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
//! GitHub webhook HMAC-SHA256 verification, over the exact received bytes.
//!
//! Why: the same check exists twice already — `trusty-analyze`'s
//! `core/github.rs:210` and `trusty-review`'s
//! `integrations/github/webhook.rs:27` — and they disagree on what an *unset*
//! secret means: review returns 401, analyze logs a warning and processes the
//! payload anyway (ADR-0034 Context, "how they disagree"). ADR-0034 §3 puts
//! verification in exactly one place, at console, and unifies the unset-secret
//! policy to fail-closed. This module is that place. #5089 step 3 routes
//! console through it; step 4 retires the other two copies.
//!
//! What: [`verify_github_signature`] returns a three-state
//! [`SignatureVerdict`] rather than a `bool`, because "no secret is configured"
//! and "the signature is wrong" are different operator problems that both have
//! to fail closed. The digest is computed over the raw body bytes — the HMAC
//! covers the literal request body, so a caller must pass what arrived on the
//! wire, never a re-serialised value.
//!
//! Test: `tests` below — a valid signature, a wrong secret, an empty secret, a
//! missing `sha256=` prefix, non-hex, a truncated digest, and a body mutated by
//! one byte.
//!
//! [`verify_github_signature`]: crate::webhook_hmac::verify_github_signature
//! [`SignatureVerdict`]: crate::webhook_hmac::SignatureVerdict
use hmac::{Hmac, Mac};
use sha2::Sha256;
/// Algorithm name recorded in a relayed frame's provenance record.
///
/// ADR-0034 §3 requires the frame to state which algorithm was checked, so a
/// target can tell a verified delivery from an unverified one without
/// re-deriving it from the header name.
pub const HMAC_ALGORITHM: &str = "hmac-sha256";
/// The header GitHub carries the digest in.
pub const SIGNATURE_HEADER: &str = "x-hub-signature-256";
/// Outcome of verifying one delivery.
///
/// Why: a `bool` cannot distinguish "the operator never configured a secret"
/// from "someone sent a forged payload", and collapsing them is how
/// `trusty-analyze` ended up treating the first as permission to proceed.
/// Both are refusals here; only the diagnostic differs.
/// What: three variants, of which exactly one means the payload may be used.
/// Test: every `verify_github_signature_*` test asserts on a specific variant.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum SignatureVerdict {
/// The digest matched. The body is authentic.
Valid,
/// No secret is configured, so nothing can be verified. Fail closed.
SecretMissing,
/// A secret is configured and the digest did not match, was malformed, or
/// was absent.
Invalid,
}
impl SignatureVerdict {
/// True only for [`SignatureVerdict::Valid`].
///
/// Exists so a call site cannot accidentally write `!= Invalid` and let
/// `SecretMissing` through.
pub fn is_valid(self) -> bool {
matches!(self, SignatureVerdict::Valid)
}
}
/// Verify a GitHub `X-Hub-Signature-256` digest against the raw body.
///
/// Why: the single entry point for this check (ADR-0034 §3, "Verification
/// happens exactly once, at console, over the exact bytes GitHub sent"). Any
/// re-framing of the body destroys the ability to verify it, so this must run
/// before the payload is parsed, copied, or re-encoded.
///
/// What: rejects an empty or whitespace-only `secret` with
/// [`SignatureVerdict::SecretMissing`]; otherwise parses the `sha256=<hex>`
/// header, decodes the hex, and constant-time compares it against
/// `HMAC-SHA256(secret, body)` via `Mac::verify_slice`. Any parse or decode
/// failure is [`SignatureVerdict::Invalid`], never an error return, so the
/// caller emits one uniform 401 for every refusal and leaks nothing about
/// which step failed.
