Hash，一般翻译做“散列”，也有直接音译为"哈希"的，就是把任意长度的输入（又叫做预映射， pre-image），通过散列算法，变换成固定长度的输出，该输出就是散列值。这种转换是一种压缩映射，也就是，散列值的空间通常远小于输入的空间，不同的输入可能会散列成相同的输出，而不可能从散列值来唯一的确定输入值。 密码学上的 Hash 又被称为"消息摘要(message digest)。
MD5 和 SHA1 可以说是目前应用最广泛的Hash算法，而它们都是以 MD4 为基础设计的。参考相应RFC文档可以得到MD4(RFC 1320)、MD5(RFC 1321)算法的详细描述和算法的C源代码。
下面是SHA1算法的实现：
/*
* sha1.h
*
* Copyright (C) 1998, 2009
* Paul E. Jones <paulej@packetizer.com>
* All Rights Reserved
*
*****************************************************************************
* $Id: sha1.h 12 2009-06-22 19:34:25Z paulej $
*****************************************************************************
*
* Description:
* This class implements the Secure Hashing Standard as defined
* in FIPS PUB 180-1 published April 17, 1995.
*
* Many of the variable names in the SHA1Context, especially the
* single character names, were used because those were the names
* used in the publication.
*
* Please read the file sha1.c for more information.
*
*/
#ifndef _SHA1_H_
#define _SHA1_H_
/*
* This structure will hold context information for the hashing
* operation
*/
typedef struct SHA1Context
{
unsigned Message_Digest[5]; /* Message Digest (output) */
unsigned Length_Low; /* Message length in bits */
unsigned Length_High; /* Message length in bits */
unsigned char Message_Block[64]; /* 512-bit message blocks */
int Message_Block_Index; /* Index into message block array */
int Computed; /* Is the digest computed? */
int Corrupted; /* Is the message digest corruped? */
} SHA1Context;
/*
* Function Prototypes
*/
void SHA1Reset(SHA1Context *);
int SHA1Result(SHA1Context *);
void SHA1Input( SHA1Context *,
const unsigned char *,
unsigned);
#endif
/*
* sha1.c
*
* Copyright (C) 1998, 2009
* Paul E. Jones <paulej@packetizer.com>
* All Rights Reserved
*
*****************************************************************************
* $Id: sha1.c 12 2009-06-22 19:34:25Z paulej $
*****************************************************************************
*
* Description:
* This file implements the Secure Hashing Standard as defined
* in FIPS PUB 180-1 published April 17, 1995.
*
* The Secure Hashing Standard, which uses the Secure Hashing
* Algorithm (SHA), produces a 160-bit message digest for a
* given data stream. In theory, it is highly improbable that
* two messages will produce the same message digest. Therefore,
* this algorithm can serve as a means of providing a "fingerprint"
* for a message.
*
* Portability Issues:
* SHA-1 is defined in terms of 32-bit "words". This code was
* written with the expectation that the processor has at least
* a 32-bit machine word size. If the machine word size is larger,
* the code should still function properly. One caveat to that
* is that the input functions taking characters and character
* arrays assume that only 8 bits of information are stored in each
* character.
*
* Caveats:
* SHA-1 is designed to work with messages less than 2^64 bits
* long. Although SHA-1 allows a message digest to be generated for
* messages of any number of bits less than 2^64, this
* implementation only works with messages with a length that is a
* multiple of the size of an 8-bit character.
*
*/
#include "sha1.h"
/*
* Define the circular shift macro
*/
#define SHA1CircularShift(bits,word) \
((((word) << (bits)) & 0xFFFFFFFF) | \
((word) >> (32-(bits))))
/* Function prototypes */
void SHA1ProcessMessageBlock(SHA1Context *);
void SHA1PadMessage(SHA1Context *);
/*
* SHA1Reset
*
* Description:
* This function will initialize the SHA1Context in preparation
* for computing a new message digest.
*
* Parameters:
* context: [in/out]
* The context to reset.
*
* Returns:
* Nothing.
