HLEN

In cryptography, hLen denotes the length, in bytes, of the output produced by a given hash function. It is a parameter used by several PKCS standards and RFCs to describe padding, masking, and signature schemes that depend on a hash digest. For a hash function H, hLen equals the number of bytes in H’s digest.

The value of hLen depends on the chosen hash function. For example, SHA-1 yields hLen of 20 bytes, SHA-256 yields 32 bytes, SHA-384 yields 48 bytes, and SHA-512 yields 64 bytes.

In schemes such as OAEP (Optimal Asymmetric Encryption Padding) and PSS (Probabilistic Signature Scheme) defined in PKCS #1, hLen is used to compute padding lengths and to drive the mask generation function MGF1. MGF1 uses a hash function with output length hLen, and the generated mask length is specified as a multiple of hLen.

Notes: The choice of hash function—and thus hLen—affects the maximum message size, padding overhead, and security properties. Implementers should ensure the selected hash’s hLen satisfies protocol requirements and aligns with the desired security level. The term appears in standards as an explicit parameter, but in other contexts the digest length is embedded in the hash’s name.