SHA-1 v0.2 (2005-06-21)
by Michael D. Leonhard
http://tamale.net/sha1

* Introduction

This is a C++ class that implements the Secure Hash Algorithm SHA-1.  The
purpose of the algorithm is to calculate a strong hash of given bit string.
By "strong", we mean that it is very difficult to find a different bit string
that results in the same hash.  It is similarly difficult, using only the hash,
to determine the original bit string.  SHA-1 produces a 20 byte (160 bit) hash.

* Class Usage

The class is called SHA1.  The API consists of four methods:

SHA1::SHA1() is the class constructor

void SHA1::addBytes( const char* data, int num )
  Processes bytes into the hash.
  - data is a pointer to the first byte to be added. The bytes are not modified.
  - num is the number of bytes to process.

unsigned char* SHA1::getDigest()
  Completes the hashing process and returns the final hash.  The SHA1 instance
  should not be used after calling this method.
  Returns a pointer to a 20-byte hash.  Release the memory with free().

static void SHA1::hexPrinter( unsigned char* c, int l )
  Is a utility method that prints bytes to stdout in hexadecimal format.
  - c is a pointer to the first byte to be printed. The bytes are not modified.
  - l is the number of bytes to print.

* Example

The following program will print one line to stdout:
 a9 99 3e 36 47 06 81 6a ba 3e 25 71 78 50 c2 6c 9c d0 d8 9d

#include <string.h>
#include <stdlib.h>
#include "sha1.h"
int main(int argc, char *argv[])
{
	#define BYTES "abc"
	SHA1* sha1 = new SHA1();
	sha1->addBytes( BYTES, strlen( BYTES ) );
	unsigned char* digest = sha1->getDigest();
	sha1->hexPrinter( digest, 20 );
	delete sha1;
	free( digest );
}

* Why I Wrote It

I needed an SHA-1 implementation for C++ and couldn't find a free one, so I
wrote my own. I wrote the Python version first and then the C++ one.  The
source code has an open source license (the so-called "MIT License") which
allows it to be used in commercial software.

* Links

Secure Hash Standard (FIP 180-1) is the official document from the U.S.
government which defines the algorithm:
http://www.itl.nist.gov/fipspubs/fip180-1.htm

Wikipedia has a very helpful article on SHA:
http://en.wikipedia.org/wiki/SHA

The MIT License:
http://www.opensource.org/licenses/mit-license.php