Cryptographically strong

Cryptographically strong

This term cryptographically strong is often used to describe an encryption algorithm, and implies, in comparison to some other algorithm (which is thus cryptographically weak), greater resistance to attack. But it can also be used to describe hashing and unique identifier and filename creation algorithms. See for example the description of the Microsoft .NET runtime library function Path.GetRandomFileName. In this usage the term means difficult to guess.

An encryption algorithm is intended to be unbreakable (in which case it is as strong as it can ever be), but might be breakable (in which case it is as weak as it can ever be) so there is not, in principle, a continuum of strength as the idiom would seem to imply: Algorithm A is stronger than Algorithm B which is stronger than Algorithm C, and so on. The situation is made more complex, and less subsumable into a single strength metric, by the fact that there are many types of cryptanalytic attack and that any given algorithm is likely to force the attacker to do more work to break it when using one attack than another.

The usual sense in which this term is (loosely) used, is in reference to a particular attack, brute force key search — especially in explanations for newcomers to the field. Indeed, with this attack (always assuming keys to have been randomly chosen), there is a continuum of resistance depending on the length of the key used. But even so there are two major problems: many algorithms allow use of different length keys at different times, and any algorithm can forego use of the entire key length possible. Thus, Blowfish and RC5 are block cipher algorithms whose design specifically allowed for several key lengths, and who cannot therefore be said to have any particular strength with respect to brute force key search. Furthermore, US export regulations restrict key length for exportable crypto products and in several cases in the '80s and '90s (eg, famously in the case of Lotus Notes' export approval) only partial keys were used, decreasing 'strength' against brute force attack for those (export) versions. More or less the same thing happened outside the US as well, as for example in the case of more than one of the crypto algorithms in the GSM cellular telephone standard.

The term is commonly used to convey that some algorithm is suitable for some task in cryptography or information security, but also resists cryptanalysis and has no, or fewer, security weaknesses. Tasks are varied, and might include:

* generating randomness
* encrypting data
* providing a method to ensure data integrity

"Cryptographically strong" would seem to mean that the described method has some kind of maturity, perhaps even approved for use against different kinds of systematic attacks in theory and/or practice. Indeed, that the method may resist those attacks long enough to protect the information carried (and what stands behind the information) for a useful length of time. But due to the complexity and subtlety of the field, neither is almost ever the case. Since such assurances are not actually available in real practice, sleight of hand in language which implies that they are will generally be misleading.

There will be always uncertainty as advances (eg, in cryptanalytic theory or merely affordable computer capacity) may reduce the effort needed to successfully use some attack method against an algorithm.

In addition, actual use of cryptographic algorithms requires their encapsulation in a cryptosystem, and doing so often introduces vulnerabilities which are not due to faults in an algorithm. For example, essentially all algorithms require random choice of keys, and any cryptosystem which does not provide such keys will be subject to attack regardless of any attack resistant qualities of the encryption algorithm(s) used.

See also

* Cryptography
* Unsolved problems in mathematics
* Computer insecurity


Wikimedia Foundation. 2010.

Игры ⚽ Поможем решить контрольную работу

Look at other dictionaries:

  • Strong cryptography — or cryptographically strong are general terms applied cryptographic systems or components that are considered highly resistant to cryptanalysis. Demonstrating the resistance of any cryptographic scheme to attack is a complex matter, requiring… …   Wikipedia

  • Cryptographically secure pseudorandom number generator — A cryptographically secure pseudo random number generator (CSPRNG) is a pseudo random number generator (PRNG) with properties that make it suitable for use in cryptography. Many aspects of cryptography require random numbers, for example: Key… …   Wikipedia

  • Strong prime — In mathematics, a strong prime is a prime number with certain special properties. The definitions of strong primes are different in cryptography and number theory. Definition in cryptography In cryptography, a prime number p is strong if the… …   Wikipedia

  • Electronic envelope — An electronic envelope or e envelope is almost like a postal Envelope in function. Where a paper Envelope privately encloses its contents like a mail message, so an Electronic envelope privately encloses its contents like an e mail message.… …   Wikipedia

  • VEST — High Level Structure of VEST General Designers Sean O Neil First published June 13, 2005 Cipher deta …   Wikipedia

  • Random self-reducibility — (RSR): A good algorithm for the average case implies a good algorithm for the worst case. RSR is the ability to solve all instances of a problem by solving a large fraction of the instances.DefinitionIf a function f evaluating any instance x can… …   Wikipedia

  • Pearson hashing — Fast Hashing of Variable Length Text Strings . Peter K. Pearson, Communications of the ACM 33(6), 677 (1990) mdash; [http://portal.acm.org/citation.cfm?id=78978 ACM full text (requires subscription)] ] is a hash function designed for fast… …   Wikipedia

  • Silvio Micali — Infobox Scientist name = Silvio Micali image width = caption = birth date = 1954 birth place = Palermo, Italy death date = death place = residence = citizenship = United States nationality = ethnicity = field = Computer Science, Cryptography work …   Wikipedia

  • CCM mode — (Counter with CBC MAC) is a mode of operation for cryptographic block ciphers. It is an authenticated encryption algorithm designed to provide both authentication and privacy. CCM mode is only defined for block ciphers with a block length of 128… …   Wikipedia

  • Pseudorandom generator theorem — In computational complexity a distribution is considered pseudorandom if no efficient computation can distinguish it from the true uniform distribution by a non negligible advantage. Formally, a family of distributions Dn is pseudorandom if for… …   Wikipedia

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”