mtcs 0.3.0

Offline verification of Merkle Tree Certificates (draft-ietf-plants-merkle-tree-certs)
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

//! Cosignature verification (Sections 5.3.1 and 5.3.3 of the draft).
//!
//! The signed message is a `CosignedSubtree`; for certificate use the
//! timestamp is zero (Section 6.2). Signature algorithms are identified by
//! the cosigner's configured public key, following Section 5.3.3.

use crate::der::read_element;
use crate::id::TrustAnchorId;
use crate::tree::{HASH_SIZE, Subtree};
use crate::{Error, Result};

/// A cosigner signature algorithm (Section 5.3.3 of the draft).
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
#[non_exhaustive]
pub enum SignatureAlgorithm {
    /// ECDSA over P-256 with SHA-256.
    EcdsaP256Sha256,
    /// ECDSA over P-384 with SHA-384.
    EcdsaP384Sha384,
    /// Ed25519 (RFC 8032).
    Ed25519,
    /// ML-DSA-44 (FIPS 204).
    MlDsa44,
    /// ML-DSA-65 (FIPS 204).
    MlDsa65,
    /// ML-DSA-87 (FIPS 204).
    MlDsa87,
}

impl SignatureAlgorithm {
    /// Identify an algorithm by its X.509 AlgorithmIdentifier OID, as carried
    /// in the CA's MTCCertificationAuthority extension or configured directly.
    pub fn from_algorithm_oid(oid: &str) -> Option<Self> {
        Some(match oid {
            "1.2.840.10045.4.3.2" => Self::EcdsaP256Sha256,
            "1.2.840.10045.4.3.3" => Self::EcdsaP384Sha384,
            "1.3.101.112" => Self::Ed25519,
            "2.16.840.1.101.3.4.3.17" => Self::MlDsa44,
            "2.16.840.1.101.3.4.3.18" => Self::MlDsa65,
            "2.16.840.1.101.3.4.3.19" => Self::MlDsa87,
            _ => return None,
        })
    }
}

/// The message a cosigner signs over a subtree: a `CosignedSubtree` with the
/// draft's domain separation label. `timestamp` MUST be zero for cosignatures
/// embedded in certificates; non-zero values are for checkpoint cosignatures
/// (tlog-cosignature) and require `start == 0`.
pub(crate) fn cosigned_subtree_message(
    cosigner: &TrustAnchorId,
    log: &TrustAnchorId,
    timestamp: u64,
    subtree: Subtree,
    subtree_hash: &[u8; HASH_SIZE],
) -> Result<Vec<u8>> {
    if timestamp != 0 && subtree.start != 0 {
        return Err(Error::Malformed("timestamped non-checkpoint cosignature"));
    }
    let cosigner_name = cosigner.note_name();
    let log_origin = log.note_name();
    if cosigner_name.len() > 255 || log_origin.len() > 255 {
        return Err(Error::Malformed("cosigner name"));
    }
    let mut message = Vec::with_capacity(
        12 + 1 + cosigner_name.len() + 8 + 1 + log_origin.len() + 16 + HASH_SIZE,
    );
    message.extend_from_slice(b"subtree/v1\n\0");
    message.push(cosigner_name.len() as u8);
    message.extend_from_slice(&cosigner_name);
    message.extend_from_slice(&timestamp.to_be_bytes());
    message.push(log_origin.len() as u8);
    message.extend_from_slice(&log_origin);
    message.extend_from_slice(&subtree.start.to_be_bytes());
    message.extend_from_slice(&subtree.end.to_be_bytes());
    message.extend_from_slice(subtree_hash);
    Ok(message)
}

/// Extract the raw public key bytes from a SubjectPublicKeyInfo, checking the
/// SPKI algorithm (and, for ECDSA, the curve OID) against the signature
/// algorithm the cosigner is configured with.
fn spki_key_bytes(algorithm: SignatureAlgorithm, spki_der: &[u8]) -> Result<&[u8]> {
    let (spki, rest) = read_element(spki_der).ok_or(Error::UnsupportedKey("SPKI"))?;
    if spki.tag != 0x30 || !rest.is_empty() {
        return Err(Error::UnsupportedKey("SPKI"));
    }
    let (algorithm_element, rest) =
        read_element(spki.content).ok_or(Error::UnsupportedKey("SPKI algorithm"))?;
    let (key, rest) = read_element(rest).ok_or(Error::UnsupportedKey("SPKI key"))?;
    if !rest.is_empty() || key.tag != 0x03 || key.content.is_empty() || key.content[0] != 0 {
        return Err(Error::UnsupportedKey("SPKI key"));
    }
    let (algorithm_oid, rest) = read_element(algorithm_element.content)
        .ok_or(Error::UnsupportedKey("SPKI algorithm OID"))?;
    if algorithm_oid.tag != 0x06 {
        return Err(Error::UnsupportedKey("SPKI algorithm OID"));
    }
    let dotted = crate::id::oid_content_to_dotted(algorithm_oid.content)?;
    let expected = match algorithm {
        SignatureAlgorithm::EcdsaP256Sha256 => "1.2.840.10045.2.1",
        SignatureAlgorithm::EcdsaP384Sha384 => "1.2.840.10045.2.1",
        SignatureAlgorithm::Ed25519 => "1.3.101.112",
        SignatureAlgorithm::MlDsa44 => "2.16.840.1.101.3.4.3.17",
        SignatureAlgorithm::MlDsa65 => "2.16.840.1.101.3.4.3.18",
        SignatureAlgorithm::MlDsa87 => "2.16.840.1.101.3.4.3.19",
    };
    if dotted != expected {
        return Err(Error::UnsupportedKey("SPKI algorithm mismatch"));
    }
    if let SignatureAlgorithm::EcdsaP256Sha256 | SignatureAlgorithm::EcdsaP384Sha384 = algorithm {
        let (curve, _) = read_element(rest).ok_or(Error::UnsupportedKey("curve OID"))?;
        if curve.tag != 0x06 {
            return Err(Error::UnsupportedKey("curve OID"));
        }
        let curve = crate::id::oid_content_to_dotted(curve.content)?;
        let expected_curve = match algorithm {
            SignatureAlgorithm::EcdsaP256Sha256 => "1.2.840.10045.3.1.7",
            SignatureAlgorithm::EcdsaP384Sha384 => "1.3.132.0.34",
            _ => unreachable!(),
        };
        if curve != expected_curve {
            return Err(Error::UnsupportedKey("curve mismatch"));
        }
    } else if !rest.is_empty() {
        // Ed25519 and ML-DSA algorithm parameters MUST be absent.
        return Err(Error::UnsupportedKey("unexpected algorithm parameters"));
    }
    Ok(&key.content[1..])
}

