Which hashing algorithm produces a 128-bit output and is sometimes used to represent passwords, though it is not an authentication code?

MD5, or Message Digest Algorithm 5, is a widely recognized hashing algorithm known for producing a 128-bit output. It takes an input (or message) and generates a fixed-size string of characters, which is typically presented as a hexadecimal number. This characteristic makes MD5 notable for various applications, including representing passwords in hashed form.

Despite its usefulness, MD5 is not considered a secure way to handle passwords due to vulnerabilities that allow for collision attacks, where two different inputs can produce the same hash output. This undermines its reliability as an authentication mechanism. While it can create hashes quickly, its susceptibility to such attacks and the evolution of hashing technologies have led to the recommendation of stronger algorithms for password storage, such as bcrypt or Argon2.

Other options do not match the specified attributes. SHA-256, for example, produces a 256-bit output and is part of the SHA-2 family of cryptographic hash functions. Poly1305 is not a hashing algorithm but rather a message authentication code that is used to ensure data integrity and authenticity. HMAC is also a method that utilizes hashes to provide data integrity and authentication but does not specifically produce a 128-bit output on its own, as it depends on the underlying hash function used