Dart

AEAD AES 128-bit GCM

See more Encryption Examples

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". Dart

import 'package:chilkat/chilkat.dart';

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

  final crypt = CkCrypt2();

  // 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 = 128;

  // This is the 128-bit AES secret key (in hex format)
  final k = 'feffe9928665731c6d6a8f9467308308';

  // This is the 16-byte initialization vector:
  final 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.
  final aad = 'feedfacedeadbeeffeedfacedeadbeefabaddad2';

  // The plain-text bytes (in hex format) to be encrypted.
  final pt = 'd9313225f88406e5a55909c5aff5269a86a7a9531534f7da2e4c303d8a318a721c3c0c95956809532fcf0e2449a6b525b16aedf5aa0de657ba637b39';

  // The expected cipher text (in hex format)
  final ct = '42831ec2217774244b7221b784d0d49ce3aa212f2c02a4e035c17e2329aca12e21d514b25466931c7d8f6a5aac84aa051ba30b396a0aac973d58e091';

  // The expected authenticated tag given the above inputs.
  final 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".
  crypt.encodingMode = 'hex';

  // Set the secret key and IV
  crypt.setEncodedIV(iv, 'hex');
  crypt.setEncodedKey(k, 'hex');

  // Set the additional authenticated data (AAD)
  crypt.setEncodedAad(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.
  final String ctResult;
  try {
    ctResult = crypt.encryptEncoded(pt);
  } on ChilkatException catch (e) {
    print(e.lastErrorText);
    return;
  }

  // Examine the result.  It should be the same (case insensitive) as our expected result:
  print('computed result: $ctResult');
  print('expected result: $ct');

  // Examine the authenticated tag. It should be the same (case insensitive) as our expected authenticated tag:
  final tResult = crypt.getEncodedAuthTag('hex');
  print('computed authTag: $tResult');
  print('expected authTag: $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.
  crypt.setEncodedAuthTag(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.
  String ptResult;
  try {
    ptResult = crypt.decryptEncoded(ct);
  } on ChilkatException catch (e) {
    // Failed.  The resultant authenticated tag did not equal the expected authentication tag.
    print(e.lastErrorText);
    return;
  }

  // Examine the decrypted result.  It should be the same as our expected plaintext (case insensitive)
  print('plaintext decrypted: $ptResult');
  print('plaintext expected:  $pt');

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

  crypt.setEncodedAuthTag(tInvalid, 'hex');

  try {
    ptResult = crypt.decryptEncoded(ct);
  } on ChilkatException catch (e) {
    // Failed.  The resultant authenticated tag did not equal the expected authentication tag.
    print(e.lastErrorText);
  }
}