Add ES256 and PS256 signing algorithm support

[zshenker]

Summary

The Common Access Token spec supports the ES256 and PS256 signing algorithms in addition to HS256. This PR adds both signing and verification for those two asymmetric algorithms. HS256 (HMAC-SHA256) behavior is unchanged.

Algorithm	COSE id	Structure	sign() key	verify() key
HmacSha256	5	COSE_Mac0	raw symmetric key	raw symmetric key
Es256	-7	COSE_Sign1	PKCS#8 DER private	SPKI DER public
Ps256	-37	COSE_Sign1	PKCS#8 DER private	SPKI DER public

Changes

• Algorithm enum (header.rs): adds Es256 / Ps256, updates identifier()/from_identifier().
• Crypto primitives (utils.rs) via RustCrypto (p256, rsa, sha2): ES256 is ECDSA P-256 + SHA-256 (deterministic RFC 6979, 64-byte r || s signatures); PS256 is RSASSA-PSS + SHA-256/MGF1 using OsRng for the salt (WASM-friendly via getrandom).
• Token wiring (token.rs): sign()/verify() dispatch per algorithm; asymmetric algorithms reuse the existing COSE_Sign1 input and emit CWT tag 61 + COSE_Sign1 tag 18.
• Key format: existing &[u8] API preserved — PKCS#8 DER private keys for signing, SPKI DER public keys for verifying. New Error::InvalidKey (error.rs) for malformed keys.
• Docs/example: new examples/asymmetric_signing.rs, README signing-algorithms section, updated crate docs.
• Deps: bump thiserror to 2; bump rust-version to 1.88.0 — the real minimum. Because Cargo.lock isn't committed, downstream users re-resolve to the latest deps, which pull minicbor-derive 0.19.4 (uses let-chains, stabilized in 1.88) and zeroize 1.9 (edition 2024, needs 1.85). 1.85/1.86 fail to compile minicbor-derive; 1.88 builds and tests cleanly. A pinned MSRV CI job (dtolnay/rust-toolchain@1.88.0) guards against silent drift. Note this MSRV reality predates the PR: main already depended on minicbor 2.x, so its declared 1.60 was already stale — this PR just makes the metadata honest. The crypto stack uses current stable RustCrypto versions (the newer 0.14/0.10 releases are pre-1.0 release candidates).

Review-driven fixes (commit 079409e)

Grounded in RFC 9052 (COSE structures) and RFC 9053 (algorithm registry):

• COSE protected-header interop (addresses the open ES256/PS256 review threads): from_bytes() previously discarded the original protected-header bytes and sign1_input/mac0_input re-encoded the decoded header map. COSE signs the exact serialized protected bstr (RFC 9052 §4.4 — a re-encode must be byte-identical), so verifying a token whose producer used a different-but-valid CBOR encoding (map ordering, integer width) failed. Token now preserves original_protected_bytes and reuses them in sign1_input/mac0_input/to_bytes (mirroring the existing payload-bytes handling). Fixes the bug for all algorithms — HMAC had the same latent issue, masked by its Sign1/Mac0 fallback.
• Single source of truth for algorithm → structure: new AlgorithmClass { Mac, Signature } + Algorithm::class() (RFC 9053 §2/§3, RFC 9052 §8.1/§8.2). to_bytes() now selects the COSE tag (Mac0=17 / Sign1=18) via an exhaustive match instead of a Some(_) catch-all, so a future MAC algorithm can no longer be silently mis-tagged as COSE_Sign1 — adding a variant is a compile error until its class is declared. is_mac() is reimplemented in terms of class().
• Dedup: the byte-identical sign1_input/mac0_input collapse into one cose_input(context) helper; the context string ("Signature1"/"MAC0", RFC 9052 §4.4/§6.3) comes from AlgorithmClass::context().

Lossy-claim verification regression (commit ac8fa56)

Follow-up review caught a regression in the payload-bytes caching introduced above. get_payload_bytes() reused the producer's original signed payload bytes only when the full decoded map equaled the current claims projection. But Claims is a lossy projection: a registered claim carried with an unexpected CBOR type is dropped on decode — most notably aud (key 3) encoded as a CBOR array of audiences, which RFC 8392/7519 permit but RegisteredClaims.aud: Option<String> cannot hold. For such an unmutated, validly-signed external token the comparison failed, the payload was re-encoded without the dropped claim, and the genuine signature no longer matched — silently rejecting previously-accepted tokens (reject-valid, not accept-forged).

