ECB, CFB, OFB cipher modes in Openssl

Question

I know that when I use CBC mode with Openssl, I can give as an input which is a multiple of a block size. But how about other modes? ECB, CFB, OFB? I saw a doc but its not all clear to me. Should I call them in a loop? Lets say, ECB. It encrypts 64 bits at a time. So a pseudocode would look like this (should look like this)? ``` int len = 512, c = 0; unsigned char in[len], out[len]; while(c < len) { Aes_ecb_encrypt(in+c, out+c, &enckey, AES_ENCRYPT); c += 8; } ``` But with the above code it doesnt encrpyt good. When I change `c += 8;` into `c += 16;` its ok then. Whats is good way of doing this? I mean, we all know that 8x8 = 64 bits so this should be correct, but it isnt, the encryption/decryption is working only when I have `c += 16;`. What about other cipher modes? Sample for ECB mode (notice, that the question is also about other modes too;)): ``` #include #include #include #include #include // a simple hex-print rou…

Accepted Answer

Lets say, ECB. It encrypts 64 bits at a time

AES is a 128-bit block cipher. It encrypts/decrypts 128-bit at a time. It is a standard. AES_encrypt/AES_decrypt block expects 128 bits or 16 bytes length of the input and output data.

But with the above code it doesnt encrpyt good. When I change c += 8; into c += 16; its ok then. Whats is good way of doing this? I mean, we all know that 8x8 = 64 bits so this should be correct, but it isnt, the encryption/decryption is working only when I have c += 16;

That is why it is working fine on

c+=16

Apart from this, there are few problems in your code.

```
unsigned char enc_out[encslength];
```

Please keep the size of dec_out of encslength since you are decrypting encslength bytes not inputslength in your code.

```
unsigned char dec_out[encslength];
memset(enc_out, 0, sizeof(enc_out));
memset(dec_out, 0, sizeof(dec_out));

AES_KEY enc_key, dec_key;
AES_set_encrypt_key(aes_key, keylength, &enc_key);
```

In this section, increase c by 16 as AES is 128 block cipher.

```
long c = 0;
while(c < encslength)
{
    AES_ecb_encrypt(paddedinput+c, enc_out+c, &enc_key, AES_ENCRYPT);
    c += 16;
}
```

Similar change here:

```
c = 0;
AES_set_decrypt_key(aes_key, keylength, &dec_key);
while(c < encslength)
{
    AES_ecb_encrypt(enc_out+c, dec_out+c, &dec_key, AES_DECRYPT);
    c += 16;
}
```

About other modes:

- CFB mode There is no need to pad the data. Use AES_cfb128_encrypt and initialization vector (IV) along with key.

For encryption:

```
AES_cfb128_encrypt (paddedinput, enc_out, inputlength, &enc_key, iv, AES_ENCRYPT);
```

For decryption:

```
AES_cfb128_decrypt (enc_out, dec_out, inputlength, &enc_key, iv, AES_ENCRYPT);
```

- OFB mode Similarly on OFB mode, there is no need to pad the data. Use AES_ofb128_encrypt and IV along with key.

For encryption:

```
//Initialize num to 0.
num = 0;
AES_ofb128_encrypt (paddedinput, enc_out, inputlength, &enc_key, iv, &num);
```

For decryption:

```
num = 0;
AES_ofb128_encrypt (enc_out, dec_out, inputlength, &enc_key, iv, &num);
```

You may need to tweak the code some bit as per your need.

Please read more about block cipher mode of operation.

ECB, CFB, OFB cipher modes in Openssl

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