ppoppo_token/
signing_key.rs

1//! Opaque Ed25519 signing key with associated `kid`.
2//!
3//! Wraps `jsonwebtoken::EncodingKey` so the public surface stays free of
4//! `jsonwebtoken::*` (M51 boundary). The `kid` travels with the key
5//! because JWT kids are issuer-chosen — there's no library-derived
6//! identifier the way PASETO has PASERK pid. Pairing kid+key here lets
7//! the engine reject mismatched configurations (`IssueError::KeyMismatch`)
8//! before any encoding work happens.
9//!
10//! Companion: [`ed25519_public_from_pem`] derives the 32-byte public key
11//! half from the same PEM that builds a `SigningKey`. PAS uses it at boot
12//! to populate the JWKS document without re-encoding the key material.
13
14use jsonwebtoken::{DecodingKey, EncodingKey};
15
16use crate::{IssueError, KeySet};
17
18pub struct SigningKey {
19    inner: EncodingKey,
20    kid: String,
21}
22
23impl SigningKey {
24    /// Parse an Ed25519 private key from PEM and pair it with the given
25    /// `kid`. Returns `KeyParse` on any decode failure — the variant
26    /// name is the audit signal; the wrapped string carries the
27    /// library's diagnostic for incident response.
28    pub fn from_ed25519_pem(pem: &[u8], kid: impl Into<String>) -> Result<Self, IssueError> {
29        let inner =
30            EncodingKey::from_ed_pem(pem).map_err(|e| IssueError::KeyParse(e.to_string()))?;
31        Ok(Self {
32            inner,
33            kid: kid.into(),
34        })
35    }
36
37    /// Returns the `kid` associated with this signer.
38    pub fn kid(&self) -> &str {
39        &self.kid
40    }
41
42    /// Test-only constructor — returns a `(SigningKey, KeySet)` pair
43    /// where the KeySet already carries the matching decoding key under
44    /// the same kid. Round-trip integration tests rely on this contract:
45    /// `issue` with the signing half and `verify` against the key set
46    /// must agree without manual wiring.
47    ///
48    /// The PEM constants are deterministic across runs (checked-in test
49    /// material; same key as `tests/jwt_negative.rs::TEST_PRIVATE_KEY_PEM`),
50    /// so failures are reproducible. The private key has no production
51    /// value.
52    #[allow(clippy::expect_used)]
53    pub fn test_pair() -> (Self, KeySet) {
54        const TEST_PRIVATE_KEY_PEM: &[u8] = b"-----BEGIN PRIVATE KEY-----
55MC4CAQAwBQYDK2VwBCIEIG+00IvEd4uv6IWtGFVUEBVdqnXiuI/ESQHu6rmcDvAs
56-----END PRIVATE KEY-----
57";
58        const TEST_PUBLIC_KEY_PEM: &[u8] = b"-----BEGIN PUBLIC KEY-----
59MCowBQYDK2VwAyEAh//e6j3It3xhjghg8Kpn2pM0jMCH/cvemGu4vv7D1Q4=
60-----END PUBLIC KEY-----
61";
62        const TEST_KID: &str = "k4.test.0";
63
64        let signer = Self::from_ed25519_pem(TEST_PRIVATE_KEY_PEM, TEST_KID)
65            .expect("checked-in test PEM should always parse");
66        let mut key_set = KeySet::new();
67        let dec = DecodingKey::from_ed_pem(TEST_PUBLIC_KEY_PEM)
68            .expect("checked-in test PEM should always parse");
69        key_set.insert(TEST_KID, dec);
70        (signer, key_set)
71    }
72
73    #[allow(dead_code)] // wired up by `engine::encode::issue` in commit 3.3
74    pub(crate) fn encoding(&self) -> &EncodingKey {
75        &self.inner
76    }
77}
78
79/// Derive the 32-byte Ed25519 public key from a PKCS8-encoded private
80/// PEM. Used by PAS at boot to populate `/.well-known/jwks.json` from the
81/// same key material that produces issuance signatures, so issuer and
82/// publisher cannot drift.
83///
84/// Internals: `ed25519-compact` parses the PEM and exposes the matching
85/// public key. We avoid `jsonwebtoken`'s own EncodingKey here because it
86/// does not expose pubkey extraction on its public API. `ed25519-compact`
87/// is the smallest pure-Rust path; it carries no `unsafe`, no I/O, no
88/// global state.
89///
90/// Errors as `IssueError::KeyParse` to match `from_ed25519_pem`'s error
91/// shape (operators see one variant for "private PEM didn't load");
92/// the wrapped string carries the underlying library's diagnostic.
93pub fn ed25519_public_from_pem(pem: &[u8]) -> Result<[u8; 32], IssueError> {
94    let pem_str = std::str::from_utf8(pem)
95        .map_err(|e| IssueError::KeyParse(format!("PEM utf8: {e}")))?;
96    let secret = ed25519_compact::SecretKey::from_pem(pem_str)
97        .map_err(|e| IssueError::KeyParse(format!("ed25519 pem decode: {e}")))?;
98    Ok(*secret.public_key())
99}
100
101#[cfg(test)]
102mod tests {
103    use super::*;
104
105    #[test]
106    fn public_from_pem_matches_test_pair_public_key() {
107        // The test_pair public key bytes come from a known DER blob in
108        // signing_key.rs. The PEM-side derivation must agree with the
109        // public PEM that test_pair already encodes — this is the
110        // round-trip that PAS depends on at boot for JWKS construction.
111        const TEST_PRIVATE_KEY_PEM: &[u8] = b"-----BEGIN PRIVATE KEY-----
112MC4CAQAwBQYDK2VwBCIEIG+00IvEd4uv6IWtGFVUEBVdqnXiuI/ESQHu6rmcDvAs
113-----END PRIVATE KEY-----
114";
115        // SPKI DER for the matching public key. Last 32 bytes = raw pk.
116        const TEST_PUBLIC_KEY_SPKI_B64: &str =
117            "MCowBQYDK2VwAyEAh//e6j3It3xhjghg8Kpn2pM0jMCH/cvemGu4vv7D1Q4=";
118
119        use base64::Engine as _;
120        let spki = base64::engine::general_purpose::STANDARD
121            .decode(TEST_PUBLIC_KEY_SPKI_B64)
122            .expect("test SPKI must decode");
123        let expected: [u8; 32] = spki[12..].try_into().expect("SPKI carries 32-byte pk");
124
125        let derived = ed25519_public_from_pem(TEST_PRIVATE_KEY_PEM)
126            .expect("checked-in test PEM must derive");
127        assert_eq!(
128            derived, expected,
129            "PEM-derived public key must match the test_pair fixture",
130        );
131    }
132
133    #[test]
134    fn public_from_pem_rejects_non_pem() {
135        let err = ed25519_public_from_pem(b"not a pem at all").expect_err("garbage must reject");
136        match err {
137            IssueError::KeyParse(_) => {}
138            other => panic!("expected KeyParse, got {other:?}"),
139        }
140    }
141}
ppoppo_token/signing_key.rs

ppoppo_token/
signing_key.rs
