Visual FoxPro

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". Visual FoxPro

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

lnSuccess = 0

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

loCrypt = CreateObject('Chilkat.Crypt2')

* 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