I am trying to write a set of basic encrypt and decrypt functions in C using OpenSSL and I am losing the last block of my plain text when decrypting. I think it’s probably a padding issue but I am not sure how to diagnose it since I can’t seem to get any information out of the OpenSSL call as to why it’s returning an incorrect length.

At the moment EVP_DecryptUpdate returns a length of 32 when I think it should return 48. Additionally to this even though the length value passed to EVP_DecryptFinal_ex is 32 if your array is 49 long or any other size that is not a multiple of 16 it will return a value indicating there was an error.

#include <stdio.h>
#include <string.h>

#include <openssl/conf.h>
#include <openssl/evp.h>
#include <openssl/err.h>

int ecb_encrypt(unsigned char *plain_bytes, unsigned char *key, unsigned char *cipher_bytes, size_t plain_bytes_len, size_t *cipher_bytes_len) {
    EVP_CIPHER_CTX *ctx;
    int len = 0;

    if(!(ctx = EVP_CIPHER_CTX_new()))
        return 4;

    if(1 != EVP_EncryptInit_ex(ctx, EVP_aes_128_ecb(), NULL, key, NULL))
        return 5;
    
    if(1 != EVP_EncryptUpdate(ctx, cipher_bytes, &len, plain_bytes, plain_bytes_len))
        return 6;
    *cipher_bytes_len = len;

    if(1 != EVP_EncryptFinal_ex(ctx, cipher_bytes + *cipher_bytes_len, &len))
        return 7;

    EVP_CIPHER_CTX_free(ctx);
    return 0;
}

int ecb_decrypt(unsigned char *cipher_bytes, unsigned char *key, unsigned char *plain_bytes, size_t cipher_bytes_len, size_t *plain_bytes_len) {
    EVP_CIPHER_CTX *ctx;
    int len = 0;

    if(!(ctx = EVP_CIPHER_CTX_new()))
        return 4;

    if(1 != EVP_DecryptInit_ex(ctx, EVP_aes_128_ecb(), NULL, key, NULL))
        return 5;

    if(1 != EVP_DecryptUpdate(ctx, plain_bytes, &len, cipher_bytes, cipher_bytes_len))
        return 6;
    *plain_bytes_len = len;
    printf("len = %dn", len);
    printf("plain_bytes_len = %lun", *plain_bytes_len);
    if(1 != EVP_DecryptFinal_ex(ctx, plain_bytes + *plain_bytes_len, &len))
        return 7;
    
    EVP_CIPHER_CTX_free(ctx);
    return 0;
}

int main() {
    unsigned char plain_text[] = "AAAAAAAAAAAAAAAABBBBBBBBBBBBBBBBCCCCCCCCCCCCCCCC";
    unsigned char key[]        = "YELLOW SUBMARINE";
    unsigned char cipher_bytes[48];
    size_t cipher_bytes_len = 0;

    ecb_encrypt(plain_text, key, (unsigned char *) &cipher_bytes, 48, &cipher_bytes_len);
    printf("cipher_bytes_len = %lun", cipher_bytes_len);

    //decrypt
    size_t plain_bytes_len = 0;
    unsigned char plain_bytes[cipher_bytes_len];

    ecb_decrypt(cipher_bytes, key, (unsigned char *) &plain_bytes, cipher_bytes_len, &plain_bytes_len);

    printf("plain_bytes_len = %lun", plain_bytes_len);
    for(size_t i = 0; i < plain_bytes_len; i++){
        printf("%c", plain_bytes[i]);
    }
       
    return 0;
}

Basically I am losing my C’s and I am not sure why it’s getting the length wrong.

Output:

cipher_bytes_len = 48
len = 32
plain_bytes_len = 32
plain_bytes_len = 32
AAAAAAAAAAAAAAAABBBBBBBBBBBBBBBBcipher_bytes_len = 48
len = 32
plain_bytes_len = 32
plain_bytes_len = 32
AAAAAAAAAAAAAAAABBBBBBBBBBBBBBB