SHA256Init(3) calculate the NIST Secure Hash Standard (version 2)

Other Alias

SHA256Update, SHA256Pad, SHA256Final, SHA256Transform, SHA256End, SHA256File, SHA256FileChunk, SHA256Data


Lb libmd


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


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

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

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

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

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

Fn SHA256FileChunk behaves like Fn SHA256File but calculates the digest only for that portion of the file starting at Fa offset and continuing for Fa length bytes or until end of file is reached, whichever comes first. A zero Fa length can be specified to read until end of file. A negative Fa length or Fa offset will be ignored.

The Fn SHA256Data function calculates the digest of an arbitrary string and returns the result via Fn SHA256End .

For each of the Fn SHA256End , Fn SHA256File , Fn SHA256FileChunk , and Fn 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.


The following code fragment will calculate the SHA-256 digest for the string Qq abc , which is ``0xba7816bf8f01cfea414140de5dae2223b00361a396177a9cb410ff61f20015ad''
SHA2_CTX ctx;
uint8_t results[SHA256_DIGEST_LENGTH];
char *buf;
int n;
buf = "abc";
n = strlen(buf);
SHA256Update(&ctx, (uint8_t *)buf, n);
SHA256Final(results, &ctx);
/* Print the digest as one long hex value */
for (n = 0; n < SHA256_DIGEST_LENGTH; n++)
        printf("%02x", results[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));


The SHA2 functions appeared in Ox 3.4 .


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

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


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.