(C#) AES-GCM Encryption / Decryption

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Demonstrates AES-GCM encryption. Afterwards, the encrypted bytes, the authentication tag, and the IV are concatenated into one byte array and encoded to base64. For decryption, we decode the base64, extract the IV, authentication tag, and encrypted bytes, and then perform AES-GCM decryption.

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// This example assumes the Chilkat API to have been previously unlocked.
// See Global Unlock Sample for sample code.

Chilkat.Crypt2 crypt = new Chilkat.Crypt2();

crypt.CryptAlgorithm = "aes";
crypt.CipherMode = "gcm";
crypt.KeyLength = 256;

string K = "000102030405060708090A0B0C0D0E0F000102030405060708090A0B0C0D0E0F";
string IV = "000102030405060708090A0B0C0D0E0F";
string AAD = "feedfacedeadbeeffeedfacedeadbeefabaddad2";
string PT = "This is the text to be AES-GCM encrypted.";

crypt.SetEncodedIV(IV,"hex");
crypt.SetEncodedKey(K,"hex");

bool success = crypt.SetEncodedAad(AAD,"hex");

// Return the encrypted bytes as base64
crypt.EncodingMode = "base64";
crypt.Charset = "utf-8";
string cipherText = crypt.EncryptStringENC(PT);
if (crypt.LastMethodSuccess != true) {
    Debug.WriteLine(crypt.LastErrorText);
    return;
}

// Get the GCM authenticated tag computed when encrypting.
string authTag = crypt.GetEncodedAuthTag("base64");

Debug.WriteLine("Cipher Text: " + cipherText);
Debug.WriteLine("Auth Tag: " + authTag);

// Let's send the IV, CipherText, and AuthTag to the decrypting party.
// We'll send them concatenated like this: [IV || Ciphertext || AuthTag]
// In base64 format.
Chilkat.BinData bdEncrypted = new Chilkat.BinData();
bdEncrypted.AppendEncoded(IV,"hex");
bdEncrypted.AppendEncoded(cipherText,"base64");
bdEncrypted.AppendEncoded(authTag,"base64");

string concatenatedGcmOutput = bdEncrypted.GetEncoded("base64");
Debug.WriteLine("Concatenated GCM Output: " + concatenatedGcmOutput);

// Sample output so far:

// -------------------------------------------------------------------------------------
// Now let's GCM decrypt...
// -------------------------------------------------------------------------------------

Chilkat.Crypt2 decrypt = new Chilkat.Crypt2();

// The values shared and agreed upon by both sides beforehand are: algorithm, cipher mode, secret key, and AAD.
// Sometimes the IV can be a value already known and agreed upon, but in this case the encryptor sends the IV to the decryptor.
decrypt.CryptAlgorithm = "aes";
decrypt.CipherMode = "gcm";
decrypt.KeyLength = 256;
decrypt.SetEncodedKey(K,"hex");
decrypt.SetEncodedAad(AAD,"hex");

Chilkat.BinData bdFromEncryptor = new Chilkat.BinData();
bdFromEncryptor.AppendEncoded(concatenatedGcmOutput,"base64");

int sz = bdFromEncryptor.NumBytes;

// Extract the parts.
string extractedIV = bdFromEncryptor.GetEncodedChunk(0,16,"hex");
string extractedCipherText = bdFromEncryptor.GetEncodedChunk(16,sz - 32,"base64");
string expectedAuthTag = bdFromEncryptor.GetEncodedChunk(sz - 16,16,"base64");

// Before GCM decrypting, we must set the authenticated tag to the value that is expected.
// The decryption will fail if the resulting authenticated tag is not equal to the expected result.
success = decrypt.SetEncodedAuthTag(expectedAuthTag,"base64");

// Also set the IV.
decrypt.SetEncodedIV(extractedIV,"hex");

// Decrypt..
decrypt.EncodingMode = "base64";
decrypt.Charset = "utf-8";
string decryptedText = decrypt.DecryptStringENC(extractedCipherText);
if (decrypt.LastMethodSuccess != true) {
    // Failed.  The resultant authenticated tag did not equal the expected authentication tag.
    Debug.WriteLine(decrypt.LastErrorText);
    return;
}

Debug.WriteLine("Decrypted: " + decryptedText);

// Sample output:

// Cipher Text: cYspSW4GuSj0Msho4OgZZ0AwspDEpTF5Br8NlA+qT3f+g3nQo+xalmU=
// Auth Tag: z/N82vdj/ZsM0WnHNCnPPw==
// Concatenated GCM Output: AAECAwQFBgcICQoLDA0OD3GLKUluBrko9DLIaODoGWdAMLKQxKUxeQa/DZQPqk93/oN50KPsWpZlz/N82vdj/ZsM0WnHNCnPPw==
// Decrypted: This is the text to be AES-GCM encrypted.