(Mono C#) AES GCM Encrypt and Decrypt a File

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Demonstrates how to AES GCM encrypt and decrypt a file.

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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();

// Set the encryption algorithm to "AES"	
crypt.CryptAlgorithm = "aes";

// Indicate that the Galois/Counter Mode (GCM) should be used:
crypt.CipherMode = "gcm";

// KeyLength may be 128, 192, 256
crypt.KeyLength = 256;

// This is the 256-bit AES secret key (in hex format)
string K = "000102030405060708090A0B0C0D0E0F000102030405060708090A0B0C0D0E0F";

// This is the 16-byte initialization vector (in hex format)
string IV = "000102030405060708090A0B0C0D0E0F";

// This is the OPTIONAL additional data (in hex format) to be used as input to the GCM AEAD algorithm,
// but is not included in the output.  It plays a role in the computation of the
// resulting authenticated tag.
string AAD = "feedfacedeadbeeffeedfacedeadbeefabaddad2";

// Set the secret key and IV
crypt.SetEncodedIV(IV,"hex");
crypt.SetEncodedKey(K,"hex");

// Set the additional authenticated data (AAD)
bool success = crypt.SetEncodedAad(AAD,"hex");

// Encrypt a file.
string inFile = "qa_data/hamlet.xml";
string outFile = "c:/temp/qa_output/hamlet_aes_gcm.enc";
success = crypt.CkEncryptFile(inFile,outFile);
if (success == false) {
    Debug.WriteLine(crypt.LastErrorText);
    return;
}

// Get the authentication tag in hex format
string authTag = crypt.GetEncodedAuthTag("hex");
Debug.WriteLine("authentication tag = " + authTag);

// Decrypt..

// Before decrypting, you must provide the expected authentication tag.
// The decrypt will fail if the resulting authentication tag computed while decrypting is not equal to the
// expected authentication tag.
crypt.SetEncodedAuthTag(authTag,"hex");

inFile = outFile;
outFile = "c:/temp/qa_output/hamlet_restored.xml";
success = crypt.CkDecryptFile(inFile,outFile);
if (success == false) {
    Debug.WriteLine(crypt.LastErrorText);
    return;
}

Debug.WriteLine("Success.");

// --------------------------------------------------------------------------------------------
// About AES-GCM:

// AES-GCM (Advanced Encryption Standard - Galois/Counter Mode) is a widely-used
// encryption mode that provides both confidentiality (encryption) and
// integrity/authentication (data integrity verification) in one operation. It is
// commonly used in secure communications due to its efficiency and strong security
// properties.
// 
// Key Concepts:
// 
//     AES (Advanced Encryption Standard):
// 
//         AES is a symmetric encryption algorithm, meaning the same key is used
//         for both encryption and decryption.
// 
//         It operates on fixed-size blocks of data (128 bits) using key sizes of
//         128, 192, or 256 bits.
// 
//         In AES-GCM, AES is used to perform the actual data encryption.
// 
//     GCM (Galois/Counter Mode):
// 
//         Counter Mode (CTR): GCM uses counter mode for encryption. In this mode,
//         a nonce (or initialization vector, IV) and a counter are combined and encrypted
//         with AES. The result is XORed with the plaintext to produce the ciphertext. 
// 
//         Galois Mode (GMAC): GCM also includes an authentication mechanism based
//         on a Galois field. It generates an authentication tag, which ensures the
//         integrity of both the ciphertext and any additional data (called AAD -
//         Additional Authenticated Data). This tag is verified during decryption to ensure
//         that the data hasn't been tampered with.
// 
// Key Features:
// 
//     Confidentiality (Encryption):
// 
//         The plaintext is encrypted using AES in counter mode. Each block of
//         plaintext is XORed with the output of AES applied to a combination of the IV and
//         an incremented counter.
// 
//     Integrity (Authentication):
// 
//         In addition to encryption, GCM provides authentication for both the
//         encrypted data (ciphertext) and any Additional Authenticated Data (AAD), such as
//         headers or metadata that need to be protected but not encrypted.
// 
//         The authentication tag is generated using a Galois field multiplication
//         of the ciphertext and AAD. This ensures that any changes to the encrypted
//         message or the AAD will be detected during decryption.
// 
// Key Components:
// 
//     - Plaintext: The data you want to encrypt.
//     - Ciphertext: The encrypted data.
//     - Key: A symmetric key used for both encryption and decryption.
//     - Nonce/IV: A unique value used for each encryption to ensure security. It is not secret but should never be reused with the same key.
//     - AAD (Additional Authenticated Data): Optional data that is not encrypted but needs to be authenticated (e.g., headers).
//     - Authentication Tag: A tag generated to verify the integrity and authenticity of the ciphertext and AAD