Classic ASP

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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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<%
success = 0

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

set crypt = Server.CreateObject("Chilkat.Crypt2")

'  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 <> 1) Then
    Response.Write "<pre>" & Server.HTMLEncode( crypt.LastErrorText) & "</pre>"
    Response.End
End If

'  Examine the result.  It should be the same (case insensitive) as our expected result:
Response.Write "<pre>" & Server.HTMLEncode( "computed result: " & ctResult) & "</pre>"
Response.Write "<pre>" & Server.HTMLEncode( "expected result: " & CT) & "</pre>"

'  Examine the authenticated tag. It should be the same (case insensitive) as our expected authenticated tag:
tResult = crypt.GetEncodedAuthTag("hex")
Response.Write "<pre>" & Server.HTMLEncode( "computed authTag: " & tResult) & "</pre>"
Response.Write "<pre>" & Server.HTMLEncode( "expected authTag: " & T) & "</pre>"

'  -------------------------------------------------------------------------------------
'  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 <> 1) Then
    '  Failed.  The resultant authenticated tag did not equal the expected authentication tag.
    Response.Write "<pre>" & Server.HTMLEncode( crypt.LastErrorText) & "</pre>"
    Response.End
End If

'  Examine the decrypted result.  It should be the same as our expected plaintext (case insensitive)
Response.Write "<pre>" & Server.HTMLEncode( "plaintext decrypted: " & ptResult) & "</pre>"
Response.Write "<pre>" & Server.HTMLEncode( "plaintext expected:  " & PT) & "</pre>"

'  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 <> 1) Then
    '  Failed.  The resultant authenticated tag did not equal the expected authentication tag.
    Response.Write "<pre>" & Server.HTMLEncode( crypt.LastErrorText) & "</pre>"
End If


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