## One Way Hash functions — OpenSSL

Hash or Fingerprint generation functions are always an interesting Chapter in Cryptography as they are the basics of the most cryptographic protocols.
Definition for a Cryptographic Hash Functions
=============================
Let us consider that M is the message and h is it’s hash value after applying the hash function H. Or, it can be stated mathematically as: H (M) :=h. Where H will also satisfy the following characteristics.
1. Given M, it is easy to compute h
2. Given h, it is hard to compute M such that H(M) = h
3. Given M, it is hard to find another messag, M1 , such that H(M) = H(M1)

SHA-*, MD* and RIPEMD-* are the most popular Hash functions. OpenSSL provides both generic APIs to these Hash functions also provides direct APIs. Accessing through generic API (the EVP) is preferred.

Now let us first use the Hash APIs from OpenSSL and generate fingerprint or calculate hash value. The alorithm, that we will use, is MD5. MD5 (MD stands for Message Digest ) is One way hash algorithm from Ron Rivest. There are test vectors in MD5 RFC, which we can use to calculate (or validate) the hash values. The following figure shows the process of calculation MD5 hash using OpenSSL :- Below is an example to calculate message digest using MD5 algorithm.

Code Snippet :

```openSSlExmpleHashMd5.cpp

```
```
#include <stdio.h>
#include <string.h>
#include <openssl/evp.h>

int main(int argc, char *argv[  ]) {

int i,j;
const int	totalTestVectors = 7;
/*
The ouput length for the claculated
digest. This will be fixed for a parti-
cular Hash algorithm and will very algo-
rithm to algorithm.
*/
unsigned int	outputLength;
/*
The Message Digest Context object, which
will hold the intermediate state.
*/
EVP_MD_CTX    messageDigestContext;

/*
The buffer to store message digest after
computing. 64 bytes is enough for any hash
function. For MD5 128/8 would be fine as
MD5 has 128 bit output length.
Someone can directly use the 128/8 as
the size but if you change the Hash
algorithm, for example SHA1,has 160 bit
output, the length need to be changed to
160/8 bytes.
*/

unsigned char messageDigest[EVP_MAX_MD_SIZE];

/*
Hashes will be calculated for the following strings.
These strings are from the MD5 RFC.
*/
const char *strMessages[] =
{	"", // Input String : 1
"a", // Input String : 2
"abc",// Input String : 3
"message digest",// Input String : 4
"abcdefghijklmnopqrstuvwxyz",// Input String : 5
"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789",// Input String : 6
"12345678901234567890123456789012345678901234567890123456789012345678901234567890"// Input String : 7
};
/*
For each Input string the Expected Output are (from RFC,
this is to make sure that, this implementation is not bogus):
1: d41d8cd98f00b204e9800998ecf8427e
2: 0cc175b9c0f1b6a831c399e269772661
3: 900150983cd24fb0d6963f7d28e17f72
4: f96b697d7cb7938d525a2f31aaf161d0
5: f96b697d7cb7938d525a2f31aaf161d0
6: d174ab98d277d9f5a5611c2c9f419d9f
7: 57edf4a22be3c955ac49da2e2107b67a
*/

/*
Initialize the Message Digest Context
Calculate Message Digest
*/
//EVP_DigestInit(&amp;amp;amp;messageDigestContext, EVP_md5());
for (j=0; j < totalTestVectors; j++)
{
EVP_DigestInit(&messageDigestContext, EVP_md5()); // for SHA1 use EVP_sha1()
EVP_DigestUpdate(&messageDigestContext, strMessages[j], strlen(strMessages[j]));
EVP_DigestFinal(&messageDigestContext, messageDigest, &outputLength);
printf("Test Vector : \"%s\" \n, Digest : = \"",strMessages[j]);
for (i = 0;  i < outputLength;  i++) printf("%02x", messageDigest[i]);
printf("\"\n");
}
return 0;
}

```

The above code work fine on windows or Linux but you should have already openssl libs with you or you can download it.