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packages/security-stack/crypto/crypto-rsa/src/encoding/emsa-pss-verify.md

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+# emsa-pss-verify
+
+## Purpose
+
+Implements the EMSA-PSS verification operation defined in RFC 8017
+§9.1.2. Given the message digest, the candidate encoded message `EM`,
+the encoded-message bit length, the expected salt length, and the
+hash + MGF1 hash pair, this function returns whether `EM` is a valid
+PSS encoding of the digest.
+
+## Features
+
+- RFC 8017 §9.1.2 conformant — full structure, padding, and trailer checks
+- Returns `false` on every malformed input rather than throwing — the
+  signature scheme decides how to escalate
+- Constant-time final compare via `constantTimeEqual` from `@suaveplan/crypto-utils`
+- Validates leftmost-bit clearance (RFC 8017 §9.1.2 step 6)
+- Validates the `0xbc` trailer
+- Validates the all-zero PS region and the `0x01` separator
+
+## Usage: Basic Example
+
+```ts
+import { emsaPssVerify } from "@suaveplan/crypto-rsa/encoding/emsa-pss-verify";
+import { emsaPssEncode } from "@suaveplan/crypto-rsa/encoding/emsa-pss-encode";
+import { sha256 } from "@suaveplan/crypto-hash";
+
+const mHash = sha256(new TextEncoder().encode("hello"));
+const salt = new Uint8Array(32);
+const em = emsaPssEncode(mHash, 2047, salt, "SHA-256", "SHA-256");
+console.log(emsaPssVerify(mHash, em, 2047, 32, "SHA-256", "SHA-256")); // true
+```
+
+## Usage: Advanced Example
+
+A bit-flipped EM is rejected at one of several gates: the trailer check,
+the leftmost-bit check, the PS region check, the separator check, or
+the final salt+hash compare. All of these gates fail closed (return
+`false`) without revealing which gate caught the divergence.
+
+```ts
+import { emsaPssVerify } from "@suaveplan/crypto-rsa/encoding/emsa-pss-verify";
+import { emsaPssEncode } from "@suaveplan/crypto-rsa/encoding/emsa-pss-encode";
+import { sha256 } from "@suaveplan/crypto-hash";
+
+const mHash = sha256(new TextEncoder().encode("hello"));
+const em = emsaPssEncode(
+    mHash,
+    2047,
+    new Uint8Array(32),
+    "SHA-256",
+    "SHA-256"
+);
+const tampered = new Uint8Array(em);
+tampered[0] = tampered[0] ^ 0x01;
+console.log(emsaPssVerify(mHash, tampered, 2047, 32, "SHA-256", "SHA-256"));
+// false
+```
+
+## API Reference
+
+### `emsaPssVerify(mHash, em, emBits, saltLength, hash, mgfHash): boolean`
+
+- `mHash: Uint8Array` — `Hash(message)`; length must equal `hashOutputLength(hash)` (otherwise returns `false`)
+- `em: Uint8Array` — candidate encoded message; length must equal `ceil(emBits / 8)` (otherwise returns `false`)
+- `emBits: number` — encoded-message bit length, typically `modulusBits - 1`
+- `saltLength: number` — expected salt length, must match the value used by `emsaPssEncode`
+- `hash: RsaHashAlgorithm` — message hash; also used to digest `M'`
+- `mgfHash: RsaHashAlgorithm` — MGF1 hash for `dbMask`
+- **returns** `true` when `EM` is a valid PSS encoding of `mHash` under the triple, `false` otherwise
+
+## Implementation Notes
+
+- The function never throws on encoding-level failures — every
+  malformed input returns `false`. This matches the RFC 8017 §9.1.2
+  contract: the caller is responsible for distinguishing between
+  expected verify failures and protocol errors.
+- The final `H == H'` comparison is constant-time. The earlier
+  structural checks (trailer byte, PS region, separator byte) short-
+  circuit on first failure — that is acceptable because the public
+  inputs determining those bytes are known to the verifier; only the
+  hash output carries the secret-derived signal that timing attacks
+  would target.
+- The `leftmostBitsToClear` check (RFC 8017 §9.1.2 step 6) prevents a
+  trivial signature-malleability attack where an attacker flips bits
+  in the cleared region without being detected.
+- Returning `false` on a length mismatch (`em.length !== emLen` or
+  `mHash.length !== hLen`) preserves the same constant-effort failure
+  shape as a structurally-valid mismatch.