feather_reader/oauth/

jwt.rs

//! ES256 JWS signing — the one signing primitive the OAuth flow needs.
//!
//! Two things are signed with the confidential client's key:
//!
//! * the **client assertion** (`private_key_jwt`), proving to the PDS's token
//!   endpoint that we are the client named by `client_id`, and
//! * **DPoP proofs**, proving possession of the per-session DPoP key.
//!
//! Both are compact JWS: `base64url(header) . base64url(payload) . base64url(sig)`.
//!
//! The detail worth stating, because getting it wrong produces a signature that
//! verifies nowhere: JOSE ECDSA signatures are the **fixed-width `r || s`**
//! form (64 bytes for P-256), *not* ASN.1 DER. A DER signature is the default
//! output of many ECDSA APIs and is silently accepted by nothing.

use anyhow::{anyhow, bail, Context as _, Result};
use base64::engine::general_purpose::URL_SAFE_NO_PAD;
use base64::Engine;
use p256::ecdsa::signature::{Signer as _, Verifier as _};
use p256::ecdsa::{Signature, SigningKey as EcdsaSigningKey, VerifyingKey};
use serde_json::Value;

use super::keys::SigningKey;

/// The only algorithm this module signs or accepts.
const ALG: &str = "ES256";

/// The JOSE signing input: `base64url(header) . base64url(payload)`.
fn signing_input(header: &Value, claims: &Value) -> Result<String> {
    let header = serde_json::to_vec(header).context("serializing the JWS header")?;
    let claims = serde_json::to_vec(claims).context("serializing the JWS claims")?;
    Ok(format!(
        "{}.{}",
        URL_SAFE_NO_PAD.encode(header),
        URL_SAFE_NO_PAD.encode(claims)
    ))
}

/// Sign `header`/`claims` as a compact ES256 JWS.
///
/// `p256`'s `Signature` is the fixed-width `r || s` form that JOSE requires, so
/// `to_bytes()` is 64 bytes. Reaching for `to_der()` here would produce a
/// signature no verifier accepts — see the module docs.
///
/// Signing is deterministic (RFC 6979): the nonce `k` is derived from the key
/// and the message rather than drawn from an RNG, so there is no nonce-reuse
/// failure mode here of the kind AES-GCM has.
pub fn sign(key: &SigningKey, header: &Value, claims: &Value) -> Result<String> {
    // `alg` is owned here, not taken from the caller: RFC 7515 §4.1.1 makes it
    // REQUIRED, and a header advertising anything other than what actually
    // signed is an algorithm-confusion bug. Same reasoning as `guarded_post`
    // owning the Content-Type.
    let mut header = header.clone();
    header
        .as_object_mut()
        .ok_or_else(|| anyhow!("JWS header must be a JSON object"))?
        .insert("alg".into(), Value::String(ALG.into()));

    let input = signing_input(&header, claims)?;
    let signer = EcdsaSigningKey::from(key.secret());
    let signature: Signature = signer.sign(input.as_bytes());
    Ok(format!(
        "{input}.{}",
        URL_SAFE_NO_PAD.encode(signature.to_bytes())
    ))
}

/// Verify a compact ES256 JWS against `key`'s public half.
///
/// Used by the tests and available for verifying anything we minted; the PDS
/// verifies our assertions itself via the published JWKS.
pub fn verify(key: &SigningKey, jws: &str) -> Result<()> {
    let parts: Vec<&str> = jws.split('.').collect();
    if parts.len() != 3 {
        bail!("malformed JWS: expected 3 segments, got {}", parts.len());
    }
    // Check the advertised algorithm before spending a verification. The header
    // is covered by the signature, so a mismatch would fail anyway — but saying
    // so explicitly means a caller can never be handed a verified-looking JWS
    // whose header claims an algorithm we did not check.
    let header: Value = serde_json::from_slice(
        &URL_SAFE_NO_PAD
            .decode(parts[0])
            .context("header is not valid base64url")?,
    )
    .context("JWS header is not valid JSON")?;
    match header.get("alg").and_then(Value::as_str) {
        Some(ALG) => {}
        other => bail!("unsupported JWS alg {other:?}; only {ALG} is accepted"),
    }

    let raw = URL_SAFE_NO_PAD
        .decode(parts[2])
        .context("signature is not valid base64url")?;
    let signature =
        Signature::from_slice(&raw).map_err(|err| anyhow!("not a valid P-256 signature: {err}"))?;

    let input = format!("{}.{}", parts[0], parts[1]);
    let verifier = VerifyingKey::from(key.secret().public_key());
    verifier
        .verify(input.as_bytes(), &signature)
        .map_err(|_| anyhow!("JWS signature does not verify"))
}

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

    const KID: &str = "featherreader-oauth-1";

    fn decode_part(part: &str) -> Value {
        serde_json::from_slice(&URL_SAFE_NO_PAD.decode(part).unwrap()).unwrap()
    }