/// Verify a cosignature over a subtree. Returns an error for unrecognized
/// algorithms, malformed keys, or invalid signatures.
pub(crate) fn verify_cosignature(
    cosigner: &TrustAnchorId,
    log: &TrustAnchorId,
    algorithm: SignatureAlgorithm,
    spki_der: &[u8],
    subtree: Subtree,
    subtree_hash: &[u8; HASH_SIZE],
    signature: &[u8],
) -> Result<()> {
    let message = cosigned_subtree_message(cosigner, log, 0, subtree, subtree_hash)?;
    let key = spki_key_bytes(algorithm, spki_der)?;
    let invalid = || Error::InvalidCosignature(cosigner.clone());
    match algorithm {
        SignatureAlgorithm::EcdsaP256Sha256 => {
            verify_ecdsa_p256(key, &message, signature).map_err(|_| invalid())
        }
        SignatureAlgorithm::EcdsaP384Sha384 => {
            verify_ecdsa_p384(key, &message, signature).map_err(|_| invalid())
        }
        SignatureAlgorithm::Ed25519 => {
            verify_ed25519(key, &message, signature).map_err(|_| invalid())
        }
        SignatureAlgorithm::MlDsa44 => {
            verify_ml_dsa::<ml_dsa::MlDsa44>(key, &message, signature).map_err(|_| invalid())
        }
        SignatureAlgorithm::MlDsa65 => {
            verify_ml_dsa::<ml_dsa::MlDsa65>(key, &message, signature).map_err(|_| invalid())
        }
        SignatureAlgorithm::MlDsa87 => {
            verify_ml_dsa::<ml_dsa::MlDsa87>(key, &message, signature).map_err(|_| invalid())
        }
    }
}

fn verify_ecdsa_p256(
    key: &[u8],
    message: &[u8],
    signature: &[u8],
) -> core::result::Result<(), signature::Error> {
    use signature::Verifier as _;
    let key =
        p256::ecdsa::VerifyingKey::from_sec1_bytes(key).map_err(|_| signature::Error::new())?;
    let signature =
        p256::ecdsa::Signature::from_der(signature).map_err(|_| signature::Error::new())?;
    key.verify(message, &signature)
}

fn verify_ecdsa_p384(
    key: &[u8],
    message: &[u8],
    signature: &[u8],
) -> core::result::Result<(), signature::Error> {
    use signature::Verifier as _;
    let key =
        p384::ecdsa::VerifyingKey::from_sec1_bytes(key).map_err(|_| signature::Error::new())?;
    let signature =
        p384::ecdsa::Signature::from_der(signature).map_err(|_| signature::Error::new())?;
    key.verify(message, &signature)
}

fn verify_ed25519(
    key: &[u8],
    message: &[u8],
    signature: &[u8],
) -> core::result::Result<(), signature::Error> {
    use signature::Verifier as _;
    let key_bytes: &[u8; 32] = key.try_into().map_err(|_| signature::Error::new())?;
    let key =
        ed25519_dalek::VerifyingKey::from_bytes(key_bytes).map_err(|_| signature::Error::new())?;
    let signature =
        ed25519_dalek::Signature::from_slice(signature).map_err(|_| signature::Error::new())?;
    key.verify(message, &signature)
}

fn verify_ml_dsa<P: ml_dsa::MlDsaParams>(
    key: &[u8],
    message: &[u8],
    signature: &[u8],
) -> core::result::Result<(), signature::Error> {
    use signature::Verifier as _;
    let encoded_key =
        ml_dsa::EncodedVerifyingKey::<P>::try_from(key).map_err(|_| signature::Error::new())?;
    let key = ml_dsa::VerifyingKey::<P>::decode(&encoded_key);
    let encoded_signature =
        ml_dsa::EncodedSignature::<P>::try_from(signature).map_err(|_| signature::Error::new())?;
    let signature =
        ml_dsa::Signature::<P>::decode(&encoded_signature).ok_or_else(signature::Error::new)?;
    key.verify(message, &signature)
}