Cryptographic test vector

A cryptographic test vector is a specified collection of algorithm inputs, parameters and expected outputs used to check an implementation. Exact byte encodings are part of the vector: a key displayed in hexadecimal represents bytes, not necessarily the displayed characters. A known-answer test compares a computed result with the supplied answer. RFC 4231 provides such vectors for several HMAC instantiations.^[ref]

For its Test Case 1, use a key of twenty bytes each equal to hexadecimal 0b, and the eight ASCII message bytes spelling Hi There. HMAC with SHA-256 must produce

b0344c61d8db38535ca8afceaf0bf12b881dc200c9833da726e9376c2e32cff7.

The output has 64 hexadecimal digits, hence 32 bytes. The key has twenty bytes; using the forty ASCII characters obtained by writing 0b twenty times supplies a different key. Adding a newline to the message supplies different message bytes too.^[ref]

A good vector identifies the complete algorithm variant. An unkeyed SHA-256 digest of that message is not the expected HMAC result, and SHA-512/256 is a different hash operation despite its equal digest length. HKDF additionally needs its salt, initial material, context and requested output length; its primary specification supplies all those fields together with the expected extraction and expansion results.^[ref]

Passing a vector establishes agreement for those inputs. It does not prove collision resistance, key secrecy or a cryptographic protocol's security. Nor does one short input exercise every implementation path. Vectors covering long keys, truncation and boundary lengths can reveal mistakes that a single example misses; RFC 4231 includes separate cases for those behaviours.^[ref]