*
* Comments:
*
*/
void SHA1Reset(SHA1Context *context)
{
context->Length_Low = 0;
context->Length_High = 0;
context->Message_Block_Index = 0;
context->Message_Digest[0] = 0x67452301;
context->Message_Digest[1] = 0xEFCDAB89;
context->Message_Digest[2] = 0x98BADCFE;
context->Message_Digest[3] = 0x10325476;
context->Message_Digest[4] = 0xC3D2E1F0;
context->Computed = 0;
context->Corrupted = 0;
}
/*
* SHA1Result
*
* Description:
* This function will return the 160-bit message digest into the
* Message_Digest array within the SHA1Context provided
*
* Parameters:
* context: [in/out]
* The context to use to calculate the SHA-1 hash.
*
* Returns:
* 1 if successful, 0 if it failed.
*
* Comments:
*
*/
int SHA1Result(SHA1Context *context)
{
if (context->Corrupted)
{
return 0;
}
if (!context->Computed)
{
SHA1PadMessage(context);
context->Computed = 1;
}
return 1;
}
/*
* SHA1Input
*
* Description:
* This function accepts an array of octets as the next portion of
* the message.
*
* Parameters:
* context: [in/out]
* The SHA-1 context to update
* message_array: [in]
* An array of characters representing the next portion of the
* message.
* length: [in]
* The length of the message in message_array
*
* Returns:
* Nothing.
*
* Comments:
*
*/
void SHA1Input( SHA1Context *context,
const unsigned char *message_array,
unsigned length)
{
if (!length)
{
return;
}
if (context->Computed || context->Corrupted)
{
context->Corrupted = 1;
return;
}
while(length-- && !context->Corrupted)
{
context->Message_Block[context->Message_Block_Index++] =
(*message_array & 0xFF);
context->Length_Low += 8;
/* Force it to 32 bits */
context->Length_Low &= 0xFFFFFFFF;
if (context->Length_Low == 0)
{
context->Length_High++;
/* Force it to 32 bits */
context->Length_High &= 0xFFFFFFFF;
if (context->Length_High == 0)
{
/* Message is too long */
context->Corrupted = 1;
}
}
if (context->Message_Block_Index == 64)
{
SHA1ProcessMessageBlock(context);
}
message_array++;
}
}
/*
* SHA1ProcessMessageBlock
*
* Description:
* This function will process the next 512 bits of the message
* stored in the Message_Block array.
*
* Parameters:
* None.
*
* Returns:
* Nothing.
*
* Comments:
* Many of the variable names in the SHAContext, especially the
* single character names, were used because those were the names
* used in the publication.
*
*
*/
void SHA1ProcessMessageBlock(SHA1Context *context)
{
const unsigned K[] = /* Constants defined in SHA-1 */
{
0x5A827999,
0x6ED9EBA1,
0x8F1BBCDC,
0xCA62C1D6
};
int t; /* Loop counter */
unsigned temp; /* Temporary word value */
unsigned W[80]; /* Word sequence */
unsigned A, B, C, D, E; /* Word buffers */
/*
* Initialize the first 16 words in the array W
*/
for(t = 0; t < 16; t++)
{
W[t] = ((unsigned) context->Message_Block[t * 4]) << 24;
W[t] |= ((unsigned) context->Message_Block[t * 4 + 1]) << 16;
W[t] |= ((unsigned) context->Message_Block[t * 4 + 2]) << 8;
W[t] |= ((unsigned) context->Message_Block[t * 4 + 3]);
}
for(t = 16; t < 80; t++)
{
W[t] = SHA1CircularShift(1,W[t-3] ^ W[t-8] ^ W[t-14] ^ W[t-16]);
}
A = context->Message_Digest[0];
B = context->Message_Digest[1];
C = context->Message_Digest[2];
D = context->Message_Digest[3];
E = context->Message_Digest[4];
for(t = 0; t < 20; t++)
{
temp = SHA1CircularShift(5,A) +
((B & C) | ((~B) & D)) + E + W[t] + K[0];
temp &= 0xFFFFFFFF;
E = D;
D = C;
C = SHA1CircularShift(30,B);
B = A;
A = temp;
}
for(t = 20; t < 40; t++)
{
