All of the following are valid cryptographic hash functions EXCEPT:

RIPEMD (RACE Integrity Primitives Evaluation Message Digest) is a legitimate cryptographic hash function producing fixed-length digests, developed as part of the EU RIPE project.

RC4 (Rivest Cipher 4) is a symmetric stream cipher used for encryption, meaning it produces variable-length encrypted output and is reversible with a key - properties fundamentally incompatible with a hash function, which must be one-way and produce a fixed-length digest.

SHA-512 is a widely used cryptographic hash function from NIST's Secure Hash Algorithm family, producing a 512-bit fixed-length digest.

MD4 is an early cryptographic hash function developed by Ron Rivest that produces a 128-bit digest, making it a hash function even though it is now considered cryptographically broken.