AES CTR OpenSSL command line does not match EVP_aes_128_ctr C code

Question

CTR-AES256 Encrypt does not match OpenSSL -aes-256-ctr <-- this post did not help

I tried the following C implementation of Openssl EVP function for AES-128-CTR encryption but the results I am getting are incorrect compared to the command line OpenSSL result.

The odd part is that when I try with a larger size of Plaintext (600 bytes or more), only last 600 bytes of cipher is different between C code and Command line. If required I can paste that result here as well.

C code implementation of AES-128-CTR

static const unsigned char key[16] = {
    0x00, 0x01, 0x02, 0x03, 
    0x04, 0x05, 0x06, 0x07, 
    0x08, 0x09, 0x0a, 0x0b, 
    0x0c, 0x0d, 0x0e, 0x0f, 
};

static const unsigned char iv[16] = {
    0x01, 0x23, 0x45, 0x67, 
    0x89, 0xab, 0xcd, 0xef, 
    0x88, 0x88, 0x88, 0x88, 
    0xC0, 0x00, 0x00, 0x00, 
};

FILE *fp_output = fopen("cipherCode.bin", "wb");

// Encrypt Plaintext

EVP_CIPHER_CTX *ctx;
int outlen;
unsigned char cipher[size];

if(!(ctx = EVP_CIPHER_CTX_new())) handleErrors();

if(!(EVP_EncryptInit_ex(ctx, EVP_aes_128_ctr(), NULL, key, iv))) handleErrors();

if(!(EVP_EncryptUpdate(ctx, cipher, &outlen, plaintext, size))) handleErrors();

if(!(EVP_EncryptFinal_ex(ctx, cipher + outlen, &outlen))) handleErrors();

/*---Edit----

// EVP_CIPHER_CTX_set_padding(ctx, 0); <-- removed this as it isnt necessary 

-----------*/

EVP_CIPHER_CTX_free(ctx);

// Write result cipher into output file
fwrite((unsigned char *)&cipher[0], outlen, 1, fp_output);
fclose(fp_output);

OpenSSL command line:

openssl enc -aes-128-ctr -in plaintext.bin -out cipherCL.bin -K 000102030405060708090a0b0c0d0e0f -iv 0123456789abcdef88888888c0000000 -p -nopad

Same Plaintext, key and IV used for both.

Input:

Plaintext:

0000000 ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff

Ouput:

Hexdiff (shortened for clarity):

Visuel HexDiff v 0.0.53 by tTh 2007                             dec   7bits  

0   00 00 00 00 00 00 00 00 10 90 66 01 00 00 00 00              f     

** cipherCode.bin                                    16        0   0%      

0   1e a4 43 3f d8 4c 8c b7 1a e7 f0 af 85 0c d2 c2      C? L

** cipherCL.bin                                   16        0   0%

Include in your code excerpt how you are declaring key and iv. — Luke Joshua Park
Also show how your code writes the output file; a 'ciphertext' having three-quarters of its bytes zero is effectively impossible. FYI calling _set_padding after doing encryption (or decryption) is useless, but CTR is a stream mode and doesn't use padding anyway. — dave_thompson_085
@LukeJoshuaPark dave_thompson_085 Please check the edits. Thanks. — embedevug

embedevug embedevug · Accepted Answer · 2018-06-18T01:39:59

I found the issue in my program. I was not defining the cipher variable as static. Now that I define it static, the correct cipher data is written to the file.

Why static worked? I called a ciphering function to calculate the cipher and then return the cipher. Since the cipher was not declared as static, it lost its value after exiting the function, thus the data returned was not the same as the data in the cipher. After declaring cipher as static, the value of cipher retained after function call and wrote correct information in the file.

AES CTR OpenSSL command line does not match EVP_aes_128_ctr C code

2 Answers