SipHash
SipHash is an Add-Rotate-Xor (ARX) based family of pseudorandom functions created by Jean-Philippe Aumasson and Daniel J. Bernstein in 2012, in response to a spate of "hash flooding" denial-of-service attacks in late 2011.
Although designed for use as a hash function in the computer science sense, SipHash is a fundamentally different from cryptographic hash functions like SHA in that it is only suitable as a message authentication code: a keyed hash function like HMAC. That is, SHA is designed so that it is difficult for an attacker to find two messages X and Y such that SHA(X) = SHA(Y), even though anyone may compute SHA(X). SipHash instead guarantees that, having seen Xi and SipHash(Xi, k), an attacker who does not know the key k cannot find (any information about) k or SipHash(Y, k) for any message Y ∉ {Xi} which they have not seen before.
SipHash computes 64-bit message authentication code from a variable-length message and 128-bit secret key. It was designed to be efficient even for short inputs, with performance comparable to non-cryptographic hash functions, such as CityHash, thus can be used to prevent denial-of-service attacks against hash tables ("hash flooding"), or to authenticate network packets.
An unkeyed hash function such as SHA is only collision-resistant if the entire output is used. If used to generate a small output, such as an index into a hash table of practical size, then no algorithm can prevent collisions; an attacker need only make as many attempts as there are possible outputs.
For example, suppose a network server is designed to be able to handle up to a million requests at once. It keeps track of incoming requests in a hash table with two million entries, using a hash function to map identifying information from each request to one of the two million possible table entries. An attacker who knows the hash function need only feed it arbitrary inputs; one out of two million will have a specific hash value. If the attacker now sends a few hundred requests all chosen to have the same hash value to the server, that will produce a large number of hash collisions, slowing (or possibly stopping) the server with an effect similar to a packet flood of many million requests.
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