Collisions

An ideal hash function is free of collisions. Collisions are instances in which two inputs result in the same output. Collisions weaken a hashing algorithm, as it is possible to get the expected result with the wrong input. As hashing algorithms are used in the digital signatures of root certificates, password storage, and blockchain signing, a hash function having many collisions could allow a malicious hacker to retrieve passwords from password hashes that could be used to access other accounts. A weak hashing algorithm, rife with collisions, could aid in a man-in-the-middle attack, allowing an attacker to spoof a Secure Sockets Layer (SSL) certificate perfectly.

MD5, the algorithm used in the above example, is regarded as inadequate for cryptographic hashing. Blockchains thankfully largely use more secure hash functions, such as SHA-256 and RIPEMD-160.