decryption fails with: Invalid_Authentication_Tag: GCM tag check failed

[MohammadMD1383]

this is my code:

	vector<uint8_t> encrypt_aes256_gcm(vector<uint8_t> buffer, const uint8_t* key) {
		const auto enc = Botan::AEAD_Mode::create("AES-256/GCM", Botan::Cipher_Dir::Encryption);
		enc->set_key(key, 32);

		Botan::AutoSeeded_RNG rng;
		const auto nonce = rng.random_vec(12);
		enc->start(nonce);
		enc->finish(buffer, buffer.size());

		vector<uint8_t> result(nonce.size() + buffer.size());
		std::ranges::copy(nonce, result.begin()); // Add nonce at the start
		std::ranges::copy(buffer, result.begin() + nonce.size()); // Add ciphertext+tag
		return std::move(result);
	}

	vector<uint8_t> decrypt_aes256_gcm(vector<uint8_t> buffer, const uint8_t* key) {
		const auto dec = Botan::AEAD_Mode::create("AES-256/GCM", Botan::Cipher_Dir::Decryption);
		dec->set_key(key, 32);

		vector nonce(buffer.begin(), buffer.begin() + 12);
		buffer.erase(buffer.begin(), buffer.begin() + 12);

		dec->start(nonce);
		dec->finish(buffer);
		return std::move(buffer);
	}

vector<uint8_t> to_bytes(string_view str) {
    return vector<uint8_t>(str.begin(), str.end());
}

string to_string(const vector<uint8_t>& bytes) {
    return string(bytes.begin(), bytes.end());
}

int main() {
    const string sample_text = "سلام دنیا"; // Hello World in Persian
    const uint8_t key[32] = {
        0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08,
        0x09, 0x0A, 0x0B, 0x0C, 0x0D, 0x0E, 0x0F, 0x10,
        0x11, 0x12, 0x13, 0x14, 0x15, 0x16, 0x17, 0x18,
        0x19, 0x1A, 0x1B, 0x1C, 0x1D, 0x1E, 0x1F, 0x20
    };

    cout << "Original text: " << sample_text << endl;

    auto encrypted = encrypt_aes256_gcm(to_bytes(sample_text), key);
    cout << "Encrypted size: " << encrypted.size() << " bytes" << endl;

    auto decrypted = decrypt_aes256_gcm(encrypted, key);
    cout << "Decrypted text: " << to_string(decrypted) << endl;

    return 0;
}

please help me!

[MohammadMD1383]

enc->finish(buffer, buffer.size());

the problem was this
I removed 2nd parameter and everything works fine. why???!!!

[reneme]

Hi Mohammad. What was going wrong in the first place? Did you have see compiler errors or did the code behave unexpectedly?

Anyway: enc->finish() expects a vector as the first parameter (as it might have to re-allocate its length to accomodate the authentication tag). The second parameter (called offset) is fairly technical and allows operating on only a portion of the passed-in vector. Typical uses won't need that and should leave it out (as you already found yourself).

[MohammadMD1383]

the problem was runtime error indicating gcm tag verification failure.

I was getting wrong the offset as vector size. I made a mistake.

thanks for the reply!