(Excel) 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".

Download Excel Class Modules Chilkat Excel Class Modules

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

Dim crypt As Chilkat.Crypt2
Set crypt = Chilkat.NewCrypt2

' 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)

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".
crypt.EncodingMode = "hex"

' Set the secret key and IV
crypt.SetEncodedIV IV,"hex"
crypt.SetEncodedKey K,"hex"

' Set the additional authenticated data (AAD)

success = 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.

ctResult = crypt.EncryptEncoded(PT)
If (crypt.LastMethodSuccess <> True) Then
    Debug.Print crypt.LastErrorText
    Exit Sub
End If

' Examine the result.  It should be the same (case insensitive) as our expected result:
Debug.Print "computed result: "; ctResult
Debug.Print "expected result: "; CT

' Examine the authenticated tag. It should be the same (case insensitive) as our expected authenticated tag:

tResult = crypt.GetEncodedAuthTag("hex")
Debug.Print "computed authTag: "; tResult
Debug.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.
success = 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.

ptResult = crypt.DecryptEncoded(CT)
If (crypt.LastMethodSuccess <> True) Then
    ' Failed.  The resultant authenticated tag did not equal the expected authentication tag.
    Debug.Print crypt.LastErrorText
    Exit Sub
End If

' Examine the decrypted result.  It should be the same as our expected plaintext (case insensitive)
Debug.Print "plaintext decrypted: "; ptResult
Debug.Print "plaintext expected:  "; PT

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

tInvalid = "ffaabbbc3221a5db94fae95ae7121a47"

success = crypt.SetEncodedAuthTag(tInvalid,"hex")

ptResult = crypt.DecryptEncoded(CT)
If (crypt.LastMethodSuccess <> True) Then
    ' Failed.  The resultant authenticated tag did not equal the expected authentication tag.
    Debug.Print crypt.LastErrorText
End If