com"
//!              string ML-DSA-44 public key (1312 B) || Ed25519 public key (32 B)
//! signature:   string "ssh-mldsa44-ed25519@openssh.com"
//!              string ML-DSA-44 signature (2420 B) || Ed25519 signature (64 B)
//! private key: ML-DSA-44 seed (32 B) || Ed25519 seed (32 B)
//! ```
//!
//! Both components sign `M' = "CompositeAlgorithmSignatures2025" || label ||
//! u8(0) || SHA512(message)`; ML-DSA additionally takes the label as its
//! context string.

use alloc::vec::Vec;

use purecrypto::ec::{Ed25519PrivateKey, Ed25519PublicKey, Ed25519Signature};
use purecrypto::hash::{Digest, Sha512};
use purecrypto::mldsa::{MlDsa44PrivateKey, MlDsa44PublicKey};
use purecrypto::zeroize::Zeroizing;

use super::{HostKey, HostKeyAlgorithm, HostKeyVerify};
use crate::error::{Error, Result};
use crate::format::{Reader, Writer};

/// Marker for the `ssh-mldsa44-ed25519@openssh.com` algorithm.
pub struct SshMlDsa44Ed25519;

impl HostKeyAlgorithm for SshMlDsa44Ed25519 {
    const NAME: &'static str = "ssh-mldsa44-ed25519@openssh.com";
}

/// ML-DSA-44 public key length.
const MLDSA_PK: usize = 1312;
/// ML-DSA-44 signature length.
const MLDSA_SIG: usize = 2420;
/// Composite public key length (ML-DSA-44 || Ed25519).
pub const PUBLIC_KEY_BYTES: usize = MLDSA_PK + 32;
/// Composite private key length (two 32-byte seeds).
pub const PRIVATE_KEY_BYTES: usize = 64;
/// Composite signature length.
pub const SIGNATURE_BYTES: usize = MLDSA_SIG + 64;

const PREFIX: &[u8] = b"CompositeAlgorithmSignatures2025";
const LABEL: &[u8] = b"COMPSIG-MLDSA44-Ed25519-SHA512";

/// The message both components sign (empty composite context).
fn m_prime(msg: &[u8]) -> Vec<u8> {
    let mut m = Vec::with_capacity(PREFIX.len() + LABEL.len() + 1 + 64);
    m.extend_from_slice(PREFIX);
    m.extend_from_slice(LABEL);
    m.push(0);
    m.extend_from_slice(Sha512::digest(msg).as_ref());
    m
}

/// The composite public key for a private key (two seeds).
pub fn public_from_seeds(seeds: &[u8; PRIVATE_KEY_BYTES]) -> Vec<u8> {
    let mut mldsa_seed = Zeroizing::new([0u8; 32]);
    mldsa_seed.copy_from_slice(&seeds[..32]);
    let mut ed_seed = Zeroizing::new([0u8; 32]);
    ed_seed.copy_from_slice(&seeds[32..]);
    let (_, mldsa_pk) = MlDsa44PrivateKey::from_seed(&mldsa_seed);
    let ed_pk = Ed25519PrivateKey::from_bytes(*ed_seed).public_key();
    let mut pk = Vec::with_capacity(PUBLIC_KEY_BYTES);
    pk.extend_from_slice(mldsa_pk.to_bytes());
    pk.extend_from_slice(&ed_pk.to_bytes());
    pk
}

/// An `ssh-mldsa44-ed25519@openssh.com` key: a verifier, and a signer when
/// built from the private seeds.
pub struct MlDsa44Ed25519HostKey {
    seeds: Option<Zeroizing<[u8; PRIVATE_KEY_BYTES]>>,
    public: Vec<u8>,
}

impl MlDsa44Ed25519HostKey {
    /// A signing key from its private part (ML-DSA seed || Ed25519 seed).
    pub fn from_seeds(seeds: [u8; PRIVATE_KEY_BYTES]) -> Self {
        let public = public_from_seeds(&seeds);
        MlDsa44Ed25519HostKey {
            seeds: Some(Zeroizing::new(seeds)),
            public,
        }
    }

    /// A verify-only key from the raw composite public key.
    pub fn from_public(raw: &[u8]) -> Result<Self> {
        if raw.len() != PUBLIC_KEY_BYTES {
            return Err(Error::Format("mldsa44-ed25519: public key length"));
        }
        Ok(MlDsa44Ed25519HostKey {
            seeds: None,
            public: raw.to_vec(),
        })
    }

    /// The raw composite public key.
    pub fn public_bytes(&self) -> &[u8] {
        &self.public
    }

