SHA2(3)

NAME
SHA256Init, SHA256Update, SHA256Pad, SHA256Final, SHA256Transform, SHA256End, SHA256File, SHA256FileChunk, SHA256Data - calculate the NIST Secure Hash Standard (version 2)

LIBRARY
libmd

SYNOPSIS
#include <sys/types.h>
#include <sha2.h>
void
SHA256Init(SHA2_CTX *context);
void
SHA256Update(SHA2_CTX *context, const uint8_t *data, size_t len);
void
SHA256Pad(SHA2_CTX *context);
void
SHA256Final(uint8_t digest[SHA256_DIGEST_LENGTH], SHA2_CTX *context);
void
SHA256Transform(uint32_t state[8], const uint8_t buffer[SHA256_BLOCK_LENGTH]);
char *
SHA256End(SHA2_CTX *context, char *buf);
char *
SHA256File(const char *filename, char *buf);
char *
SHA256FileChunk(const char *filename, char *buf, off_t offset, off_t length);
char *
SHA256Data(uint8_t *data, size_t len, char *buf);
void
SHA384Init(SHA2_CTX *context);
void
SHA384Update(SHA2_CTX *context, const uint8_t *data, size_t len);
void
SHA384Pad(SHA2_CTX *context);
void
SHA384Final(uint8_t digest[SHA384_DIGEST_LENGTH], SHA2_CTX *context);
void
SHA384Transform(uint64_t state[8], const uint8_t buffer[SHA384_BLOCK_LENGTH]);
char *
SHA384End(SHA2_CTX *context, char *buf);
char *
SHA384File(char *filename, char *buf);
char *
SHA384FileChunk(char *filename, char *buf, off_t offset, off_t length);
char *
SHA384Data(uint8_t *data, size_t len, char *buf);
void
SHA512Init(SHA2_CTX *context);
void
SHA512Update(SHA2_CTX *context, const uint8_t *data, size_t len);
void
SHA512Pad(SHA2_CTX *context);
void
SHA512Final(uint8_t digest[SHA512_DIGEST_LENGTH], SHA2_CTX *context);
void
SHA512Transform(uint64_t state[8], const uint8_t buffer[SHA512_BLOCK_LENGTH]);
char *
SHA512End(SHA2_CTX *context, char *buf);
char *
SHA512File(char *filename, char *buf);
char *
SHA512FileChunk(char *filename, char *buf, off_t offset, off_t length);
char *
SHA512Data(uint8_t *data, size_t len, char *buf);

DESCRIPTION
The SHA2 functions implement the NIST Secure Hash Standard, FIPS PUB 180-2. The SHA2 functions are used to generate a condensed representation of a message called a message digest, suitable for use as a digital signature. There are three families of functions, with names corresponding to the number of bits in the resulting message digest. The SHA-256 functions are limited to processing a message of less than 2^64 bits as input. The SHA-384 and SHA-512 functions can process a message of at most 2^128 - 1 bits as input.

The SHA2 functions are considered to be more secure than the sha1(3) functions with which they share a similar interface. The 256, 384, and 512-bit versions of SHA2 share the same interface. For brevity, only the 256-bit variants are described below.

The SHA256Init() function initializes a SHA2_CTX context for use with SHA256Update() and SHA256Final(). The SHA256Update() function adds data of length len to the SHA2_CTX specified by context. SHA256Final() is called when all data has been added via SHA256Update() and stores a message digest in the digest parameter.

The SHA256Pad() function can be used to apply padding to the message digest as in SHA256Final(), but the current context can still be used with SHA256Update().

The SHA256Transform() function is used by SHA256Update() to hash 512-bit blocks and forms the core of the algorithm. Most programs should use the interface provided by SHA256Init(), SHA256Update(), and SHA256Final() instead of calling SHA256Transform() directly.

The SHA256End() function is a front end for SHA256Final() which converts the digest into an ASCII representation of the digest in hexadecimal.

The SHA256File() function calculates the digest for a file and returns the result via SHA256End(). If SHA256File() is unable to open the file, a NULL pointer is returned.

SHA256FileChunk() behaves like SHA256File() 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 SHA256Data() function calculates the digest of an arbitrary string and returns the result via SHA256End().

For each of the SHA256End(), SHA256File(), SHA256FileChunk(), and SHA256Data() functions the buf parameter should either be a string large enough to hold the resulting digest (e.g. SHA256_DIGEST_STRING_LENGTH, SHA384_DIGEST_STRING_LENGTH, or SHA512_DIGEST_STRING_LENGTH, depending on the function being used) 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 SHA-256 digest for the string "abc", which is “0xba7816bf8f01cfea414140de5dae2223b00361a396177a9cb410ff61f20015ad”.

    SHA2_CTX ctx;
    uint8_t results[SHA256_DIGEST_LENGTH];
    char *buf;
    int n;

    buf = "abc";
    n = strlen(buf);
    SHA256Init(&ctx);
    SHA256Update(&ctx, (uint8_t *)buf, n);
    SHA256Final(results, &ctx);

    /* Print the digest as one long hex value */
    printf("0x");
    for (n = 0; n < SHA256_DIGEST_LENGTH; n++)
    	printf("%02x", results[n]);
    putchar('\n');

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

    uint8_t output[SHA256_DIGEST_STRING_LENGTH];
    char *buf = "abc";

    printf("0x%s\n", SHA256Data(buf, strlen(buf), output));

SEE ALSO
cksum(1), md4(3), md5(3), rmd160(3), sha1(3) Secure Hash Standard, FIPS PUB 180-2.

HISTORY
The SHA2 functions appeared in OpenBSD 3.4.

AUTHORS
This implementation of the SHA functions was written by Aaron D. Gifford.

The SHA256End(), SHA256File(), SHA256FileChunk(), and SHA256Data() helper functions are derived from code written by Poul-Henning Kamp.

CAVEATS
This implementation of the Secure Hash Standard 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 (i.e. an array of 32-bit integers) it will be necessary to perform byte swapping on little endian machines such as the i386, alpha, and vax.