• Fix: detect mutation by comparing the current claims projection against a baseline_payload_projection captured at decode time (Claims::from_map(decoded).to_map()), instead of against the full original map. Both sides drop the same information, so an unmutated lossy token still matches and keeps its byte-faithful signed bytes, while a real mutation still forces a re-encode. The eager re-encode on the cache-hit path is also dropped.
• Scope: correctness only — the array aud now verifies and round-trips byte-faithfully, but is still not readable via the typed audience() accessor. Exposing it would change RegisteredClaims.aud's type (breaking), so it's deferred to a future breaking release.
• Tests: test_aud_as_array_verifies_and_roundtrips, test_cti_as_text_verifies_and_roundtrips (verify + byte-faithful round-trip; fail without the fix), and test_aud_as_array_mutation_is_reflected (mutation on a lossy token is still reflected and breaks the original MAC).

ES256 signature malleability — canonical low-S (commit f54f710)

Follow-up review flagged that ES256 signatures were not normalized to "low-S". ECDSA is malleable: for a valid (r, s), the pair (r, n - s) verifies over the same message. RustCrypto's p256 neither emits low-S on signing nor rejects high-S on verifying, so a third party could derive a second valid signature for an unchanged token, and the signature bytes could not serve as a stable token identity (e.g. a dedup/replay key).

• Fix (utils.rs): compute_es256 normalizes to low-S before encoding; verify_es256 rejects high-S (non-canonical) signatures. Per RFC 9053 §2.1 / SEC1 and strict COSE/WebCrypto verifiers, low-S is the canonical form.
• Tests: test_es256_signatures_are_low_s (every emitted signature is low-S across many messages) and test_es256_high_s_signature_is_rejected (constructs the malleable (r, n - s) twin of a valid signature and asserts it is rejected).

PS256 minimum RSA key size + tag-mapping cleanup (commit 5b04be7)

Final review pass. The most material finding was that PS256 imposed no floor on the RSA modulus size: neither the DER decoders nor PSS verification reject undersized keys, so a token signed under a weak key (e.g. 512- or 1024-bit, where a 512-bit modulus is factorable on commodity hardware) would sign and verify normally.

• Fix (utils.rs): compute_ps256 and verify_ps256 reject any RSA key whose modulus is below 2048 bits (MIN_RSA_KEY_BITS) with Error::InvalidKey, on both the signing and verification paths. CHANGELOG security note added. Note this is a behavioral tightening — a consumer relying on sub-2048-bit keys would now be rejected (acceptable for this breaking 0.3.0 release).
• Tests: test_ps256_undersized_key_is_rejected_on_sign / ..._on_verify using a real 1024-bit key pair.
• Cleanup: the COSE structure-tag mapping (Mac0=17 / Sign1=18) moves onto AlgorithmClass::cbor_tag(), beside context(), so both wire facts the class determines stay co-located; to_bytes() calls it instead of inline literals. Added comments explaining why the Es256/Ps256 arms in verify/sign are kept separate, and marked the intentionally-duplicated demo keys across the two examples and tests.

Reject untagged to_bytes() with no algorithm (commit af2542d)

Final review comment. to_bytes() previously emitted a bare, untagged COSE array when the protected header carried no algorithm — output the crate's own verify() then rejects with InvalidFormat, so it silently produced an unverifiable token. That state is only reachable by hand-building a Token via Token::new without an algorithm (the sign() path already requires one).

• Fix (token.rs): to_bytes() now returns Error::InvalidFormat in that case, symmetric with verify()/sign(), so the caller bug surfaces instead of being masked. Test test_to_bytes_without_algorithm_errors; CHANGELOG note added.
• The other comment in that review (algorithm-confusion in verify) is the already-tracked FOLLOWUPS.md Security item, consciously deferred — see below.

Deferred follow-ups

FOLLOWUPS.md records review findings intentionally left out of this PR to keep its scope focused: an algorithm-confusion hardening for verify (decoder-trust design that deserves its own PR — note it is not blocked on a breaking release, an additive verify_with_algorithm entrypoint would suffice) and a COSE-tag/alg cross-check on decode, the array-valued aud readability work (breaking — see above), two hot-path allocation costs in the payload/protected-header caching (the comparison rebuild and the cache-hit clone), and assorted cleanups.

Tests (tests.rs)

Round-trip sign/verify, COSE_Sign1 tag bytes, wrong-key & cross-algorithm rejection, payload tampering, PSS salt randomization, invalid-key errors, the PS256 undersized-key rejection (sign + verify), identifier round-trips, the class()/context() mapping, a non-canonical protected-header regression test (mints an external ES256 token with alg=-7 in 2-byte form and asserts both verify and a byte-faithful to_bytes() round-trip; fails without the interop fix), the lossy-claim round-trip regression tests above, and the ES256 low-S canonicalization/rejection tests.

