feat(stdlib): Crypto.affine — HMAC-SHA256, RS256, base64url for Deno-ESM

[hyperpolymath]

Summary

Adds the host-binding surface needed to express the GitHub-App webhook signature + JWT pattern in AffineScript. The motivating consumer is the Oikos eco/econ analysis bot (currently ReScript+Deno; the port to AffineScript was blocked on these primitives — see oikos PR #32 for the ReScript implementation that defines the contract).

Three families of externs in stdlib/Crypto.affine:

Extern	Purpose	Effect row
hmac_sha256_verify(secret, body, hex_sig) -> Bool	Constant-time HMAC-SHA256 verify via crypto.subtle.verify. Hex signature input matches GitHub's X-Hub-Signature-256 after sha256= is stripped. Fail-closed on malformed hex.	/ Async
rs256_sign(pkcs8_pem, payload) -> Bytes	RSASSA-PKCS1-v1_5 over SHA-256 (RS256 for JWT). PKCS#8 PEM input as produced by openssl pkcs8 -topk8 -nocrypt. PKCS#1 is rejected at the runtime shim's PEM decoder with a clear error.	/ Async
base64url_encode_string, base64url_encode_bytes, base64url_decode	JOSE-style base64url for JWT header/payload/signature assembly.	pure

Design notes

• Lowering: lib/codegen_deno.ml adds 5 entries to deno_builtins plus runtime helpers in the inlined prelude. HMAC/RS256 emit (await __as_*(...)) hooking into the existing fd_is_async async-fn detection (same shape as the issue stdlib: portable Http.fetch (body/headers/methods/response) #160 http_request lowering). Base64url emits plain expressions.
• Portability: all shims route through globalThis.crypto.subtle so they run unmodified on Deno, Node 18+, and browser ESM.
• Effect row: Async only, not a bespoke Crypto effect. Random is reserved (lib/effect.ml:139) but not yet wired, and these primitives are deterministic — introducing a fresh effect would have no consumer. Following the precedent set by Http.affine ("a bespoke effect Http; would not be visible to importing modules' effect checker"), Async is both correct and the only cross-module-sound choice.
• hmac_sha256_verify returns Bool, not Result: GitHub webhook verification has exactly two outcomes (accept / reject) and the caller treats them as boolean. Returning Result would force callers into a match arm that always discards the error type. Malformed hex / wrong length / non-hex character all fail-closed to false.
• rs256_sign PKCS#1 rejection at the boundary: PKCS#1 (-----BEGIN RSA PRIVATE KEY-----) is what GitHub ships; the user has to convert to PKCS#8. Rejecting PKCS#1 with "expected PKCS#8 PEM" at the shim is much friendlier than the opaque subtle.importKey "unsupported key" error that would otherwise reach the caller.

Test plan

• dune build — clean, only pre-existing menhir warnings (per justfile masking policy)
• dune runtest — 340 tests, 2 pre-existing failures unchanged (walker-phase2c-parity, E2E Node-CJS Codegen; neither touches Crypto or the Deno-ESM path; confirmed by stashing the diff and re-running on plain main)
• ./tools/run_codegen_deno_tests.sh — 7/7 harnesses pass including the new crypto_primitives one
• New tests/codegen-deno/crypto_primitives.{affine,harness.mjs} covers: happy-path HMAC verify, wrong-key reject, malformed-hex fail-closed, non-hex char fail-closed, RS256 sign with arg threading, PKCS#1 rejection with clear message, JWT-header b64u known fixture (eyJhbGciOiJSUzI1NiIsInR5cCI6IkpXVCJ9), >>>> -> Pj4-Pg for the +// -> -/_ rewrite + padding strip, non-aligned-length round-trip.

Downstream

Once this lands, the planned Oikos bot port (~2 weeks, scoped in oikos/bot-integration/ROADMAP.md) becomes mechanical translation. Any future GitHub/GitLab webhook receiver or OAuth2 / JWT client in AffineScript will reuse the same surface.

🤖 Generated with Claude Code

[hyperpolymath]

Closing per architectural decision (see oikos thread): the inline-JS shim approach in this PR conflicts with the Hyperpolymath RSR Standard (Idris2 ABI + Zig FFI + C ABI consumed cross-language).

A replacement will be filed once the survey of existing RSR crypto libraries is complete. The replacement will:

• Declare HMAC-SHA256 / RS256 / base64url externs in stdlib/Crypto.affine pointing at a C ABI (hpm-crypto-rsr or whichever library the survey identifies)
• Have Idris2 ABI definitions with safety proofs
• Have a Zig FFI implementation using std.crypto

Branch left in place at feat/crypto-stdlib-hmac-rs256-base64url for reference / cherry-pickable test fixtures. Will delete after the replacement lands.

[github-actions]

🔍 Hypatia Security Scan

Findings: 82 issues detected

Severity	Count
🔴 Critical	4
🟠 High	11
🟡 Medium	67

⚠️ Action Required: Critical security issues found!

View findings

[
  {
    "reason": "Action actions/checkout@v6 needs attention",
    "type": "unpinned_action",
    "file": "publish-jsr.yml",
    "action": "pin_sha",
    "rule_module": "workflow_audit",
    "severity": "medium"
  },
  {
    "reason": "Action denoland/setup-deno@v2 needs attention",
    "type": "unpinned_action",
    "file": "publish-jsr.yml",
    "action": "pin_sha",
    "rule_module": "workflow_audit",
    "severity": "medium"
  },
  {
    "reason": "Issue in affine-vscode-publish.yml",
    "type": "unknown",
    "file": "affine-vscode-publish.yml",
    "action": "flag",
    "rule_module": "workflow_audit",
    "severity": "medium"
  },
  {
    "reason": "Issue in casket-pages.yml",
    "type": "unknown",
    "file": "casket-pages.yml",
    "action": "flag",
    "rule_module": "workflow_audit",
    "severity": "medium"
  },
  {
    "reason": "Issue in casket-pages.yml",
    "type": "unknown",
    "file": "casket-pages.yml",
    "action": "flag",
    "rule_module": "workflow_audit",
    "severity": "medium"
  },
  {
    "reason": "Issue in ci.yml",
    "type": "unknown",
    "file": "ci.yml",
    "action": "flag",
    "rule_module": "workflow_audit",
    "severity": "medium"
  },
  {
    "reason": "Issue in ci.yml",
    "type": "unknown",
    "file": "ci.yml",
    "action": "flag",
    "rule_module": "workflow_audit",
    "severity": "medium"
  },
  {
    "reason": "Issue in ci.yml",
    "type": "unknown",
    "file": "ci.yml",
    "action": "flag",
    "rule_module": "workflow_audit",
    "severity": "medium"
  },
  {
    "reason": "Issue in ci.yml",
    "type": "unknown",
    "file": "ci.yml",
    "action": "flag",
    "rule_module": "workflow_audit",
    "severity": "medium"
  },
  {
    "reason": "Issue in ci.yml",
    "type": "unknown",
    "file": "ci.yml",
    "action": "flag",
    "rule_module": "workflow_audit",
    "severity": "medium"
  }
]

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