    /// The raw composite signature over `msg`.
    fn sign_raw(&self, msg: &[u8]) -> Result<Vec<u8>> {
        let seeds = self
            .seeds
            .as_ref()
            .ok_or(Error::Crypto("mldsa44-ed25519: no private key"))?;
        let m = m_prime(msg);
        let mut mldsa_seed = Zeroizing::new([0u8; 32]);
        mldsa_seed.copy_from_slice(&seeds[..32]);
        let mut ed_seed = Zeroizing::new([0u8; 32]);
        ed_seed.copy_from_slice(&seeds[32..]);
        let (mldsa_sk, _) = MlDsa44PrivateKey::from_seed(&mldsa_seed);
        #[cfg(feature = "std")]
        let mldsa_sig = mldsa_sk.sign(&mut purecrypto::rng::OsRng, &m, LABEL);
        #[cfg(not(feature = "std"))]
        let mldsa_sig = mldsa_sk.sign_deterministic(&m, LABEL);
        let mldsa_sig = mldsa_sig.map_err(|_| Error::Crypto("mldsa44-ed25519: sign"))?;
        let ed_sig = Ed25519PrivateKey::from_bytes(*ed_seed).sign(&m);
        let mut sig = Vec::with_capacity(SIGNATURE_BYTES);
        sig.extend_from_slice(&mldsa_sig);
        sig.extend_from_slice(&ed_sig.to_bytes());
        Ok(sig)
    }

    /// Whether `sig` (raw composite) is valid over `msg`: both halves must be.
    fn verify_raw(&self, msg: &[u8], sig: &[u8]) -> bool {
        if sig.len() != SIGNATURE_BYTES {
            return false;
        }
        let Ok(mldsa_pk) = MlDsa44PublicKey::from_bytes(&self.public[..MLDSA_PK]) else {
            return false;
        };
        let mut ed_pk = [0u8; 32];
        ed_pk.copy_from_slice(&self.public[MLDSA_PK..]);
        let mut ed_sig = [0u8; 64];
        ed_sig.copy_from_slice(&sig[MLDSA_SIG..]);
        let m = m_prime(msg);
        let mldsa_ok = mldsa_pk.verify(&sig[..MLDSA_SIG], &m, LABEL);
        let ed_ok = Ed25519PublicKey::from_bytes(ed_pk)
            .verify(&m, &Ed25519Signature::from_bytes(ed_sig))
            .is_ok();
        mldsa_ok & ed_ok
    }
}

impl HostKey for MlDsa44Ed25519HostKey {
    fn algorithm(&self) -> &'static str {
        SshMlDsa44Ed25519::NAME
    }

    fn public_blob(&self) -> Vec<u8> {
        let mut w = Writer::with_capacity(4 + 31 + 4 + PUBLIC_KEY_BYTES);
        w.write_string(SshMlDsa44Ed25519::NAME.as_bytes());
        w.write_string(&self.public);
        w.into_vec()
    }

    fn sign(&self, msg: &[u8]) -> Result<Vec<u8>> {
        let sig = self.sign_raw(msg)?;
        let mut w = Writer::with_capacity(4 + 31 + 4 + SIGNATURE_BYTES);
        w.write_string(SshMlDsa44Ed25519::NAME.as_bytes());
        w.write_string(&sig);
        Ok(w.into_vec())
    }
}

impl HostKeyVerify for MlDsa44Ed25519HostKey {
    fn algorithm(&self) -> &'static str {
        SshMlDsa44Ed25519::NAME
    }

    fn verify(&self, msg: &[u8], sig_blob: &[u8]) -> Result<()> {
        let mut r = Reader::new(sig_blob);
        if r.read_string()? != SshMlDsa44Ed25519::NAME.as_bytes() {
            return Err(Error::Format(
                "mldsa44-ed25519: signature algorithm mismatch",
            ));
        }
        let sig = r.read_string()?;
        if !r.is_empty() {
            return Err(Error::Format("mldsa44-ed25519: signature trailing data"));
        }
        if self.verify_raw(msg, sig) {
            Ok(())
        } else {
            Err(Error::BadSignature)
        }
    }

    fn from_public_blob(blob: &[u8]) -> Result<Self> {
        let mut r = Reader::new(blob);
        if r.read_string()? != SshMlDsa44Ed25519::NAME.as_bytes() {
            return Err(Error::Format("mldsa44-ed25519: public key type mismatch"));
        }
        let raw = r.read_string()?;
        if !r.is_empty() {
            return Err(Error::Format("mldsa44-ed25519: public key trailing data"));
        }
        Self::from_public(raw)
    }
}

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

    #[test]
    fn sign_verify_roundtrip() {
        let mut seeds = [0u8; 64];
        for (i, b) in seeds.iter_mut().enumerate() {
            *b = i as u8;
        }
        let k = MlDsa44Ed25519HostKey::from_seeds(seeds);
        let sig = k.sign(b"hello").unwrap();
        let v = MlDsa44Ed25519HostKey::from_public_blob(&k.public_blob()).unwrap();
        v.verify(b"hello", &sig).unwrap();
        assert!(v.verify(b"hellO", &sig).is_err());
        // Either half alone is not enough.
        let mut bad = sig.clone();
        let n = bad.len();
        bad[n - 1] ^= 1;
        assert!(v.verify(b"hello", &bad).is_err());
        let mut bad = sig;
        bad[40] ^= 1;
        assert!(v.verify(b"hello", &bad).is_err());
    }
}