Verification

• cargo test — 55 lib tests + 73 doctests pass
• cargo clippy --all-targets --all-features -- -D warnings — clean
• cargo fmt --check — clean
• cargo run --example asymmetric_signing — ES256 and PS256 both sign and verify

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR extends the Common Access Token crate to support asymmetric signing and verification using ES256 (P-256 ECDSA) and PS256 (RSA-PSS), in addition to the existing HS256 (HMAC-SHA256) support. It updates the algorithm model, token encoding/verification dispatch, cryptographic primitives, and adds tests/examples/docs to cover the new functionality.

Changes:

• Add Algorithm::{Es256, Ps256} (+ COSE identifiers) and use is_mac() to select COSE_Mac0 vs COSE_Sign1.
• Implement ES256/PS256 signing + verification utilities using RustCrypto (p256, rsa, sha2, rand_core).
• Add asymmetric round-trip tests and new examples/docs demonstrating key formats and COSE tags.

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/utils.rs	Adds ES256/PS256 compute + verify helpers with PKCS#8/SPKI DER key parsing.
src/token.rs	Tags tokens as COSE_Mac0 (HS256) vs COSE_Sign1 (ES256/PS256) and dispatches sign/verify accordingly.
src/tests.rs	Adds asymmetric algorithm tests (round-trips, tag bytes, wrong-key, tampering, invalid-key errors).
src/header.rs	Extends Algorithm enum with identifiers and is_mac() helper.
src/constants.rs	Adds COSE algorithm identifier constants for ES256/PS256.
src/error.rs	Introduces Error::InvalidKey for malformed DER keys.
src/lib.rs	Updates crate-level docs to mention ES256/PS256 support.
README.md	Documents the new algorithms, key formats, and COSE_Sign1 usage.
examples/asymmetric_signing.rs	Adds an example of signing/verifying with ES256/PS256.
examples/sample_es256_ps256_tokens.rs	Adds an example to mint and print sample ES256/PS256 tokens.
Cargo.toml	Adds crypto dependencies and bumps rust-version / thiserror.

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[jedisct1]

Thanks for putting this together. I reviewed the PR description, the changed files, the existing review comments, the token/header/utils implementation around the diff, and the current CI results. This is an automated review by Swival (https://swival.dev) using only open-source models, intended to help both the PR author and the maintainers.

My recommendation is not to merge this yet.

The main blocker is that the PR advertises an MSRV that the dependency graph no longer satisfies. Cargo.toml:14 sets rust-version = "1.72.0", while Cargo.toml:24-30 adds the RustCrypto stack through p256, rsa, sha2, and rand_core. With Rust/Cargo 1.72.0 on this branch, cargo check fails before compiling this crate because the resolver selects zeroize v1.9.0, pulled through the new crypto dependencies, and that crate's manifest uses the 2024 edition, which Cargo 1.72 cannot parse.

I also verified that the branch does build and test successfully on the latest local toolchain, and cargo fmt --check plus cargo clippy --all-targets -- -D warnings pass there. That does not cover the compatibility contract in Cargo.toml, though. Since this is a library crate and Cargo.lock is not committed, downstream users building with the declared Rust version can hit the same dependency resolution failure. That makes the MSRV bump inaccurate and creates a real compatibility regression for users who rely on the crate metadata.

Before this merges, the PR should either constrain the new dependency graph so it actually builds with Rust 1.72, or raise rust-version to the real minimum supported compiler and make that intentional in the PR. If the project wants to keep the lower MSRV, a direct dependency constraint or a different crypto dependency choice may be needed, because simply having CI pass on latest stable does not prove the published crate is usable with its declared minimum.

I would keep the PR open because the core ES256 and PS256 direction appears plausible and the new tests exercise the main signing and verification paths. The compatibility issue above needs to be fixed first, and the existing comments about the embedded demo private keys in the examples are also worth addressing before merge, even though I see those as secondary to the MSRV regression.

[zshenker]

Thanks for the thorough reviews. Pushed db41b80 addressing all the feedback.

MSRV (the blocker)

You're right that the declared 1.72 is inaccurate. I dug into the resolution and the true floor is actually higher than 1.85: because Cargo.lock isn't committed, downstream users re-resolve to the latest deps, which pulls minicbor-derive 0.19.4. That crate uses let-chains (stabilized in 1.88), and zeroize 1.9 is edition-2024 (needs 1.85). 1.85 and 1.86 both fail to compile minicbor-derive; 1.88 builds and tests cleanly.

• Set rust-version = "1.88.0" — the real minimum, verified by building/testing with cargo +1.88.0 against a freshly re-resolved (latest) lockfile.
• Added a pinned MSRV CI job (dtolnay/rust-toolchain@1.88.0) that builds + tests, so the declared MSRV can't silently rot again. The existing CI only ran latest stable, which is exactly why this slipped through.

