hLenaffects

hLenaffects refers to the impact of the hash length, hLen, on cryptographic constructions that rely on hash functions. In many schemes, hLen is the number of bytes produced by the chosen hash function, such as SHA-256 producing 32 bytes.

In RSA OAEP and related padding schemes, hLen plays a direct role in encoding overhead and capacity. The maximum plaintext length that can be encrypted with a k-octet RSA modulus is approximately k minus 2*hLen minus 2. Therefore, increasing hLen reduces the maximum message size that can be encrypted in a single operation, while decreasing hLen increases the allowable message size but can weaken security margins. OAEP relies on a mask generation function (MGF1) that uses the hash output, so the hash length also governs the length of masks that can be produced and the overall padding structure.

In key derivation and pseudorandom generation, such as HKDF, hLen is the length of the derived key material produced in each expansion block. To obtain a target length L, the algorithm uses ceil(L/hLen) rounds of expansion. A larger hLen allows longer keys per expansion but increases the per-round computational cost; a smaller hLen requires more rounds for the same total length.

Security and design considerations: choosing a hash function with an appropriate hLen is important for meeting security goals. A too-short hLen can erode collision resistance and preimage security, and in padding schemes it also reduces safety margins or message capacity. When selecting parameters, it is important to follow protocol specifications and align hLen with the desired security level and practical constraints, such as message size and performance.