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SHA

 

Description

Secure Hash Algorithm (SHA)

 

The SHA-1 Hash encryption algorithm specifies a Secure Hash Algorithm, which can be used to generate a condensed representation of a message called a message digest. The algorithm is required for use with the Digital Signature Algorithm (DSA) as specified in the Digital Signature Standard (DSS) and whenever a secure hash algorithm is required. Both the transmitter and intended receiver of a message in computing and verifying a digital signature uses this method.

 

The same SHA-1 algorithm, but employing a variable key size, is used to create the SHA-2 family of functions. The four hash functions that comprise SHA-2 are SHA-224, SHA-256, SHA-384, and SHA-512, with the numeric portion of the name indicating the number of bits in the key. SHA-2 functions are more secure than SHA-1 although not as widely used currrently.

 

SHA-1 Hash is used for computing a condensed representation of a message or a data file. When a message of any length < 2 64 bits is input, the Hash algorithm produces a 160-bit output called a message digest. The message digest can then be input to the Digital Signature Algorithm (DSA), which generates or verifies the signature for the message. Signing the message digest rather than the message often improves the efficiency of the process because the message digest is usually much smaller in size than the message. The same hash algorithm must be used by the verifier of a digital signature as was used by the creator of the digital signature.

 

The SHA-1 Hash is called secure because it is computationally infeasible to find a message which corresponds to a given message digest, or to find two different messages which produce the same message digest. Any change to a message in transit will, with very high probability, result in a different message digest, and the signature will fail to verify. SHA-1 is a technical revision of SHA (FIPS 180). A circular left shift operation has been added to the SHA (FIPS 180). SHA-1 improves the security provided by the SHA standard. The SHA-1 is based on principles similar to those used by the MD4 message digest algorithm.

 

Features

 

  • The algorithm is used to compute a message digest for a message or data file that is provided as input.
  • The message or data file should be considered to be a bit string.
  • The length of the message is the number of bits in the message (the empty message has length 0).
  • If the number of bits in a message is a multiple of 8, for compactness we can represent the message in hex.
  • The purpose of message padding is to make the total length of a padded message a multiple of 512.
  • The purpose of message padding is to make the total length of a padded message a multiple of 512.
  • As a summary, a “1″ followed by m “0″s followed by a 64-bit integer are appended to the end of the message to produce a padded message of length 512 * n.
  • The 64-bit integer is l, the length of the original message.
  • The padded message is then processed by the SHA-1 as n 512-bit blocks.

 

 

RFCs and references

  • US Secure Hash Algorithm 1 (SHA1) - RFC 3174
  • IP Authentication using Keyed SHA1 with Interleaved Padding (IP-MAC) - RFC 2841
  • Using HMAC-SHA-256, HMAC-SHA-384, and HMAC-SHA-512 with IPsec - RFC 4868
  • SECURE HASH STANDARD
  • US Secure Hash Algorithms (SHA and HMAC-SHA) - RFC 4634

 

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