Worth noting: this MSRV reality predates the PR — main already depends on minicbor 2.x, so its declared 1.60 was already stale. This PR just makes the metadata honest.

Demo keys in examples

Added prominent ⚠️ DEMO KEYS — DO NOT USE IN PRODUCTION warnings next to the private-key constants in both examples/asymmetric_signing.rs and examples/sample_es256_ps256_tokens.rs, noting they're publicly known. Kept the keys embedded so the examples remain runnable with no setup.

Test fixes (Copilot)

• test_es256_wrong_key_fails: the old bogus key was malformed base64, so the wrong-key assertion never ran behind if let Ok(...). Replaced it with a real, non-matching P-256 SPKI key decoded via .expect(...), so the rejection path is always exercised.
• test_es256_tampered_payload_fails: switched len() - 70 to checked_sub(70).expect(...) to avoid panic-on-underflow.

Verification

• cargo fmt --check, cargo clippy --all-targets — clean
• cargo test — 42 lib tests + 73 doctests pass
• cargo +1.88.0 build && test with a re-resolved latest lockfile — passes; cargo +1.85.0 is correctly rejected
• cargo run --example asymmetric_signing — ES256 and PS256 sign/verify

[Copilot]

Pull request overview

Copilot reviewed 12 out of 12 changed files in this pull request and generated 2 comments.

[zshenker]

Good catch — the PR description was stale. The code is correct at 1.88.0; I've updated the description to match and explain why (uncommitted Cargo.lock → downstream re-resolves to minicbor-derive 0.19.4 which needs let-chains/1.88, and zeroize 1.9 needs edition 2024/1.85). The pinned msrv CI job already builds+tests on 1.88.0, so all three (Cargo.toml, CI, description) are now consistent.

[zshenker]

Updated the comment to make the untagged case explicit (7b85884): the None branch (no algorithm in the header) intentionally emits the bare COSE array untagged, and from_bytes accepts both tagged and untagged input. The comment now documents that.

[Copilot]

Pull request overview

Copilot reviewed 12 out of 12 changed files in this pull request and generated 1 comment.

[zshenker]

Fixed in 158bb17. The test now mutates a byte inside the custom string claim contents (XOR with 0x01 to stay in ASCII so the text stays valid UTF-8 and the CBOR structure is intact). It then asserts from_bytes succeeds before asserting verify fails, so the test can no longer pass without exercising signature rejection.

[jedisct1]

Thanks for the follow-up work here. I reviewed the updated PR description, the current diff, the previous discussion and review comments, the affected token/header/utils paths, the new examples and docs, and the current CI state. I also ran the branch locally with rustup run stable cargo test, rustup run stable cargo fmt --check, rustup run stable cargo clippy --all-targets -- -D warnings, and both new examples. Those all passed for me. This is an automated review by Swival (https://swival.dev) using only open-source models, intended to help both the author and the maintainers.

My recommendation is not to merge this quite yet, although I do think the ES256 and PS256 implementation direction now looks broadly reasonable and worth continuing.

The remaining blocker for me is the public compatibility story. This PR changes two public enums that are currently exhaustive: Algorithm in src/header.rs:43-50 gains Es256 and Ps256, and Error in src/error.rs:7-31 gains InvalidKey. Downstream users can legally have exhaustive matches over both of those enums today, and those users will fail to compile once these variants are added. The PR also intentionally raises rust-version from 1.60 to 1.88 in Cargo.toml:14, which is another compatibility break even though the description now explains why that is necessary.

That may all be acceptable for this feature, but it should not be accidental. Cargo.toml:3 still says version = "0.2.7", so if this is released as another 0.2.x patch, users with normal compatible-version requirements on 0.2 can pick it up and get source breaks. Before merge, I think the PR should either make this a clearly intentional 0.3.0-style breaking release, or the maintainers should separate the code change from release versioning in a way that leaves an explicit note that this cannot be shipped as a 0.2.x patch. Another option would be to first make these public enums non-exhaustive in a deliberate breaking release, but as the code stands, the compatibility impact needs to be handled directly.

There is also one smaller unresolved test-quality issue from the current review thread. test_es256_tampered_payload_fails in src/tests.rs:1595-1613 still accepts Token::from_bytes returning Err and then performs no assertion. That means the test can pass without proving that a structurally valid token with a tampered payload is rejected by ES256 verification. This is not the main reason I am requesting changes, but for a crypto behavior change it would be much better to mutate a known payload byte, assert that decoding succeeds, and then assert that verification fails.

I would keep the PR open rather than close it. The core wiring in src/token.rs now follows the existing COSE input pattern, the new helpers in src/utils.rs use the expected key formats and signature encodings, CI is green including the new MSRV job, and the earlier demo-key and stale-description feedback has been addressed. Once the compatibility/release handling is made explicit, and ideally the tamper test is tightened, I would be comfortable re-reviewing this for merge.

