(Swift 3,4,5...) 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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func chilkatTest() {
    // This example assumes the Chilkat API to have been previously unlocked.
    // See Global Unlock Sample for sample code.

    let crypt = CkoCrypt2()!

    // 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)
    var K: String? = "000102030405060708090A0B0C0D0E0F000102030405060708090A0B0C0D0E0F"

    // This is the 16-byte initialization vector (in hex format)
    var IV: String? = "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.
    var AAD: String? = "feedfacedeadbeeffeedfacedeadbeefabaddad2"

    // Set the secret key and IV
    crypt.setEncodedIV(IV, encoding: "hex")
    crypt.setEncodedKey(K, encoding: "hex")

    // Set the additional authenticated data (AAD)
    var success: Bool = crypt.setEncodedAad(AAD, encoding: "hex")

    // Encrypt a file.
    var inFile: String? = "qa_data/hamlet.xml"
    var outFile: String? = "c:/temp/qa_output/hamlet_aes_gcm.enc"
    success = crypt.ckEncryptFile(inFile, destFile: outFile)
    if success == false {
        print("\(crypt.lastErrorText!)")
        return
    }

    // Get the authentication tag in hex format
    var authTag: String? = crypt.getEncodedAuthTag("hex")
    print("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, encoding: "hex")

    inFile = outFile
    outFile = "c:/temp/qa_output/hamlet_restored.xml"
    success = crypt.ckDecryptFile(inFile, destFile: outFile)
    if success == false {
        print("\(crypt.lastErrorText!)")
        return
    }

    print("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

}