C++ Openssl Encryption routines generates different output on each run

Question

I have an application that has to Encrypt data.

However, when I run the following it returns me different output within the same execution. When I repeat the execution, the pattern of output is exactly the same, see end of my question.

int Encrypt(unsigned char *plaintext, int plaintext_len, unsigned char *key, unsigned char *iv, unsigned char *ciphertext) {
    EVP_CIPHER_CTX *ctx;
    int len;
    int ciphertext_len;
    if (!(ctx = EVP_CIPHER_CTX_new())) handleErrors();
    if (1 != EVP_EncryptInit_ex(ctx, EVP_aes_256_cfb(), NULL, key, iv)) {
        handleErrors();
        }
    if (1 != EVP_EncryptUpdate(ctx, ciphertext, &len, plaintext, plaintext_len)) {
        handleErrors();
        }
    ciphertext_len = len;
    if (1 != EVP_EncryptFinal_ex(ctx, ciphertext + len, &len)) {
        handleErrors();
        }
    ciphertext_len += len;
    EVP_CIPHER_CTX_free(ctx);
    return ciphertext_len;
    }
string EncryptThis(string Data, string Key, string IV, long long Buffer) {
    unsigned char *key = (unsigned char *)Key.c_str();
    unsigned char *iv = (unsigned char *)IV.c_str();
    unsigned char *plaintext = (unsigned char *)Data.c_str();
    unsigned char* ciphertext = new unsigned char[Buffer];
    ERR_load_crypto_strings();
    OpenSSL_add_all_algorithms();
    //cout << Data << endl; //ALWAYS THE SAME
    //cout << Key << endl; //ALWAYS THE SAME
    //cout << IV << endl; //ALWAYS THE SAME


    int ciphertext_len = Encrypt(plaintext, (int)strlen((char *)plaintext), key, iv, ciphertext);
    string Encrypted = base64_encode(reinterpret_cast<const unsigned char *>(ciphertext), ciphertext_len);
    EVP_cleanup();
    ERR_free_strings();
    return Encrypted;
    }
int main() {
    cout << EncryptThis("ThisisATest", "Test", "Test2", 11) << endl;
    cout << EncryptThis("ThisisATest", "Test", "Test2", 11) << endl;
    cout << EncryptThis("ThisisATest", "Test", "Test2", 11) << endl;
    }

In this test the results where:

mJOjOk2wIe5D4oA=
GTvRQb8IZtFYbLI=
plOJA083gOSfVrs=

However, when I run the same application again, it returns me exact the same output even in the same order. So it seems to be a problem with the repetition within the execution of the script?

To clarify. When this application would be called Test.exe and I execute the application three times, it would output the following:

//execute Test.exe
mJOjOk2wIe5D4oA=
GTvRQb8IZtFYbLI=
plOJA083gOSfVrs=

//Execute Test.exe
mJOjOk2wIe5D4oA=
GTvRQb8IZtFYbLI=
plOJA083gOSfVrs=

//execute Test.exe
mJOjOk2wIe5D4oA=
GTvRQb8IZtFYbLI=
plOJA083gOSfVrs=

How is this possible and where is my error?

Answer 1

The key and IV are to short. The key for EVP_aes_256_cfb needs to be 256-bits (32-bytes) and the IV needs to be the block size, 128-bits (16-bytes) for AES.

Since the key and IV are to short the extra length is made up somehow, there is no standard defined. For one or both what ever bytes follow are used. On call to main the memory is the same each time (possible zeroed) so the same results. On the internal calls the memory has been changed by execution but in the same way each time so the output is repeated.

The buffer is too short. it needs to be the length of the data to be encrypted + the block length: 11 + 16 -> 27. It is best if the encryption function (Encrypt) create the buffer, not the caller. The EVP_EncryptUpdate function will return the final length of the encrypted+padding.

Solution: use full length values for the key and IV and handle the buffer internally with the correct length.

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Hint 1: CBC mode is more generally used that CFB mode.

Hint 2: Use a random IV, just prefix the encrypted data with the IV for use in decryption, it does not need to be secret. That way the user need not provide the IV.

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Note: If you stil have problems with full length key and IV update the question and supply the valuies at the call to and after EVP_EncryptUpdate.