Unique: It's not possible to generate the same hash from two different messages. This is a characteristic of a well-built, robust cryptographic hash function. It is possible that some hash functions can derive the same hash for different messages, and this is called a hash collision. Hash functions have evolved over time, primarily due to vulnerabilities and weaknesses that were found. Modern cryptographic hash functions are known for strong collision resistance. Much work has been put into the attacking of cryptographic hash functions in particular, as they are considered key building blocks of modern cryptography.