[zshenker]

Thanks @jedisct1 — addressed both points in 158bb17:

Compatibility / release story. You're right that this can't ship as a 0.2.x patch. I've made the breaking release explicit:

• Bumped version to 0.3.0 in Cargo.toml (and the README dependency requirement to "0.3").
• Additionally marked both public enums #[non_exhaustive] — Algorithm in src/header.rs and Error in src/error.rs — so that future variant additions (more algorithms, more error cases) are no longer source-breaking. Downstream exhaustive matches over these enums will now require a wildcard arm, which is the one-time break captured by the 0.3.0 bump. The MSRV 1.88 requirement is likewise covered by this minor-version breaking release.

Tamper test. Tightened test_es256_tampered_payload_fails: it now flips a byte inside the custom string claim's contents (keeping CBOR/UTF-8 valid), asserts that Token::from_bytes still succeeds, and then asserts that verify fails — so it can no longer pass without proving ES256 rejects a structurally valid but tampered payload.

cargo fmt --check, cargo clippy --all-targets -- -D warnings, and the full test suite all pass locally.

[jedisct1]

Thanks for the follow-up changes here. I reviewed the current PR description, the latest diff at 158bb17, the earlier discussion and review comments, and the touched implementation paths in src/header.rs, src/token.rs, src/utils.rs, src/error.rs, the new tests, examples, README, and CI workflow. This is an automated review by Swival (https://swival.dev) using only open-source models, intended to help both the pull request author and the maintainers.

My recommendation is to merge this PR now.

The previous compatibility blocker looks resolved in the current revision. Cargo.toml now makes the breaking release explicit with version = "0.3.0" and rust-version = "1.88.0", and the new MSRV job in .github/workflows/ci.yml checks that compiler version directly. The public enum changes in src/header.rs and src/error.rs are still source-breaking for users with exhaustive matches, but that is now treated as part of the 0.3.0 compatibility boundary, and marking both enums #[non_exhaustive] prevents the same kind of break from recurring for future algorithm or error additions.

The ES256 and PS256 wiring fits the existing token structure. TokenBuilder::sign now signs HMAC tokens with the existing COSE_Mac0 input and asymmetric tokens with COSE_Sign1 input, while Token::verify dispatches by the protected alg header. The new helpers in src/utils.rs use the expected key formats documented in the README and examples: PKCS#8 DER private keys for signing and SPKI DER public keys for verification. The examples also now clearly warn that the embedded private keys are demo material and publicly known.

I also rechecked the test coverage that came up in the earlier comments. The wrong-key ES256 test now uses a valid non-matching P-256 public key, and test_es256_tampered_payload_fails now mutates a byte inside a decodable payload and asserts that decoding still succeeds before verification fails. That means the tamper test is actually exercising signature rejection rather than passing because the token became malformed.

For validation, GitHub Actions shows both build and msrv passing on the latest head. Locally, I ran rustup run stable cargo fmt --check, rustup run stable cargo test, rustup run stable cargo clippy --all-targets -- -D warnings, rustup run stable cargo run --example asymmetric_signing, and rustup run stable cargo run --example sample_es256_ps256_tokens. All of those passed.

I do not see any remaining correctness, regression, or maintainability issue that should block this. The change is substantial and intentionally breaking, but the current PR makes that explicit and backs the new behavior with focused tests and runnable examples.

[Copilot]

Pull request overview

Copilot reviewed 13 out of 13 changed files in this pull request and generated 2 comments.

[zshenker]

Fixed in 079409e. The root cause was exactly this: from_bytes() decoded the protected header into a map and discarded the original bytes, then sign1_input() re-encoded that map — so a token whose producer used a different-but-valid CBOR encoding (map ordering, integer width) reconstructed a different signed input and failed.

The fix preserves the original bytes: Token now carries original_protected_bytes, populated by from_bytes(), and a new protected_bytes() helper returns those exact bytes when present (mirroring the existing original_payload_bytes handling). sign1_input(), mac0_input(), and to_bytes() all use it, so verification and re-encoding are byte-faithful to the producer's encoding per RFC 9052 §4.4 ("re-encoded byte string is identical to the decoded byte string").

This applies to all algorithms, not just ES256 — HMAC had the same latent bug, previously masked by its Sign1/Mac0 fallback. New regression test test_es256_verifies_noncanonical_protected_header mints an external token encoding alg=-7 in the non-canonical 2-byte form (0x38 0x06) and asserts both verify and a byte-faithful to_bytes() round-trip; it fails without the fix.

