The md5_calc() function computes an MD5 digest on a single message block. The inlen-byte block is pointed to by input, and the 16-byte MD5 digest is written to output.

The MD5Init(), MD5Update(), and MD5Final() functions allow an MD5 digest to be computed over multiple message blocks; between blocks, the state of the MD5 computation is held in an MD5 context structure, allocated by the caller. A complete digest computation consists of one call to MD5Init(), one or more calls to MD5Update(), and one call to MD5Final(), in that order.

The MD5Init() function initializes the MD5 context structure pointed to by context.

The MD5Update() function computes a partial MD5 digest on the inlen-byte message block pointed to by input, and updates the MD5 context structure pointed to by context accordingly.

The MD5Final() function generates the final MD5 digest, using the MD5 context structure pointed to by context; the 16-byte MD5 digest is written to output. After calling MD5Final(), the state of the context structure is undefined; it must be reinitialized with MD5Init() before being used again.

The following is a sample function that must authenticate a message that is found in multiple buffers. The calling function provides an authentication buffer that will contain the result of the MD5 digest.

int
AuthenticateMsg(unsigned char *auth_buffer, struct iovec 
                *messageIov, unsigned int num_buffers)
{
    MD5_CTX md5_context;
    unsigned int i;

    MD5Init(&md5_context);

    for(i=0, i<num_buffers; i++
    {
         MD5Update(&md5_context, messageIov->iov_base,
                   messageIov->iov_len);
         messageIov += sizeof(struct iovec);
    }

    MD5Final(auth_buffer, &md5_context);

    return 0;
}

Since the buffer to be computed is contiguous, the md5_calc() function can be used to generate the MD5 digest.

int AuthenticateMsg(unsigned char *auth_buffer, unsigned
                    char *buffer, unsigned int length)
{
    md5_calc(buffer, auth_buffer, length);

    return (0);
}