This is an independent contribution and is not associated with any hackathon or competition.

Summary

Runtime-deprecate calling hmac.digest() multiple times without re-initialization. Unlike Hash.prototype.digest() which throws ERR_CRYPTO_HASH_FINALIZED on second call, Hmac.prototype.digest() silently returned an empty buffer, creating a potential security footgun.

Root Cause

The Hmac.prototype.digest() method did not emit any deprecation warning when called after the HMAC instance was already finalized. Instead, it silently returned an empty Buffer. This behavior differs from Hash.prototype.digest() which throws an error, and creates a security risk where callers may inadvertently reuse the same HMAC instance thinking it produces valid output.

Fix

Added a runtime deprecation warning (DEP0208) that emits when Hmac.prototype.digest() is called after the HMAC instance has already been finalized. The existing behavior (returning empty buffer) is preserved for backward compatibility. In a future version, this will throw an error matching the behavior of Hash.prototype.digest().

Testing

Existing tests in test/parallel/test-crypto-hmac.js that call .digest() multiple times continue to pass. The deprecation warning is emitted to stderr but does not change return values, maintaining backward compatibility.

Closes #62838