[zshenker]

Fixed in 079409e, same fix as the ES256 thread above. PS256 verification goes through the shared sign1_input(), which now uses the preserved original protected-header bytes (original_protected_bytes on Token, via protected_bytes()) rather than re-encoding the decoded header map — so verification reproduces the exact signed protected bstr per RFC 9052 §4.4.

[Copilot]

Pull request overview

Copilot reviewed 13 out of 13 changed files in this pull request and generated 1 comment.

[zshenker]

Fixed in 978ea48. to_bytes() now encodes the payload via get_payload_bytes() — the same preserved-original-bytes helper that signing and verification already use — so a decoded token's payload bstr is byte-faithful to the producer's encoding rather than re-encoded from the claims map. This closes the asymmetric gap where the protected header was preserved but the payload wasn't.

Added a round-trip regression test (test_decoded_token_reencodes_without_breaking_signature) using the external COSE_Mac0 fixture: decode → verify → to_bytes() → re-parse → verify. It fails without this fix (the re-encoded token no longer verifies) and passes with it.

[jedisct1]

Thanks for the latest updates. I reviewed the new commits on top of the earlier approval, the current PR discussion, the unresolved review comments, the token/header/utils changes, the examples and docs, and the current CI state. This is an automated review by Swival (https://swival.dev) using only open-source models, intended to help both the author and the maintainers.

I would not merge this revision yet. The ES256 and PS256 additions still look broadly worthwhile, and the new protected-header preservation fix is in the right direction, but it leaves the same class of round-trip bug on the payload side.

In src/token.rs:82, Token::to_bytes() still rebuilds the payload from self.claims.to_map() and encode_map(...). For tokens created by this crate that is fine, but for tokens decoded from the wire, from_bytes() already preserves original_payload_bytes, and verification uses those bytes through get_payload_bytes() at src/token.rs:693. That means a decoded token can verify correctly, then to_bytes() can emit a different payload bstr while keeping the existing signature or MAC. The re-encoded token is then invalid.

This is not just theoretical. I reproduced it with the existing external MAC0 fixture from test_mac0_token_verification_with_original_bytes in src/tests.rs:551. The decoded token verifies with testSecret, but after let reencoded = token.to_bytes()?, parsing reencoded and verifying it fails with SignatureVerification. That is a regression in a public serialization path, and it is especially risky because the new protected-header fix now makes to_bytes() appear byte-faithful for one signed field while still changing the other signed field.

The fix should be small and consistent with the rest of the PR: Token::to_bytes() should use the same preserved payload bytes that signing and verification use, probably by calling self.get_payload_bytes()? instead of rebuilding the claims map there. Please also add a regression test that decodes an externally produced token, verifies it, serializes it again, and verifies the serialized result. The existing fixture in src/tests.rs:551 is a good candidate because it already demonstrates why preserving original payload bytes matters.

I ran rustup run stable cargo fmt -- --check, rustup run stable cargo test, rustup run stable cargo clippy --all-targets -- -D warnings, and both new examples successfully. I also ran the round-trip repro described above, and that is the reason for this request for changes.

My merge recommendation is request changes for now. I would keep the PR open because the change is valuable and the remaining blocker looks focused, but this should be fixed before merging.

[zshenker]

Follow-up: mutation safety for to_bytes() (pushed in d814b78, bdae217)

While reviewing the payload-byte preservation fix, we found a related integrity issue that goes a bit beyond the original round-trip report, and hardened it proactively.

The issue. Token.claims and Token.header are public fields. Once to_bytes() reused the cached original_payload_bytes/original_protected_bytes from a decoded token, mutating a claim after decoding and then calling to_bytes() would silently re-emit the producer's original bytes — discarding the mutation. The caller would believe they minted a token with the new claims, but the bytes on the wire carried the old ones.

The fix. get_payload_bytes() and protected_bytes() no longer blindly trust the cache. Each decodes the cached bytes and compares them by logical content against the current claims/header:

• Unchanged → reuse the producer's exact original bytes (byte-faithful round-trip — the interop guarantee from the earlier fix is fully preserved).
• Mutated → re-encode from the current state.

This makes the behavior fail-safe: a mutated decoded token now emits the mutated claims, and because the payload changed, the original MAC/signature no longer matches — so the token fails verification rather than passing with stale claims. A rejected token is safe; a token that verifies with the wrong claims would not be.

Tests.

• test_mutated_claims_are_reflected_in_to_bytes — mutates iss on a decoded token, asserts the re-encoded token carries the mutation and fails verification with the original key.
• test_unmutated_decoded_token_reuses_original_bytes — asserts a byte-identical re-encode for the unmutated case (guards the non-canonical-encoding interop guarantee).

