MCrypt in C, why isn't the encrypting not changing the plaintext into a hash/encryption data?

Question

I have this code that I got from https://gist.github.com/bricef/2436364 to encrypt and decrypt data using mcrypt, but the thing is, I want to encrypt the plaintext into a text generated by the encrypting, so I can after encrypting pass that text to be decrypted.

But when I execute the code below, I get this output:

plain:   myText
encrypt: myText
decrypt: myText

it didn't change anything from plain to encryption, am I doing something wrong? How do I get the value generated by the encryption so I can give back to the function to make it decrypt afterwards. I want a big hash sequence to make it more secure that can be decrypted with the same private key I used to encrypt it.

 int encrypt(void* buffer, int buffer_len, char* IV, char* key, int key_len)
    {
        MCRYPT td = mcrypt_module_open("rijndael-128", NULL, "cbc", NULL);
        int blocksize = mcrypt_enc_get_block_size(td);
        int n_blocks = buffer_len / blocksize;
        int i = 0;
        if (buffer_len % blocksize != 0)
            return 1;

        mcrypt_generic_init(td, key, key_len, IV);
        for (i = 0; i < n_blocks; i++)
            mcrypt_generic(td, ((unsigned char*)buffer) + (i * blocksize), blocksize);
        mcrypt_generic_deinit(td);
        mcrypt_module_close(td);

        return 0;
    }

    int decrypt(void* buffer, int buffer_len, char* IV, char* key, int key_len){
        MCRYPT td = mcrypt_module_open("rijndael-128", NULL, "cbc", NULL);
        int blocksize = mcrypt_enc_get_block_size(td);
        int n_blocks = buffer_len / blocksize;
        int i = 0;

        if (buffer_len % blocksize != 0)
            return 1;

        mcrypt_generic_init(td, key, key_len, IV);
        for (i = 0; i < n_blocks; i++)
            mdecrypt_generic(td, ((unsigned char *)buffer) + (i * blocksize), blocksize);
        mcrypt_generic_deinit(td);
        mcrypt_module_close(td);

        return 0;
    }

    int main()
    {
        MCRYPT td, td2;
        char * plaintext = "myText";
        char* IV = "AAAAAAAAAAAAAAAA";
        char *key = "0123456789abcdef";
        int keysize = 16; /* 128 bits */
        char* buffer;
        int buffer_len = 6;


        buffer = calloc(1, buffer_len);
        strncpy(buffer, plaintext, buffer_len);

        printf("==C==\n");
        printf("plain:   %s\n", plaintext);

        encrypt(buffer, buffer_len, IV, key, keysize);
        printf("encrypt: %s\n", buffer);

        decrypt(buffer, buffer_len, IV, key, keysize);
        printf("decrypt: %s\n", buffer);

        return 0;
    }

Answer 1

In your code, with int buffer_len = 6; you never execute mcrypt_generic in encrypt. Specifically, you have the following in encrypt:

int n_blocks = buffer_len / blocksize;
...
mcrypt_generic_init (td, key, key_len, IV);
for (i = 0; i < n_blocks; i++)
mcrypt_generic (td, ((unsigned char *) buffer) + (i * blocksize),
        blocksize);

Passing buffer_len = 6 results in n_blocks = 0 due to mcrypt_enc_get_block_size (td); returning 16. The simple fix is to make int buffer_len a multiple of 16. E.g: 16, 32, 64, .... With int buffer_len = 16;, your code works fine:

output:

./bin/mcry
==C==
plain:   myText
encrypt: '���J�Ꮽ
decrypt: myText