    /// **A valid JWS with an extra segment appended must be refused.**
    ///
    /// `verify` requires EXACTLY three segments. Relaxing that to `>= 3` — a
    /// plausible-looking loosening — left the whole suite green, because the one
    /// test covering segment counts feeds `"a.b.c.d"`, which dies at base64
    /// decoding long before the count is consulted. Under the relaxed check a
    /// genuinely valid token with attacker-appended trailing data verifies.
    ///
    /// So this appends to a REAL signed JWS: every segment is well-formed and the
    /// signature covers the first two, leaving the count as the only thing that
    /// can reject it.
    #[test]
    fn a_valid_jws_with_a_trailing_segment_is_refused() {
        let key = SigningKey::generate(KID);
        let jws = sign(&key, &json!({"typ": "JWT"}), &json!({"iss": "x"})).unwrap();
        verify(&key, &jws).expect("the unmodified JWS must verify");

        let extended = format!("{jws}.AAAA");
        let err =
            verify(&key, &extended).expect_err("a JWS with a trailing segment must not verify");
        assert!(
            format!("{err:#}").contains("expected 3 segments"),
            "refused, but not by the segment count — the relaxed check is what this \
             test exists to catch: {err:#}",
        );
    }

    /// **Algorithm confusion: a signature valid over the bytes, whose header
    /// advertises a different algorithm, must not verify.**
    ///
    /// The existing pair of tests swap the header of an already-signed token,
    /// which changes the signing input — so the ECDSA check fails and the `alg`
    /// guard is never the reason. Deleting the guard entirely left all 664 tests
    /// green.
    ///
    /// This builds the hostile token properly: the header says `RS256`, and the
    /// ES256 signature is computed over THAT header, so the signature is
    /// genuinely valid and only the `alg` check can reject it. `sign` cannot be
    /// used here — it owns `alg` and overwrites it, which is the right production
    /// behaviour and exactly why the token has to be assembled by hand.
    #[test]
    fn a_signature_valid_under_a_forged_alg_header_is_refused() {
        let key = SigningKey::generate(KID);
        let header = json!({"alg": "RS256", "typ": "JWT", "kid": KID});
        let claims = json!({"iss": "https://x.example"});

        let input = signing_input(&header, &claims).unwrap();
        let signer = EcdsaSigningKey::from(key.secret());
        let signature: Signature = signer.sign(input.as_bytes());
        let forged = format!("{input}.{}", URL_SAFE_NO_PAD.encode(signature.to_bytes()));

        // The signature really is valid over these bytes — swap the header to the
        // honest algorithm and the same construction verifies.
        let honest_input =
            signing_input(&json!({"alg": "ES256", "typ": "JWT", "kid": KID}), &claims).unwrap();
        let honest_sig: Signature = signer.sign(honest_input.as_bytes());
        verify(
            &key,
            &format!(
                "{honest_input}.{}",
                URL_SAFE_NO_PAD.encode(honest_sig.to_bytes())
            ),
        )
        .expect("the same construction with an honest alg must verify");

        let err = verify(&key, &forged).expect_err("a forged alg header must not verify");
        assert!(
            format!("{err:#}").contains("unsupported JWS alg"),
            "refused, but not by the alg check — if the signature merely failed, the \
             guard could be deleted and this test would still pass: {err:#}",
        );
    }

    #[test]
    fn a_signed_jws_has_three_base64url_segments() {
        let key = SigningKey::generate(KID);
        let jws = sign(&key, &json!({"alg": "ES256"}), &json!({"iss": "x"})).unwrap();
        let parts: Vec<&str> = jws.split('.').collect();
        assert_eq!(parts.len(), 3);
        for p in &parts {
            assert!(!p.is_empty());
            assert!(!p.contains('='), "base64url in JOSE is unpadded: {p}");
            assert!(!p.contains('+') && !p.contains('/'), "not url-safe: {p}");
        }
    }

    #[test]
    fn the_header_and_payload_round_trip_verbatim() {
        let key = SigningKey::generate(KID);
        let header = json!({"alg": "ES256", "typ": "JWT", "kid": KID});
        let claims = json!({"iss": "https://x.example", "jti": "abc", "iat": 1_700_000_000});
        let jws = sign(&key, &header, &claims).unwrap();
        let parts: Vec<&str> = jws.split('.').collect();
        assert_eq!(decode_part(parts[0]), header);
        assert_eq!(decode_part(parts[1]), claims);
    }

    /// **The classic ES256 bug.** JOSE requires the fixed-width `r || s`
    /// concatenation — exactly 64 bytes for P-256 — while many ECDSA APIs return
    /// ASN.1 DER by default. For P-256 with random `r`/`s`, DER runs 70-72
    /// bytes, so the length alone discriminates.
    ///
    /// A fresh key per iteration: signing is RFC 6979-deterministic, so reusing
    /// one key would produce sixteen identical signatures and test nothing.
    #[test]
    fn the_signature_is_fixed_width_r_s_not_der() {
        for _ in 0..16 {
            let key = SigningKey::generate(KID);
            let jws = sign(&key, &json!({}), &json!({"n": 1})).unwrap();
            let sig = URL_SAFE_NO_PAD
                .decode(jws.split('.').nth(2).unwrap())
                .unwrap();
            assert_eq!(sig.len(), 64, "not a fixed-width P-256 JOSE signature");
        }
    }