New public API. Token::to_signed_payload_bytes() exposes the exact payload bstr covered by the signature/MAC (the same bytes signing and verification use). The PS256 randomization test uses it to assert two tokens with identical claims produce identical signed payloads, so any signature difference is attributable solely to the PSS salt.

cargo fmt --check, cargo clippy --all-targets -- -D warnings, and the full suite (47 lib tests + 73 doctests) all pass.

[Copilot]

Pull request overview

Copilot reviewed 13 out of 13 changed files in this pull request and generated 1 comment.

[Copilot]

Pull request overview

Copilot reviewed 13 out of 13 changed files in this pull request and generated 2 comments.

[zshenker]

@copilot The code and all the tests are compiling successfully so have you actually validated this assumption?

[Copilot]

Confirmed — the code compiles correctly as-is. protected_bytes() uses Some(ref original), which borrows rather than moves (equivalent to matching on &self.original_protected_bytes). get_payload_bytes() matches on the tuple (&self.original_payload_bytes, &self.baseline_payload_projection), making baseline: &ClaimsMap, so *baseline in the guard is a deref for PartialEq — not a move. cargo build succeeds without errors.

[Copilot]

Pull request overview

Copilot reviewed 14 out of 14 changed files in this pull request and generated 2 comments.

[zshenker]

Valid, and tracked. This is the algorithm-confusion item already recorded in FOLLOWUPS.md under Security, and it was consciously deferred for this PR (the maintainer reviewed and approved with it deferred).

To be clear about the reasoning — and I've updated the FOLLOWUPS note to say so explicitly: this is not blocked on a future breaking release. 0.3.0 is already breaking, and a minimal mitigation (an additive verify_with_algorithm(key, expected) entrypoint that rejects MAC-vs-asymmetric mismatches) would not break existing callers. What's deferred is the design decision about where the expectation should live — a dedicated entrypoint vs. threading it through VerificationOptions, plus the default-strictness question — which deserves its own focused PR rather than riding along here. The exposure and fix direction are documented so it can be picked up independently and soon.

[zshenker]

Fixed in af2542d. to_bytes() now returns Error::InvalidFormat("Missing algorithm in protected header") when the header carries no algorithm, instead of emitting a bare untagged COSE array. This is symmetric with verify() and sign(), which already require an algorithm, so the caller bug (only reachable via a hand-built Token::new) now surfaces immediately rather than producing an unverifiable token. Added test_to_bytes_without_algorithm_errors and a CHANGELOG note.

[Copilot]

Pull request overview

Copilot reviewed 14 out of 14 changed files in this pull request and generated 2 comments.

[jedisct1]

Thanks for the continued work here. I reviewed the current PR #5 head at af2542d, the changed implementation and tests around src/token.rs, src/header.rs, src/utils.rs, the README and examples, the latest discussion including the new Copilot comments, the CI state, and the stacked PR #6 context. This is an automated review by Swival (https://swival.dev) using only open-source models, intended to help both the pull request author and the maintainers.

My merge recommendation is request changes. I would not merge PR #5 as it stands.

The earlier serialization and compatibility issues look much better now. The protected-header and payload byte preservation paths in src/token.rs are covered by regression tests, the lossy-claim round-trip tests address the availability regression we discussed, ES256 low-S handling and the PS256 RSA key-size floor are both sensible, and the new to_bytes() error for a missing algorithm fixes the untagged-token corner case. I also verified the current head locally with rustup run stable cargo fmt -- --check, rustup run stable cargo test, rustup run stable cargo clippy --all-targets -- -D warnings, and both new examples. GitHub Actions is green for build and msrv.

The remaining blocker is a security issue in the public verification API. Token::verify(&[u8]) at src/token.rs:201 still chooses the cryptographic primitive from the token's own protected alg header, then applies the caller's one byte slice as whatever key type that header names. In the HMAC arm at src/token.rs:207-218, those bytes are treated as a shared MAC secret. In the ES256 and PS256 arms at src/token.rs:225-231, the same API expects public key bytes.

That becomes unsafe as soon as this PR adds asymmetric verification. A relying party following the new docs can accept ES256 or PS256 tokens by calling decoded.verify(public_key), as shown in README.md:164-191, examples/asymmetric_signing.rs:92-96, and examples/sample_es256_ps256_tokens.rs:94-96. An attacker can instead create a fresh token with alg = HmacSha256 and compute the HMAC using those public key bytes as the HMAC key. Since the public key is public, the attacker has everything needed to produce a MAC that this verify() implementation will accept.

The fact that alg is in the protected header does not prevent this attack, because the attacker is not modifying an existing ES256 token. They are minting a new HMAC token whose protected header and MAC are internally consistent, while the application believed it was verifying an asymmetric token with a public key. That is a real authentication bypass for consumers who adopt the new API exactly as the PR documents it.

