The SHA-1 is a hashing algorithm producing a 160-bit hash result from any data. It does not perform encryption.
In cryptography, SHA-1 is a cryptographic hash function designed by the United States National Security Agency and published by the United States NIST as a U.S. Federal Information Processing Standard.
SHA stands for “secure hash algorithm”. The four SHA algorithms are structured differently and are distinguished as SHA-0, SHA-1, SHA-2, and SHA-3. SHA-1 is very similar to SHA-0, but corrects an error in the original SHA hash specification that led to significant weaknesses. The SHA-0 algorithm was not adopted by many applications. SHA-2 on the other hand significantly differs from the SHA-1 hash function.
SHA-1 is the most widely used of the existing SHA hash functions, and is employed in several widely used applications and protocols.
In 2005, cryptanalysts found attacks on SHA-1 suggesting that the algorithm might not be secure enough for ongoing use. NIST required many applications in federal agencies to move to SHA-2 after 2010 because of the weakness. Although no successful attacks have yet been reported on SHA-2, they are algorithmically similar to SHA-1.
In 2012, following a long-running competition, NIST selected an additional algorithm, Keccak, for standardization as SHA-3
NOTE: A Cryptographic Hash Function is not the same as an Encryption Algorithm even thou both are Algorithms. An algorithm is defined as a step-by-step procedure for calculations. Hashing Algorithm do not encrypt the data. People sometimes will say they encrypted a password with SHA-1 but really they simply created a Message Digest of the password using SHA-1, putting the input through a series of steps to come out with the message digest or hash value.
A cryptographic hash function is a hash function; that is, an algorithm that takes an arbitrary block of data and returns a fixed-size bit string, the (cryptographic) hash value, such that any (accidental or intentional) change to the data will (with very high probability) change the hash value. The data to be encoded are often called the “message,” and the hash value is sometimes called the message digest or simply digest.
Encryption Algorithms are reversible but Hashing Algorithms are not meant to be reversible if the input is large enough.
The following are incorrect answers:
The Skipjack algorithm is a Type II block cipher with a block size of 64 bits and a key size of 80 bits that was developed by
NSA and formerly classified at the U.S. Department of Defense “Secret” level.
Twofish is a freely available 128-bit block cipher designed by Counterpane Systems (Bruce Schneier et al.).
DEA is a symmetric block cipher, defined as part of the U.S. Government’s Data Encryption Standard (DES). DEA uses a 64bit key, of which 56 bits are independently chosen and 8 are parity bits, and maps a 64-bit block into another 64-bit block.
Reference(s) used for this question: http://en.wikipedia.org/wiki/SHA-1 and SHIREY, Robert W., RFC2828: Internet Security Glossary, may 2000. and Counterpane Labs, at http://www.counterpane.com/twofish.html.