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SHA1(3) Library Functions Manual SHA1(3)


SHA1Init, SHA1Update, SHA1Pad, SHA1Final, SHA1Transform, SHA1End, SHA1File, SHA1FileChunk, SHA1Datacalculate the NIST Secure Hash Algorithm


Message Digest (MD4, MD5, etc.) Support Library (libmd, -lmd)


#include <sys/types.h>
#include <sha1.h>

SHA1Init(SHA1_CTX *context);

SHA1Update(SHA1_CTX *context, const uint8_t *data, size_t len);

SHA1Pad(SHA1_CTX *context);

SHA1Final(uint8_t digest[SHA1_DIGEST_LENGTH], SHA1_CTX *context);

SHA1Transform(uint32_t state[5], const uint8_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 uint8_t *data, size_t len, char *buf);


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.

While the SHA1 functions are considered to be more secure than the md4(3) and md5(3) functions with which they share a similar interface, they are considered insecure as of 2005, and as of 2020 chosen-prefix attacks have become practical, thus these must not be used in cryptographic contexts.

The () function initializes a SHA1_CTX context for use with (), 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 () function can be used to apply padding to the message digest as in (), but the current context can still be used with SHA1Update().

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

The () 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.

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

For each of the (), 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.


The follow code fragment will calculate the digest for the string "abc" which is ``0xa9993e364706816aba3e25717850c26c9cd0d89d''.

SHA1_CTX sha;
uint8_t results[SHA1_DIGEST_LENGTH];
char *buf;
int n;

buf = "abc";
n = strlen(buf);
SHA1Update(&sha, (uint8_t *)buf, n);
SHA1Final(results, &sha);

/* Print the digest as one long hex value */
for (n = 0; n < SHA1_DIGEST_LENGTH; n++)
	printf("%02x", results[n]);

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

uint8_t output[SHA1_DIGEST_STRING_LENGTH];
char *buf = "abc";

printf("0x%s\n", SHA1Data(buf, strlen(buf), output));


cksum(1), sha1(1), md4(3), md5(3), rmd160(3), sha2(3)

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

D. Eastlake and P. Jones, US Secure Hash Algorithm 1, RFC 3174.


The SHA-1 functions appeared in OpenBSD 2.0.


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.


This implementation of SHA-1 has not been validated by NIST and as such is not in official compliance with the standard.

If a message digest is to be copied to a multi-byte type (ie: an array of five 32-bit integers) it will be necessary to perform byte swapping on little endian machines such as the i386, alpha, and vax.

February 13, 2008 Linux 5.14.21-150500.55.52-default