SHA2(3)

NAME
SHA256_Init, SHA256_Update, SHA256_Pad, SHA256_Final, SHA256_Transform, SHA256_End, SHA256_File, SHA256_FileChunk, SHA256_Data - calculate the NIST Secure Hash Standard (version 2)

SYNOPSIS
#include <sys/types.h>
#include <sha2.h>
void
SHA256_Init(SHA256_CTX *context);
void
SHA256_Update(SHA256_CTX *context, const uint8_t *data, size_t len);
void
SHA256_Pad(SHA256_CTX *context);
void
SHA256_Final(uint8_t digest[SHA256_DIGEST_LENGTH], SHA256_CTX *context);
void
SHA256_Transform(uint32_t state[8], const uint8_t buffer[SHA256_BLOCK_LENGTH]);
char *
SHA256_End(SHA256_CTX *context, char *buf);
char *
SHA256_File(const char *filename, char *buf);
char *
SHA256_FileChunk(const char *filename, char *buf, off_t offset, off_t length);
char *
SHA256_Data(uint8_t *data, size_t len, char *buf);
void
SHA384_Init(SHA384_CTX *context);
void
SHA384_Update(SHA384_CTX *context, const uint8_t *data, size_t len);
void
SHA384_Pad(SHA384_CTX *context);
void
SHA384_Final(uint8_t digest[SHA384_DIGEST_LENGTH], SHA384_CTX *context);
void
SHA384_Transform(uint64_t state[8], const uint8_t buffer[SHA384_BLOCK_LENGTH]);
char *
SHA384_End(SHA384_CTX *context, char *buf);
char *
SHA384_File(char *filename, char *buf);
char *
SHA384_FileChunk(char *filename, char *buf, off_t offset, off_t length);
char *
SHA384_Data(uint8_t *data, size_t len, char *buf);
void
SHA512_Init(SHA512_CTX *context);
void
SHA512_Update(SHA512_CTX *context, const uint8_t *data, size_t len);
void
SHA512_Pad(SHA512_CTX *context);
void
SHA512_Final(uint8_t digest[SHA512_DIGEST_LENGTH], SHA512_CTX *context);
void
SHA512_Transform(uint64_t state[8], const uint8_t buffer[SHA512_BLOCK_LENGTH]);
char *
SHA512_End(SHA512_CTX *context, char *buf);
char *
SHA512_File(char *filename, char *buf);
char *
SHA512_FileChunk(char *filename, char *buf, off_t offset, off_t length);
char *
SHA512_Data(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 SHA256_Init() function initializes a SHA256_CTX context for use with SHA256_Update(), and SHA256_Final(). The SHA256_Update() function adds data of length len to the SHA256_CTX specified by context. SHA256_Final() is called when all data has been added via SHA256_Update() and stores a message digest in the digest parameter.

The SHA256_Pad() function can be used to apply padding to the message digest as in SHA256_Final(), but the current context can still be used with SHA256_Update().

The SHA256_Transform() function is used by SHA256_Update() to hash 512-bit blocks and forms the core of the algorithm. Most programs should use the interface provided by SHA256_Init(), SHA256_Update(), and SHA256_Final() instead of calling SHA256_Transform() directly.

The SHA256_End() function is a front end for SHA256_Final() which converts the digest into an ASCII representation of the digest in hexadecimal.

The SHA256_File() function calculates the digest for a file and returns the result via SHA256_End(). If SHA256_File() is unable to open the file, a NULL pointer is returned.

SHA256_FileChunk() behaves like SHA256_File() 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 SHA256_Data() function calculates the digest of an arbitrary string and returns the result via SHA256_End().

For each of the SHA256_End(), SHA256_File(), SHA256_FileChunk(), and SHA256_Data() 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”.

    SHA256_CTX ctx;
    uint8_t results[SHA256_DIGEST_LENGTH];
    char *buf;
    int n;
    buf = "abc";
    n = strlen(buf);
    SHA256_Init(ctx);
    SHA256_Update(ctx, (uint8_t *)buf, n);
    SHA256_Final(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:

    SHA256_CTX ctx;
    uint8_t output[SHA256_DIGEST_STRING_LENGTH];
    char *buf = "abc";
    printf("0x%s\n", SHA256_Data(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 and NetBSD 3.0.

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

The SHA256_End(), SHA256_File(), SHA256_FileChunk(), and SHA256_Data() 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 five 32-bit integers) it will be necessary to perform byte swapping on little endian machines such as the i386, alpha, and vax.