fix(auth): injective signed-payload XDR encoding (A1) — lockstep with soroban-core#38

[AquiGorka]

What

Replaces the legacy per-field bucket concatenation in buildAuthPayloadHash
(src/utils/auth/build-auth-payload.ts) with the canonical, order-sensitive XDR
encoding of the condition list, byte-identical to soroban-core#38's
hash_payload:

preimage = contractId_bytes ++ ToXdr(Vec<Condition>) ++ live_until_ledger (4-byte LE)
digest   = SHA-256(preimage)

ToXdr(Vec<Condition>) is reproduced as xdr.ScVal.scvVec(conditions.map(c => c.toScVal())).toXDR()
— the same variant-tagged, length-prefixed ScVal encoding the contract consumes.
The SDK returns the preimage; the P256 signer (crypto.subtle.sign, hash: SHA-256)
applies the SHA-256, mirroring the contract's step.

Why (A1)

The old bucketed layout concatenated deposits and withdraws into fixed buckets, so
ExtDeposit(X, a) and ExtWithdraw(X, a) over the same address/amount produced a
byte-identical preimage — a signed deposit could be reinterpreted as a withdraw. The
XDR encoding tags each enum variant and preserves list order, closing that collision
and binding the exact ordering the signer reviewed. Conditions are never sorted.

Cross-check test

src/utils/auth/build-auth-payload.unit.test.ts asserts SHA-256(SDK preimage) equals
the actual hash_payload digest emitted by the soroban-core#38 contract for fixed
inputs (known-answer vectors), covering:

• single deposit, single withdraw (SDK digest == contract digest)
• deposit vs. withdraw over the same (address, amount) → different digests (the A1 fix)
• an ordered two-condition payload (SDK digest == contract digest)
• reordering conditions → different digest

Verification

• deno fmt --check, deno lint, deno check: green
• Unit suite (incl. the new cross-check): green
• Testnet integration tests are expected RED here: they deploy the bundled
  test/contracts/*.wasm, which still carry the pre-feat(contracts): refresh artifacts for soroban-core 0.4.0 (v0.11.0) #38 hash_payload. Confirmed by
  baseline — with the old builder the withdraw flow passes against those WASMs; with this
  change it fails (the deposit/Ed25519 path is unaffected). They go green once the contract
  WASMs are rebuilt from soroban-core#38. Contract side is out of scope (feat(contracts): refresh artifacts for soroban-core 0.4.0 (v0.11.0) #38).

Coordination

Mirrors soroban-core#38 (fix/b3-hash-payload-docstring @ 97922db). Do not merge —
lands lockstep with #38 once its encoding is finalized/published. Re-sync if #38 shifts.

Bumps deno.json 0.11.1 → 0.11.2.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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