(PureBasic) 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 PureBasic Module Download Chilkat PureBasic Module

IncludeFile "CkCrypt2.pb"

Procedure ChilkatExample()

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

    crypt.i = CkCrypt2::ckCreate()
    If crypt.i = 0
        Debug "Failed to create object."
        ProcedureReturn
    EndIf

    ; Set the encryption algorithm to "AES"	
    CkCrypt2::setCkCryptAlgorithm(crypt, "aes")

    ; Indicate that the Galois/Counter Mode (GCM) should be used:
    CkCrypt2::setCkCipherMode(crypt, "gcm")

    ; KeyLength may be 128, 192, 256
    CkCrypt2::setCkKeyLength(crypt, 128)

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

    ; This is the 16-byte initialization vector:
    IV.s = "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.s = "feedfacedeadbeeffeedfacedeadbeefabaddad2"

    ; The plain-text bytes (in hex format) to be encrypted.
    PT.s = "d9313225f88406e5a55909c5aff5269a86a7a9531534f7da2e4c303d8a318a721c3c0c95956809532fcf0e2449a6b525b16aedf5aa0de657ba637b39"

    ; The expected cipher text (in hex format)
    CT.s = "42831ec2217774244b7221b784d0d49ce3aa212f2c02a4e035c17e2329aca12e21d514b25466931c7d8f6a5aac84aa051ba30b396a0aac973d58e091"

    ; The expected authenticated tag given the above inputs.
    T.s = "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".
    CkCrypt2::setCkEncodingMode(crypt, "hex")

    ; Set the secret key and IV
    CkCrypt2::ckSetEncodedIV(crypt,IV,"hex")
    CkCrypt2::ckSetEncodedKey(crypt,K,"hex")

    ; Set the additional authenticated data (AAD)
    success.i = CkCrypt2::ckSetEncodedAad(crypt,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.s = CkCrypt2::ckEncryptEncoded(crypt,PT)
    If CkCrypt2::ckLastMethodSuccess(crypt) <> 1
        Debug CkCrypt2::ckLastErrorText(crypt)
        CkCrypt2::ckDispose(crypt)
        ProcedureReturn
    EndIf

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

    ; Examine the authenticated tag. It should be the same (case insensitive) as our expected authenticated tag:
    tResult.s = CkCrypt2::ckGetEncodedAuthTag(crypt,"hex")
    Debug "computed authTag: " + tResult
    Debug "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 = CkCrypt2::ckSetEncodedAuthTag(crypt,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.s = CkCrypt2::ckDecryptEncoded(crypt,CT)
    If CkCrypt2::ckLastMethodSuccess(crypt) <> 1
        ; Failed.  The resultant authenticated tag did not equal the expected authentication tag.
        Debug CkCrypt2::ckLastErrorText(crypt)
        CkCrypt2::ckDispose(crypt)
        ProcedureReturn
    EndIf

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

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

    success = CkCrypt2::ckSetEncodedAuthTag(crypt,tInvalid,"hex")

    ptResult = CkCrypt2::ckDecryptEncoded(crypt,CT)
    If CkCrypt2::ckLastMethodSuccess(crypt) <> 1
        ; Failed.  The resultant authenticated tag did not equal the expected authentication tag.
        Debug CkCrypt2::ckLastErrorText(crypt)
    EndIf



    CkCrypt2::ckDispose(crypt)


    ProcedureReturn
EndProcedure