///
/// Test: `verify_github_signature_accepts_a_valid_digest`,
/// `verify_github_signature_rejects_a_wrong_secret`,
/// `verify_github_signature_reports_a_missing_secret`,
/// `verify_github_signature_rejects_a_missing_prefix`,
/// `verify_github_signature_rejects_non_hex`,
/// `verify_github_signature_rejects_a_truncated_digest`,
/// `verify_github_signature_rejects_a_one_byte_body_change`.
pub fn verify_github_signature(
secret: &str,
body: &[u8],
signature_header: &str,
) -> SignatureVerdict {
if secret.trim().is_empty() {
return SignatureVerdict::SecretMissing;
}
let Some(hex_sig) = signature_header.strip_prefix("sha256=") else {
return SignatureVerdict::Invalid;
};
let Ok(expected) = hex::decode(hex_sig) else {
return SignatureVerdict::Invalid;
};
let Ok(mut mac) = Hmac::<Sha256>::new_from_slice(secret.as_bytes()) else {
return SignatureVerdict::Invalid;
};
mac.update(body);
if mac.verify_slice(&expected).is_ok() {
SignatureVerdict::Valid
} else {
SignatureVerdict::Invalid
}
}
/// Produce the `sha256=<hex>` header value for `body` under `secret`.
///
/// Why: every test in this workspace that exercises a webhook path needs to
/// mint a valid signature, and three copies of that helper already exist in
/// test modules. Exposing it from the crate that owns verification keeps the
/// signing and checking halves from drifting.
/// What: `format!("sha256={}", hex(HMAC-SHA256(secret, body)))`.
/// Test: used as the input to `verify_github_signature_accepts_a_valid_digest`.
pub fn sign_github_body(secret: &str, body: &[u8]) -> String {
let mut mac = Hmac::<Sha256>::new_from_slice(secret.as_bytes())
.expect("HMAC-SHA256 accepts a key of any length");
mac.update(body);
format!("sha256={}", hex::encode(mac.finalize().into_bytes()))
}
#[cfg(test)]
mod tests {
use super::*;
const SECRET: &str = "test-hmac-key"; // pragma: allowlist secret
const BODY: &[u8] = br#"{"action":"review_requested","number":7}"#;
#[test]
fn verify_github_signature_accepts_a_valid_digest() {
let header = sign_github_body(SECRET, BODY);
assert_eq!(
verify_github_signature(SECRET, BODY, &header),
SignatureVerdict::Valid
);
}
#[test]
fn verify_github_signature_rejects_a_wrong_secret() {
let header = sign_github_body("some-other-secret", BODY); // pragma: allowlist secret
assert_eq!(
verify_github_signature(SECRET, BODY, &header),
SignatureVerdict::Invalid
);
}
#[test]
fn verify_github_signature_reports_a_missing_secret() {
let header = sign_github_body(SECRET, BODY);
// An unset secret and a whitespace-only one are the same operator
// mistake and must not be distinguishable from a wrong signature by
// the caller's response.
for empty in ["", " ", "\t\n"] {
assert_eq!(
verify_github_signature(empty, BODY, &header),
SignatureVerdict::SecretMissing,
"secret {empty:?}"
);
assert!(!verify_github_signature(empty, BODY, &header).is_valid());
}
}
#[test]
fn verify_github_signature_rejects_a_missing_prefix() {
let signed = sign_github_body(SECRET, BODY);
let bare = signed.trim_start_matches("sha256=");
assert_eq!(
verify_github_signature(SECRET, BODY, bare),
SignatureVerdict::Invalid
);
assert_eq!(
verify_github_signature(SECRET, BODY, ""),
SignatureVerdict::Invalid
);
}
#[test]
fn verify_github_signature_rejects_non_hex() {
assert_eq!(
verify_github_signature(SECRET, BODY, "sha256=zzzz"),
SignatureVerdict::Invalid
);
}
#[test]
fn verify_github_signature_rejects_a_truncated_digest() {
let signed = sign_github_body(SECRET, BODY);
let truncated = &signed[..signed.len() - 4];
assert_eq!(
verify_github_signature(SECRET, BODY, truncated),
SignatureVerdict::Invalid
);
}
#[test]
fn verify_github_signature_rejects_a_one_byte_body_change() {
let header = sign_github_body(SECRET, BODY);
let mut mutated = BODY.to_vec();
let last = mutated.len() - 1;
mutated[last] ^= 0x01;
assert_eq!(
verify_github_signature(SECRET, &mutated, &header),
SignatureVerdict::Invalid
);
}
}