Incremental hashing in PHP vs C

Question

I am trying to implement hash functionality from C to PHP but have encountred an issue. Would really appreciate to get some help with this.

This is the C-code hashing multiple times:

    SHA_CTX ctx;
    SHA1_Init(&ctx);
    SHA1_Update(&ctx, (const u_int8_t *) salt, strlen(salt));
    SHA1_Update(&ctx, (const u_int8_t *) argv[1], strlen(argv[1]));
    SHA1_Final(temp, &ctx);

But then it gets hashed again in a loop, and there is the tricky part for me to implement in php:

for (n = 0; n < 2 ; ++n) {
        SHA1_Init(&ctx);
        SHA1_Update(&ctx, (const u_int8_t *)salt, strlen(salt));
        SHA1_Update(&ctx, temp, SHA_DIGEST_LENGTH);
        SHA1_Final(temp, &ctx);
}

The SHA1_Init uses the same context &ctx in the loop. Something I fear I cannot do in php.

This is my current php code:

$ctx = hash_init('sha1');
hash_update($ctx, $salt);
hash_update($ctx, 'string');
$pw = hash_final($ctx);

for ($round = 0; $round < 2; ++$round) {
    $ctx = hash_init('sha1');
    hash_update($ctx, $salt);
    hash_update($ctx, $pw);
    $pw = hash_final($ctx);
}

From the output, I can clearly see that in the second time it hashes the hash is not the same as in C:

C:
cf584b11970312e4b973bc7b35870d7e019affcd
cb1ea097e844363e4e76d512af4245c10ade1725

PHP:
cf584b11970312e4b973bc7b35870d7e019affcd
3003969f9065d7614d7cf34675b9d9bf7584d7c3

How may I hash with the old context in php? I don't find any documentation on how to do this and I am not really sure where it goes wrong.

Would be grateful for any kind comments on how to solve this problem!

Answer 1

this is because you're use in internal cycle in C binary array (array of bytes), but in the PHP you're use string with hex-representation of this array. i think more correct is:

$salt = 'salt';
$ctx = hash_init('sha1');
hash_update($ctx, $salt);
hash_update($ctx, 'string');
$pw = hash_final($ctx, true);
for ($round = 0; $round < 2; ++$round) {
    $ctx = hash_init('sha1');
    hash_update($ctx, $salt);
    hash_update($ctx, $pw);
    $pw = hash_final($ctx, $round < 1);
}
echo $pw;