AES ECB encrypt/decrypt only decrypts the first 16 bytes

Question

I had function that decode AES 256 string but it return only 16 char

bool decrypt_block(unsigned char* cipherText, unsigned char* plainText, unsigned char* k         


        

Answer 1

AES is a block cipher. It encrypts and decrypts a block of 128 bits (16 bytes). AES_decrypt and AES_encrypt acts on a single block at a time. So, you will get only first 16 bytes. You have to manually decrypt or encrypt other blocks.

If you know the mode (like CBC, ECB etc.), you can call functions such AES_decrypt_cbc etc.

You need to modify the code as follows (I am giving just an example):

 int len = strlen(ciphertext); //or get cipher text length by any mean.
 int i;
 for(i=0; i<=len; i+=16)
    AES_decrypt(cipherText+i, plainText+i, &decKey);

If you are sure about mode, call cbc/ecb/cfb/ofb mode functions.

In case of any doubt, please let me know.

Answer 2

It appears that you are confusing key length and block size.

AES can be used with 3 different key lengths: 128-bits, 192-bits & 256-bits.

AES always uses a block size of 128 bits (16 bytes). For messages that are more than 16 bytes long, you need to decrypt (or encrypt) 16 bytes at a time and expect to get 16 bytes of output each time. (You will also need to decide which mode to use - e.g. CBC, CTR, ECB, etc.. If you're decrypting text provided by somebody else then that decision has already been taken for you. If making the decision for yourself, bear in mind that ECB is almost never the right choice.) If the message isn't a multiple of 16 bytes long, you'll need to pad it so that it is. PKCS #7 padding is the most common.

See the Wikipedia article on AES for more information.