SHA1INIT(3)

NAME
SHA1Init, SHA1Update, SHA1Pad, SHA1Final, SHA1Transform, SHA1End, SHA1File, SHA1FileChunk, SHA1Data - calculate the NIST Secure Hash Algorithm

SYNOPSIS
#include <sys/types.h>
#include <sha1.h>
void
SHA1Init(SHA1_CTX *context);
void
SHA1Update(SHA1_CTX *context, const u_int8_t *data, size_t len);
void
SHA1Pad(SHA1_CTX *context);
void
SHA1Final(u_int8_t digest[SHA1_DIGEST_LENGTH], SHA1_CTX *context);
void
SHA1Transform(u_int32_t state[5], const u_int8_t buffer[SHA1_BLOCK_LENGTH]);
char *
SHA1End(SHA1_CTX *context, char *buf);
char *
SHA1File(const char *filename, char *buf);
char *
SHA1FileChunk(const char *filename, char *buf, off_t offset, off_t length);
char *
SHA1Data(const u_int8_t *data, size_t len, char *buf);

DESCRIPTION
The SHA1 functions implement the NIST Secure Hash Algorithm (SHA-1), FIPS PUB 180-1. SHA-1 is used to generate a condensed representation of a message called a message digest. The algorithm takes a message less than 2^64 bits as input and produces a 160-bit digest suitable for use as a digital signature.

SHA-1 has been broken; it should only be used where necessary for backward compatibility. The attack on SHA-1 is in the nature of finding “collisions” — that is, multiple inputs which hash to the same value. It is still unlikely for an attacker to be able to determine the exact original input given a hash value.

The SHA1Init() function initializes a SHA1_CTX context for use with SHA1Update(), and SHA1Final(). The SHA1Update() function adds data of length len to the SHA1_CTX specified by context. SHA1Final() is called when all data has been added via SHA1Update() and stores a message digest in the digest parameter.

The SHA1Pad() function can be used to apply padding to the message digest as in SHA1Final(), but the current context can still be used with SHA1Update().

The SHA1Transform() function is used by SHA1Update() to hash 512-bit blocks and forms the core of the algorithm. Most programs should use the interface provided by SHA1Init(), SHA1Update() and SHA1Final() instead of calling SHA1Transform() directly.

The SHA1End() function is a front end for SHA1Final() which converts the digest into an ASCII representation of the 160 bit digest in hexadecimal.

The SHA1File() function calculates the digest for a file and returns the result via SHA1End(). If SHA1File() is unable to open the file, a NULL pointer is returned.

SHA1FileChunk() behaves like SHA1File() but calculates the digest only for that portion of the file starting at offset and continuing for length bytes or until end of file is reached, whichever comes first. A zero length can be specified to read until end of file. A negative length or offset will be ignored.

The SHA1Data() function calculates the digest of an arbitrary string and returns the result via SHA1End().

For each of the SHA1End(), SHA1File(), and SHA1Data() functions the buf parameter should either be a string of at least 41 characters in size or a NULL pointer. In the latter case, space will be dynamically allocated via malloc(3) and should be freed using free(3) when it is no longer needed.

EXAMPLES
The following code fragment will calculate the digest for the string "abc" which is “0xa9993e364706816aba3e25717850c26c9cd0d89d”.

    SHA1_CTX sha;
    u_int8_t results[SHA1_DIGEST_LENGTH];
    char *buf;
    int n;
    buf = "abc";
    n = strlen(buf);
    SHA1Init(&sha);
    SHA1Update(&sha, (u_int8_t *)buf, n);
    SHA1Final(results, &sha);
    /* Print the digest as one long hex value */
    printf("0x");
    for (n = 0; n < SHA1_DIGEST_LENGTH; n++)
    	printf("%02x", results[n]);
    putchar('\n');

Alternately, the helper functions could be used in the following way:

    u_int8_t output[SHA1_DIGEST_STRING_LENGTH];
    char *buf = "abc";
    printf("0x%s\n", SHA1Data(buf, strlen(buf), output));

SEE ALSO
cksum(1), sha1(1), MD5Init(3), RMD160Init(3), SHA256Init(3)

STANDARDS
J. Burrows, FIPS PUB 180-1, The Secure Hash Standard.

D. Eastlake, P. Jones, September 2001, RFC 3174, US Secure Hash Algorithm 1 (SHA1).

HISTORY
The SHA-1 functions appeared in OpenBSD 2.0.

AUTHORS
-nosplit This implementation of SHA-1 was written by Steve Reid.

The SHA1End(), SHA1File(), SHA1FileChunk(), and SHA1Data() helper functions are derived from code written by Poul-Henning Kamp.