(C) AEAD AES 128-bit GCM

Demonstrates AES encryption using the Galois/Counter Mode (GCM). GCM is an authenticated encryption mode with "additional data" (often referred to as AEAD). GCM is a cipher mode that can be applied to any symmetric encryption algorithm with a 16-byte block size, such as AES and Twofish. In GCM mode, the block encryption algorithm is transformed into a stream encryption algorithm, and therefore no padding occurs (and the PaddingScheme property does not apply). The "additional data" (known as the AAD) does not get encrypted but plays a role in the computation of the resulting "authenticated tag".

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#include <C_CkCrypt2.h>

void ChilkatSample(void)
    {
    HCkCrypt2 crypt;
    const char *K;
    const char *IV;
    const char *AAD;
    const char *PT;
    const char *CT;
    const char *T;
    BOOL success;
    const char *ctResult;
    const char *tResult;
    const char *ptResult;
    const char *tInvalid;

    // 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,128);

    // This is the 128-bit AES secret key (in hex format)
    K = "feffe9928665731c6d6a8f9467308308";

    // This is the 16-byte initialization vector:
    IV = "cafebabefacedbaddecaf888";

    // This is the additional data 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";

    // The plain-text bytes (in hex format) to be encrypted.
    PT = "d9313225f88406e5a55909c5aff5269a86a7a9531534f7da2e4c303d8a318a721c3c0c95956809532fcf0e2449a6b525b16aedf5aa0de657ba637b39";

    // The expected cipher text (in hex format)
    CT = "42831ec2217774244b7221b784d0d49ce3aa212f2c02a4e035c17e2329aca12e21d514b25466931c7d8f6a5aac84aa051ba30b396a0aac973d58e091";

    // The expected authenticated tag given the above inputs.
    T = "5bc94fbc3221a5db94fae95ae7121a47";

    // Note: The above data are the values for test vector #4 from 
    // the PDF document at: http://csrc.nist.gov/groups/ST/toolkit/BCM/documents/proposedmodes/gcm/gcm-spec.pdf

    // EncodingMode specifies the encoding of the output for
    // encryption, and the input for decryption.
    // It may be "hex", "url", "base64", or "quoted-printable".
    CkCrypt2_putEncodingMode(crypt,"hex");

    // 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");

    // For the purpose of duplicating the test vectors, we are using the EncryptEncoded method.
    // This method decodes the input string according to the encoding specified by the EncodingMode
    // property, which in this case is "hex".  The decoded bytes are encrypted using the mode specified
    // by the CipherMode property.  The resulting
    // encrypted bytes are encoded (again using the encoding mode specified by EncodingMode),
    // and the result is returned.
    // <b>Note:</b> The CipherMode property sets the block mode of operation (gcm, cfb, cbc, ofb, ecb, etc.) 
    // for any of the Chilkat encryption/decryption methods (such as EncryptBytes, EncryptString, 
    // CkEncryptFile, etc.)   Just because GCM mode is demonstrated with EncryptEncoded/DecryptEncoded,
    // does not imply that GCM mode is specific to only these methods.
    ctResult = CkCrypt2_encryptEncoded(crypt,PT);
    if (CkCrypt2_getLastMethodSuccess(crypt) != TRUE) {
        printf("%s\n",CkCrypt2_lastErrorText(crypt));
        CkCrypt2_Dispose(crypt);
        return;
    }

    // Examine the result.  It should be the same (case insensitive) as our expected result:
    printf("computed result: %s\n",ctResult);
    printf("expected result: %s\n",CT);

    // Examine the authenticated tag. It should be the same (case insensitive) as our expected authenticated tag:
    tResult = CkCrypt2_getEncodedAuthTag(crypt,"hex");
    printf("computed authTag: %s\n",tResult);
    printf("expected authTag: %s\n",T);

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

    // 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 (case insensitive) to 
    // the expected result.
    // Note: The return value of SetEncodedAuthTag indicates whether the string passed was a valid
    // representation of the encoding specified in the 2nd arg.
    success = CkCrypt2_SetEncodedAuthTag(crypt,T,"hex");

    // All of our properties (IV, secret key, cipher mode, and AAD) are already set from the code above...

    // So let's decrypt CT to and check to see if we get PT.
    ptResult = CkCrypt2_decryptEncoded(crypt,CT);
    if (CkCrypt2_getLastMethodSuccess(crypt) != TRUE) {
        // Failed.  The resultant authenticated tag did not equal the expected authentication tag.
        printf("%s\n",CkCrypt2_lastErrorText(crypt));
        CkCrypt2_Dispose(crypt);
        return;
    }

    // Examine the decrypted result.  It should be the same as our expected plaintext (case insensitive)
    printf("plaintext decrypted: %s\n",ptResult);
    printf("plaintext expected:  %s\n",PT);

    // Let's intentionally set the expected authenticated tag to an incorrect value. 
    // The decrypt operation should fail:
    tInvalid = "ffaabbbc3221a5db94fae95ae7121a47";

    success = CkCrypt2_SetEncodedAuthTag(crypt,tInvalid,"hex");

    ptResult = CkCrypt2_decryptEncoded(crypt,CT);
    if (CkCrypt2_getLastMethodSuccess(crypt) != TRUE) {
        // Failed.  The resultant authenticated tag did not equal the expected authentication tag.
        printf("%s\n",CkCrypt2_lastErrorText(crypt));
    }



    CkCrypt2_Dispose(crypt);

    }