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In cryptography, collision resistance is a property of cryptographic hash functions: a hash function H is collision-resistant if it is hard to find two inputs that hash to the same output; that is, two inputs a and b where a ≠ b but H(a) = H(b).
John Smith and Sandra Dee share the same hash value of 02, causing a hash collision. In computer science, a hash collision or hash clash [1] is when two distinct pieces of data in a hash table share the same hash value. The hash value in this case is derived from a hash function which takes a data input and returns a fixed length of bits. [2]
The compression function can either be specially designed for hashing or be built from a block cipher. A hash function built with the Merkle–Damgård construction is as resistant to collisions as is its compression function; any collision for the full hash function can be traced back to a collision in the compression function.
In cryptography, the Merkle–Damgård construction or Merkle–Damgård hash function is a method of building collision-resistant cryptographic hash functions from collision-resistant one-way compression functions. [1]: 145 This construction was used in the design of many popular hash algorithms such as MD5, SHA-1, and SHA-2.
Generally, the basic security of cryptographic hash functions can be seen from different angles: pre-image resistance, second pre-image resistance, collision resistance, and pseudo-randomness. Pre-image resistance: given a hash h, it should be hard to find any message m such that h = hash(m). This concept is related to that of the one-way function.
keyed hash function (prefix-MAC) BLAKE3: 256 bits keyed hash function (supplied IV) HMAC: KMAC: arbitrary based on Keccak MD6: 512 bits Merkle tree NLFSR: One-key MAC (OMAC; CMAC) PMAC (cryptography) Poly1305-AES: 128 bits nonce-based SipHash: 32, 64 or 128 bits non-collision-resistant PRF: HighwayHash [16] 64, 128 or 256 bits non-collision ...
Second-preimage resistance implies preimage resistance only if the size of the hash function's inputs can be substantially (e.g., factor 2) larger than the size of the hash function's outputs. [1] Conversely, a second-preimage attack implies a collision attack (trivially, since, in addition to x ′ , x is already known right from the start).
An extension of the collision attack is the chosen-prefix collision attack, which is specific to Merkle–Damgård hash functions.In this case, the attacker can choose two arbitrarily different documents, and then append different calculated values that result in the whole documents having an equal hash value.