 Collision (computer science)

Not to be confused with wireless packet collision.
In computer science, a collision or clash is a situation that occurs when two distinct pieces of data have the same hash value, checksum, fingerprint, or cryptographic digest.^{[1]}
Collisions are unavoidable whenever members of a very large set (such as all possible person names, or all possible computer files) are mapped to a relatively short bit string. This is merely an instance of the pigeonhole principle.^{[1]}
The impact of collisions depends on the application. When hash functions and fingerprints are used to identify similar data, such as homologous DNA sequences or similar audio files, the functions are designed so as to maximize the probability of collision between distinct but similar data. Checksums, on the other hand, are designed to minimize the probability of collisions between similar inputs, without regard for collisions between very different inputs.^{[citation needed]}
In most other applications, however, collisions of any kind are equally undesirable. Any collision in a hash table increases the average cost of lookup operations. When fingerprints are used to avoid unnecessary file storage or transfer, e.g. in a proxy server or backup system, a collision may cause incorrect operation and even permanent data loss. A successful collision attack on a cryptographic hash function may compromise the security of computer and communication systems. Therefore, much effort is devoted to the design of algorithms that minimize the occurrence of collisions for various applications.
In the context of cryptographic hash functions, the inability of an adversary to compute collisions efficiently is often essential to the security of the protocol. A collisionfree hash function is a type of oneway function that formalizes this property: it is not possible for a randomized polynomialtime algorithm to locate a collision of a collisionfree hash function with nonnegligible probability. It is unknown whether such a function exists.
References
 ^ ^{a} ^{b} Jered Floyd (20080718). "What do Hash Collisions Really Mean?". http://permabit.wordpress.com/: Permabits and Petabytes. http://permabit.wordpress.com/2008/07/18/whatdohashcollisionsreallymean/. Retrieved 20110324. "For the long explanation on cryptographic hashes and hash collisions, I wrote a column a bit back for SNW Online, “What you need to know about cryptographic hashes and enterprise storage”. The short version is that deduplicating systems that use cryptographic hashes use those hashes to generate shorter “fingerprints” to uniquely identify each piece of data, and determine if that data already exists in the system. The trouble is, by a mathematical rule called the “pigeonhole principle”, you can’t uniquely map any possible files or file chunk to a shorter fingerprint. Statistically, there are multiple possible files that have the same hash."
See also
 Birthday attack
 Collision attack (cryptographic hash functions)
 Collision resolution (hash tables)
Categories: Cryptographic hash functions
 Cryptographic attacks
 Hashing
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