    /// `alg` is REQUIRED by RFC 7515 §4.1.1, and a header claiming anything else
    /// while carrying an ES256 signature is an algorithm-confusion bug waiting
    /// to happen. `sign` owns the field rather than trusting the caller — the
    /// same reasoning as `guarded_post` owning Content-Type.
    #[test]
    fn sign_owns_the_alg_header_and_overrides_the_caller() {
        let key = SigningKey::generate(KID);
        for given in [json!({}), json!({"alg": "RS256"}), json!({"alg": "none"})] {
            let jws = sign(&key, &given, &json!({"x": 1})).unwrap();
            assert_eq!(decode_part(jws.split('.').next().unwrap())["alg"], "ES256");
            assert!(verify(&key, &jws).is_ok());
        }
    }

    #[test]
    fn sign_preserves_the_callers_other_header_members() {
        let key = SigningKey::generate(KID);
        let jws = sign(&key, &json!({"typ": "dpop+jwt", "kid": KID}), &json!({})).unwrap();
        let header = decode_part(jws.split('.').next().unwrap());
        assert_eq!(header["typ"], "dpop+jwt");
        assert_eq!(header["kid"], KID);
        assert_eq!(header["alg"], "ES256");
    }

    /// A signature that is valid over the bytes but whose header advertises a
    /// different algorithm must not verify — otherwise a caller could be talked
    /// into treating an ES256 signature as an RS256 one.
    #[test]
    fn verify_rejects_a_header_advertising_another_algorithm() {
        let key = SigningKey::generate(KID);
        let jws = sign(&key, &json!({}), &json!({"x": 1})).unwrap();
        let seg: Vec<&str> = jws.split('.').collect();

        for forged_header in [json!({"alg": "RS256"}), json!({"alg": "none"}), json!({})] {
            let swapped = format!(
                "{}.{}.{}",
                URL_SAFE_NO_PAD.encode(serde_json::to_vec(&forged_header).unwrap()),
                seg[1],
                seg[2]
            );
            assert!(verify(&key, &swapped).is_err(), "accepted {forged_header}");
        }
    }

    #[test]
    fn verify_rejects_a_non_json_header() {
        let key = SigningKey::generate(KID);
        let jws = sign(&key, &json!({}), &json!({"x": 1})).unwrap();
        let seg: Vec<&str> = jws.split('.').collect();
        let bad = format!(
            "{}.{}.{}",
            URL_SAFE_NO_PAD.encode(b"not json"),
            seg[1],
            seg[2]
        );
        assert!(verify(&key, &bad).is_err());
    }

    #[test]
    fn a_signature_verifies_against_the_signing_keys_public_half() {
        let key = SigningKey::generate(KID);
        let jws = sign(&key, &json!({"alg": "ES256"}), &json!({"sub": "did:plc:x"})).unwrap();
        assert!(verify(&key, &jws).is_ok());
    }

    #[test]
    fn a_signature_from_a_different_key_does_not_verify() {
        let a = SigningKey::generate(KID);
        let b = SigningKey::generate(KID);
        let jws = sign(&a, &json!({"alg": "ES256"}), &json!({"sub": "x"})).unwrap();
        assert!(verify(&b, &jws).is_err());
    }

    #[test]
    fn tampering_with_the_payload_invalidates_the_signature() {
        let key = SigningKey::generate(KID);
        let jws = sign(&key, &json!({"alg": "ES256"}), &json!({"amount": 1})).unwrap();
        let parts: Vec<&str> = jws.split('.').collect();
        let forged = URL_SAFE_NO_PAD.encode(br#"{"amount":1000000}"#);
        let tampered = format!("{}.{}.{}", parts[0], forged, parts[2]);
        assert!(verify(&key, &tampered).is_err());
    }

    #[test]
    fn a_malformed_jws_is_rejected_rather_than_panicking() {
        let key = SigningKey::generate(KID);
        for bad in [
            "",
            "onlyonepart",
            "two.parts",
            "a.b.c.d",
            "...",
            "!!!.@@@.###",
            "a.b.",
        ] {
            assert!(verify(&key, bad).is_err(), "should reject {bad:?}");
        }
    }

    /// Two signatures over the same input must not be byte-identical unless the
    /// scheme is deliberately deterministic. Either is acceptable for ES256
    /// (RFC 6979 is deterministic); this pins WHICH one we get, so a future
    /// dependency change that flips it is visible rather than silent.
    #[test]
    fn signing_is_deterministic_rfc6979() {
        let key = SigningKey::generate(KID);
        let a = sign(&key, &json!({"alg": "ES256"}), &json!({"x": 1})).unwrap();
        let b = sign(&key, &json!({"alg": "ES256"}), &json!({"x": 1})).unwrap();
        assert_eq!(a, b, "p256 signs deterministically per RFC 6979");
    }
}