main.cpp

#include <string.h>
#include <stdio.h>
#include <openssl/bio.h>
#include <openssl/evp.h>
#include <assert.h>

typedef struct {
    char *test_string;
} Config;

char compkey[] = "dsS`p%%Tu\"zT70=F*Mm7ivx[T~Mr@HcX";

// --- BASE64 --- //

size_t calcDecodeLength(const char* b64input) { //Calculates the length of a decoded string
    size_t len = strlen(b64input),
        padding = 0;

    if (b64input[len-1] == '=' && b64input[len-2] == '=') //last two chars are =
        padding = 2;
    else if (b64input[len-1] == '=') //last char is =
        padding = 1;

    return (len*3)/4 - padding;
}

int Base64Decode(char* b64message, unsigned char** buffer, size_t* length) { //Decodes a base64 encoded string
    BIO *bio, *b64;

    int decodeLen = calcDecodeLength(b64message);
    *buffer = (unsigned char*)malloc(decodeLen + 1);
    (*buffer)[decodeLen] = '\0';

    bio = BIO_new_mem_buf(b64message, -1);
    b64 = BIO_new(BIO_f_base64());
    bio = BIO_push(b64, bio);

    BIO_set_flags(bio, BIO_FLAGS_BASE64_NO_NL); //Do not use newlines to flush buffer
    *length = BIO_read(bio, *buffer, strlen(b64message));
    assert(*length == decodeLen); //length should equal decodeLen, else something went horribly wrong
    BIO_free_all(bio);

    return (0); //success
}

// --- AES 256 CBC --- //

unsigned char *aes_decrypt(EVP_CIPHER_CTX *e, unsigned char *ciphertext, int *len, unsigned char *key, unsigned char *iv) {
    int p_len = *len, f_len = 0;
    unsigned char *plaintext = (unsigned char *)malloc(p_len);
  
    EVP_CIPHER_CTX_init(e);
    EVP_DecryptInit_ex(e, EVP_aes_256_cbc(), NULL, key, iv);

    EVP_DecryptInit_ex(e, NULL, NULL, NULL, NULL);
    EVP_DecryptUpdate(e, plaintext, &p_len, ciphertext, *len);
    EVP_DecryptFinal_ex(e, plaintext+p_len, &f_len);

    *len = p_len + f_len;
    return plaintext;
}

// --- MAIN --- //

int main(int argc, char *argv[]) {
    int arg, decode_len, decrypt_len;
    Config c;
    unsigned char *decode_string, *decrypt_string;
    unsigned char iv[32], ciphertext[1024];
    EVP_CIPHER_CTX ctx;

    for (arg = 0; arg < argc; arg++) {
        if (!strcmp(argv[arg], "-t")) {
            arg++;
            c.test_string = argv[arg];
        }        
    }

    printf("\n-- CONFIG --\n");
    printf("   Test String: %s\n", c.test_string);

    Base64Decode(c.test_string, &decode_string, (size_t *)&decode_len);
    strncpy((char *)iv, (const char *)decode_string, 16);
    strcpy((char *)ciphertext, (const char *)decode_string+16);
   
    decrypt_len = decode_len-16; 
    decrypt_string = aes_decrypt(&ctx, ciphertext, &decrypt_len, (unsigned char *)compkey, iv);

    printf("   Decryted String: %s\n", (char *)decrypt_string);

    free(decrypt_string);
    free(decode_string);
    return 0;
}