temp = SHA1CircularShift(5,A) + (B ^ C ^ D) + E + W[t] + K[1];
temp &= 0xFFFFFFFF;
E = D;
D = C;
C = SHA1CircularShift(30,B);
B = A;
A = temp;
}
for(t = 40; t < 60; t++)
{
temp = SHA1CircularShift(5,A) +
((B & C) | (B & D) | (C & D)) + E + W[t] + K[2];
temp &= 0xFFFFFFFF;
E = D;
D = C;
C = SHA1CircularShift(30,B);
B = A;
A = temp;
}
for(t = 60; t < 80; t++)
{
temp = SHA1CircularShift(5,A) + (B ^ C ^ D) + E + W[t] + K[3];
temp &= 0xFFFFFFFF;
E = D;
D = C;
C = SHA1CircularShift(30,B);
B = A;
A = temp;
}
context->Message_Digest[0] =
(context->Message_Digest[0] + A) & 0xFFFFFFFF;
context->Message_Digest[1] =
(context->Message_Digest[1] + B) & 0xFFFFFFFF;
context->Message_Digest[2] =
(context->Message_Digest[2] + C) & 0xFFFFFFFF;
context->Message_Digest[3] =
(context->Message_Digest[3] + D) & 0xFFFFFFFF;
context->Message_Digest[4] =
(context->Message_Digest[4] + E) & 0xFFFFFFFF;
context->Message_Block_Index = 0;
}
/*
* SHA1PadMessage
*
* Description:
* According to the standard, the message must be padded to an even
* 512 bits. The first padding bit must be a '1'. The last 64
* bits represent the length of the original message. All bits in
* between should be 0. This function will pad the message
* according to those rules by filling the Message_Block array
* accordingly. It will also call SHA1ProcessMessageBlock()
* appropriately. When it returns, it can be assumed that the
* message digest has been computed.
*
* Parameters:
* context: [in/out]
* The context to pad
*
* Returns:
* Nothing.
*
* Comments:
*
*/
void SHA1PadMessage(SHA1Context *context)
{
/*
* Check to see if the current message block is too small to hold
* the initial padding bits and length. If so, we will pad the
* block, process it, and then continue padding into a second
* block.
*/
if (context->Message_Block_Index > 55)
{
context->Message_Block[context->Message_Block_Index++] = 0x80;
while(context->Message_Block_Index < 64)
{
context->Message_Block[context->Message_Block_Index++] = 0;
}
SHA1ProcessMessageBlock(context);
while(context->Message_Block_Index < 56)
{
context->Message_Block[context->Message_Block_Index++] = 0;
}
}
else
{
context->Message_Block[context->Message_Block_Index++] = 0x80;
while(context->Message_Block_Index < 56)
{
context->Message_Block[context->Message_Block_Index++] = 0;
}
}
/*
* Store the message length as the last 8 octets
*/
context->Message_Block[56] = (context->Length_High >> 24) & 0xFF;
context->Message_Block[57] = (context->Length_High >> 16) & 0xFF;
context->Message_Block[58] = (context->Length_High >> 8) & 0xFF;
context->Message_Block[59] = (context->Length_High) & 0xFF;
context->Message_Block[60] = (context->Length_Low >> 24) & 0xFF;
context->Message_Block[61] = (context->Length_Low >> 16) & 0xFF;
context->Message_Block[62] = (context->Length_Low >> 8) & 0xFF;
context->Message_Block[63] = (context->Length_Low) & 0xFF;
SHA1ProcessMessageBlock(context);
}
算法的使用方式：
#include <stdio.h>
#include <string.h>
#include "sha1.h"
#define TEST_STRING "abcdasfelifsljefslgfs"
int main()
{
SHA1Context sha;
SHA1Reset(&sha);
SHA1Input(&sha, (const unsigned char *) TEST_STRING, strlen(TEST_STRING));
if (!SHA1Result(&sha))
{
fprintf(stderr, "ERROR-- could not compute message digest\n");
}
else
{
printf("\t");
printf( "%08X %08X %08X %08X %08X - %s\n",
sha.Message_Digest[0],
sha.Message_Digest[1],
sha.Message_Digest[2],
sha.Message_Digest[3],
sha.Message_Digest[4]);
printf("\n");
}
return 0;
}