(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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#include <C_CkCrypt2.h>

void ChilkatSample(void)
    {
    HCkCrypt2 crypt;
    const char *K;
    const char *IV;
    const char *AAD;
    BOOL success;
    const char *inFile;
    const char *outFile;
    const char *authTag;

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

    crypt = CkCrypt2_Create();

    // Set the encryption algorithm to "AES"	
    CkCrypt2_putCryptAlgorithm(crypt,"aes");

    // Indicate that the Galois/Counter Mode (GCM) should be used:
    CkCrypt2_putCipherMode(crypt,"gcm");

    // KeyLength may be 128, 192, 256
    CkCrypt2_putKeyLength(crypt,256);

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

    // This is the 16-byte initialization vector (in hex format)
    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.
    AAD = "feedfacedeadbeeffeedfacedeadbeefabaddad2";

    // Set the secret key and IV
    CkCrypt2_SetEncodedIV(crypt,IV,"hex");
    CkCrypt2_SetEncodedKey(crypt,K,"hex");

    // Set the additional authenticated data (AAD)
    success = CkCrypt2_SetEncodedAad(crypt,AAD,"hex");

    // Encrypt a file.
    inFile = "qa_data/hamlet.xml";
    outFile = "c:/temp/qa_output/hamlet_aes_gcm.enc";
    success = CkCrypt2_CkEncryptFile(crypt,inFile,outFile);
    if (success == FALSE) {
        printf("%s\n",CkCrypt2_lastErrorText(crypt));
        CkCrypt2_Dispose(crypt);
        return;
    }

    // Get the authentication tag in hex format
    authTag = CkCrypt2_getEncodedAuthTag(crypt,"hex");
    printf("authentication tag = %s\n",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.
    CkCrypt2_SetEncodedAuthTag(crypt,authTag,"hex");

    inFile = outFile;
    outFile = "c:/temp/qa_output/hamlet_restored.xml";
    success = CkCrypt2_CkDecryptFile(crypt,inFile,outFile);
    if (success == FALSE) {
        printf("%s\n",CkCrypt2_lastErrorText(crypt));
        CkCrypt2_Dispose(crypt);
        return;
    }

    printf("Success.\n");

    // --------------------------------------------------------------------------------------------
    // 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


    CkCrypt2_Dispose(crypt);

    }