GHASHbased

GHASHbased refers to cryptographic constructions that rely on the GHASH function as the core universal hash in authenticated encryption or message authentication schemes. GHASH operates over the 128-bit finite field GF(2^128) and is parameterized by a 128-bit key H, which is typically derived as H = E_K(0^128) from the underlying block cipher key K. Data are processed in 128-bit blocks; starting with Y0 = 0, each step computes Yi = (Yi−1 XOR Xi) multiplied by H in GF(2^128). The resulting GHASH value is then combined with an encrypted nonce to form the authentication tag in GCM-based schemes.

In practical terms, GHASHbased constructions are most familiar through AES-GCM, where GHASH provides the authenticated data and ciphertext integrity, while the encryption operation (usually CTR mode) provides confidentiality. GMAC is a related usage that employs GHASH alone to generate a MAC when encryption is not required. The GHASH input includes both additional authenticated data and ciphertext blocks, together with a final block containing the bit-lengths of A and C, ensuring the tag covers all authenticated material.

Security and implementation notes emphasize the importance of nonce management: a unique IV per key is essential, as nonce reuse breaks confidentiality and integrity guarantees in GCM. GHASH multiplication over GF(2^128) can be accelerated with hardware instructions such as carry-less multiplication, and implementations often optimize data alignment and side-channel resistance. While GHASHbased schemes are widely adopted due to their performance and strong security properties when used correctly, they rely on correct nonce usage and careful implementation to prevent common pitfalls.