Unicode C++

AES GCM Encrypt and Decrypt a File

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Demonstrates how to AES GCM encrypt and decrypt a file. Unicode C++

#include <CkCrypt2W.h>

void ChilkatSample(void)
    {
    bool success = false;

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

    CkCrypt2W crypt;

    //  Set the encryption algorithm to "AES"	
    crypt.put_CryptAlgorithm(L"aes");

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

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

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

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

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

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

    //  Encrypt a file.
    const wchar_t *inFile = L"qa_data/hamlet.xml";
    const wchar_t *outFile = L"c:/temp/qa_output/hamlet_aes_gcm.enc";
    success = crypt.CkEncryptFile(inFile,outFile);
    if (success == false) {
        wprintf(L"%s\n",crypt.lastErrorText());
        return;
    }

    //  Get the authentication tag in hex format
    const wchar_t *authTag = crypt.getEncodedAuthTag(L"hex");
    wprintf(L"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.
    crypt.SetEncodedAuthTag(authTag,L"hex");

    inFile = outFile;
    outFile = L"c:/temp/qa_output/hamlet_restored.xml";
    success = crypt.CkDecryptFile(inFile,outFile);
    if (success == false) {
        wprintf(L"%s\n",crypt.lastErrorText());
        return;
    }

    wprintf(L"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.
    }