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Visual FoxPro Examples

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(Visual FoxPro) 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".

Chilkat ActiveX Downloads

ActiveX for 32-bit and 64-bit Windows

LOCAL loCrypt
LOCAL lnSuccess
LOCAL K
LOCAL lcIV
LOCAL lcAAD
LOCAL lcPT
LOCAL lcCT
LOCAL T
LOCAL lcCtResult
LOCAL lcTResult
LOCAL lcPtResult
LOCAL lcTInvalid

loCrypt = CreateObject('Chilkat_9_5_0.Crypt2')

lnSuccess = loCrypt.UnlockComponent("Anything for 30-day trial")
IF (lnSuccess <> 1) THEN
    ? loCrypt.LastErrorText
    RELEASE loCrypt
    CANCEL
ENDIF

*  Set the encryption algorithm to "AES"
loCrypt.CryptAlgorithm = "aes"

*  Indicate that the Galois/Counter Mode (GCM) should be used:
loCrypt.CipherMode = "gcm"

*  KeyLength may be 128, 192, 256
loCrypt.KeyLength = 128

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

*  This is the 16-byte initialization vector:
lcIV = "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.
lcAAD = "feedfacedeadbeeffeedfacedeadbeefabaddad2"

*  The plain-text bytes (in hex format) to be encrypted.
lcPT = "d9313225f88406e5a55909c5aff5269a86a7a9531534f7da2e4c303d8a318a721c3c0c95956809532fcf0e2449a6b525b16aedf5aa0de657ba637b39"

*  The expected cipher text (in hex format)
lcCT = "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".
loCrypt.EncodingMode = "hex"

*  Set the secret key and IV
loCrypt.SetEncodedIV(lcIV,"hex")
loCrypt.SetEncodedKey(K,"hex")

*  Set the additional authenticated data (AAD)
lnSuccess = loCrypt.SetEncodedAad(lcAAD,"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.
lcCtResult = loCrypt.EncryptEncoded(lcPT)
IF (loCrypt.LastMethodSuccess <> 1) THEN
    ? loCrypt.LastErrorText
    RELEASE loCrypt
    CANCEL
ENDIF

*  Examine the result.  It should be the same (case insensitive) as our expected result:
? "computed result: " + lcCtResult
? "expected result: " + lcCT

*  Examine the authenticated tag. It should be the same (case insensitive) as our expected authenticated tag:
lcTResult = loCrypt.GetEncodedAuthTag("hex")
? "computed authTag: " + lcTResult
? "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.
lnSuccess = loCrypt.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.
lcPtResult = loCrypt.DecryptEncoded(lcCT)
IF (loCrypt.LastMethodSuccess <> 1) THEN
    *  Failed.  The resultant authenticated tag did not equal the expected authentication tag.
    ? loCrypt.LastErrorText
    RELEASE loCrypt
    CANCEL
ENDIF

*  Examine the decrypted result.  It should be the same as our expected plaintext (case insensitive)
? "plaintext decrypted: " + lcPtResult
? "plaintext expected:  " + lcPT

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

lnSuccess = loCrypt.SetEncodedAuthTag(lcTInvalid,"hex")

lcPtResult = loCrypt.DecryptEncoded(lcCT)
IF (loCrypt.LastMethodSuccess <> 1) THEN
    *  Failed.  The resultant authenticated tag did not equal the expected authentication tag.
    ? loCrypt.LastErrorText
ENDIF

RELEASE loCrypt


 

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