FOLLOWUPS.md:8-31 acknowledges this and defers it, but I do not think this is safe to leave as a follow-up after merging PR #5. This PR is the change that introduces and documents the asymmetric verify(public_key) path, and it is already framed as the breaking 0.3.0 release. Before this lands, the branch should include an API that binds the expected algorithm or key type to verification, and the README, examples, and tests should use that safe path.

I also checked PR #6, which is directly relevant here. The late Copilot comment on the algorithm-confusion issue is valid in substance, but it seems to have missed that PR #6 is already open against this branch and implements the typed VerifyingKey / verify_with_key direction. That context matters. I would treat PR #6, or an equivalent fix, as something to fold into this branch before PR #5 merges, not as an optional later hardening item.

The two newest Copilot comments on to_signed_payload_bytes() wording and the extra claims.to_map() allocation are much smaller. The documentation wording at src/token.rs:62-71 could be clarified because byte-preserving decoded tokens are not canonical across producers, and the allocation note is a reasonable performance cleanup. Neither of those changes my merge recommendation. The security issue above does.

I would keep this PR open rather than close it, because the ES256 and PS256 implementation is substantial and now has a lot of useful test coverage. But PR #5 should not merge until the algorithm-confusion fix from PR #6, or an equivalent expected-algorithm verification design, is included in the code and docs that will land together.

[zshenker]

Pushed a follow-up commit (759948d) addressing two cleanup items from review:

• Extract RSA key-size check — the duplicated minimum-modulus check is now a shared ensure_rsa_key_bits() helper, so the 2048-bit floor and its error message can't drift between compute_ps256 and verify_ps256.
• Drop test-only public method — removed Token::to_signed_payload_bytes from the public API (it was a thin wrapper added only for a test assertion) and made get_payload_bytes pub(crate) so the PSS-randomization test calls it directly, keeping it off the semver-public surface.

cargo fmt, cargo clippy --all-targets -- -D warnings, and the full test suite (55 + 73) all pass.

[zshenker]

On the algorithm-confusion blocker: we're keeping that fix as a separate, focused change in PR #6 (alg-confusion-fix), which is already stacked on this branch and implements the typed VerifyingKey / verify_with_key direction (plus README/examples/tests using the safe path). It's tracked in FOLLOWUPS.md under Security.

Intended merge order is #6 → #5, so the asymmetric verify(public_key) path never ships without the expected-algorithm binding in place. Keeping it isolated keeps each PR independently reviewable rather than expanding the scope of this one.

[zshenker]

Correction on sequencing from my earlier comment: we're now planning to merge #5 into main first, then bring the algorithm-confusion fix in as fresh work against main (re-basing #6's typed VerifyingKey / verify_with_key change onto the post-merge main).

The safety guarantee is preserved by gating the release, not the merge order: 0.3.0 will not be published to crates.io until the algorithm-confusion fix has also landed. So the documented asymmetric verify(public_key) path never reaches released users without the expected-algorithm binding in place — the exposure window is limited to unreleased code on main. The fix remains tracked in FOLLOWUPS.md under Security.

[jedisct1]

Thanks for clarifying the sequencing. This is an automated follow-up from Swival (https://swival.dev) using only open-source models, intended to help both the author and the maintainers.

I still think merging #5 to main before the algorithm-confusion fix is a mistake, even if the crates.io release is gated.

The reason is that main is still a public integration point, not just an internal staging area. After this merge, the public repository contains docs and examples that show decoded.verify(public_key) for ES256 and PS256, including README.md:164-191, examples/asymmetric_signing.rs:92-96, and examples/sample_es256_ps256_tokens.rs:94-96. The implementation in src/token.rs:187-219 still lets the token's own alg header choose whether that same byte slice is treated as a public asymmetric key or an HMAC secret. That is the authentication bypass we discussed.

Release gating prevents this from reaching normal crates.io users, but it does not protect people who consume the repository directly with a git dependency, copy the examples from main, test against unreleased docs, or accidentally publish once the version has already been bumped to 0.3.0. It also leaves main in a known-not-shippable security state, and the only guard against release is process discipline rather than code, CI, or branch protection.

Since the typed VerifyingKey / verify_with_key fix already exists in PR #6 and is the intended design, I would strongly prefer landing that fix immediately as the next change and treating it as a hard release blocker. If it cannot land promptly, I would consider reverting the asymmetric verification docs/API exposure until it can. FOLLOWUPS.md is useful for tracking, but for this particular issue it is not enough on its own because the unsafe path is now present and documented on main.

I appreciate the desire to keep the PRs focused. My concern is only about the security boundary: a branch can be review-focused, but main should not contain a documented verification path with a known authentication bypass unless there is a mechanical guarantee that it cannot be consumed or released before the fix lands.