(Visual FoxPro) Decrypt 256-bit AES GCM Produced by Something Unknown

Demonstrates how to decrypt something produced elsewhere (unknown) with 256-bit AES GCM.

Chilkat ActiveX Downloads ActiveX for 32-bit and 64-bit Windows

LOCAL lcKeyBase64
LOCAL lcIvBase64Url
LOCAL lcCipherBase64Url
LOCAL loCrypt
LOCAL lnSuccess
LOCAL loBdEncrypted
LOCAL loBdAuthTag
LOCAL lnNumBytes
LOCAL lcAuthTagHex
LOCAL lcOriginalText

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

* We have the following to decrypt:

* Key (Base64): 
lcKeyBase64 = "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"

* IV (Base64Url):
lcIvBase64Url = "xrvaINMLqotAbWRK"

* ciphertext (base64url):
lcCipherBase64Url = "RtGNAS-zQOxSB8W0HfqJjCoyt9KgImW_l-HjVC40hOOl-RNfRF3hzDIT1kvFVF8i_KX9XmqAftb6lyq-jLCEc_MSgqt3q1ixv3Ez4SbS3G5e3qGzLwxIMi2sCt00aDNwK2ipsJ4aw8s7ePPnl4oY-y1st9rwCWR0rrgEZwS9jmS4uJWGPn9K3jbKRnMslznDbtFLNJctMVXBTP-cv47JelxLCBOQSlK29rMuEFrhHR_VQrPq6gtZaBVSXZSYT0XOklp7nu9mVhrMCRtBCC5oiu5MPE5JYx4ANo3hUY7_NyQl2bpn9GfRXrdvqRGE-gy2upj-cDkm0t_tV8xmYge9DBQTH3B_4BGl2qTk_o-m7pEmKkS8XSdQhGcuFlykqrkE8SzB5I8esfzWOM0pwxbz0H_VaylKYHY="

* For versions of Chilkat < 10.0.0, use CreateObject('Chilkat_9_5_0.Crypt2')
loCrypt = CreateObject('Chilkat.Crypt2')

loCrypt.CryptAlgorithm = "aes"
loCrypt.CipherMode = "gcm"
loCrypt.KeyLength = 256

lnSuccess = loCrypt.SetEncodedAad("random","ascii")
loCrypt.SetEncodedKey(lcKeyBase64,"base64")
loCrypt.SetEncodedIV(lcIvBase64Url,"base64url")

* The cipher text contains the 16-byte auth tag at the end.
* get it separately..
* For versions of Chilkat < 10.0.0, use CreateObject('Chilkat_9_5_0.BinData')
loBdEncrypted = CreateObject('Chilkat.BinData')
* For versions of Chilkat < 10.0.0, use CreateObject('Chilkat_9_5_0.BinData')
loBdAuthTag = CreateObject('Chilkat.BinData')
lnSuccess = loBdEncrypted.AppendEncoded(lcCipherBase64Url,"base64url")

lnNumBytes = loBdEncrypted.NumBytes
lcAuthTagHex = loBdEncrypted.GetEncodedChunk(lnNumBytes - 16,16,"hex")

? "Auth tag in hex: " + lcAuthTagHex

lnSuccess = loBdAuthTag.AppendEncoded(lcAuthTagHex,"hex")
loBdEncrypted.RemoveChunk(lnNumBytes - 16,16)

* Use this special value to tell Chilkat to ignore the auth tag.
lnSuccess = loCrypt.SetEncodedAuthTag("FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF","hex")

* Decrypt
loCrypt.EncodingMode = "base64"
lcOriginalText = loCrypt.DecryptStringENC(loBdEncrypted.GetEncoded("base64"))
IF (loCrypt.LastMethodSuccess = 0) THEN
    ? loCrypt.LastErrorText
ELSE
    ? lcOriginalText
    ? "Success."
ENDIF

* Decrypted text

RELEASE loCrypt
RELEASE loBdEncrypted
RELEASE loBdAuthTag