(Visual Basic 6.0) 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

' 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): 
Dim keyBase64 As String
keyBase64 = "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"

' IV (Base64Url):
Dim ivBase64Url As String
ivBase64Url = "xrvaINMLqotAbWRK"

' ciphertext (base64url):
Dim cipherBase64Url As String
cipherBase64Url = "RtGNAS-zQOxSB8W0HfqJjCoyt9KgImW_l-HjVC40hOOl-RNfRF3hzDIT1kvFVF8i_KX9XmqAftb6lyq-jLCEc_MSgqt3q1ixv3Ez4SbS3G5e3qGzLwxIMi2sCt00aDNwK2ipsJ4aw8s7ePPnl4oY-y1st9rwCWR0rrgEZwS9jmS4uJWGPn9K3jbKRnMslznDbtFLNJctMVXBTP-cv47JelxLCBOQSlK29rMuEFrhHR_VQrPq6gtZaBVSXZSYT0XOklp7nu9mVhrMCRtBCC5oiu5MPE5JYx4ANo3hUY7_NyQl2bpn9GfRXrdvqRGE-gy2upj-cDkm0t_tV8xmYge9DBQTH3B_4BGl2qTk_o-m7pEmKkS8XSdQhGcuFlykqrkE8SzB5I8esfzWOM0pwxbz0H_VaylKYHY="

Dim crypt As New ChilkatCrypt2

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

Dim success As Long
success = crypt.SetEncodedAad("random","ascii")
crypt.SetEncodedKey keyBase64,"base64"
crypt.SetEncodedIV ivBase64Url,"base64url"

' The cipher text contains the 16-byte auth tag at the end.
' get it separately..
Dim bdEncrypted As New ChilkatBinData
Dim bdAuthTag As New ChilkatBinData
success = bdEncrypted.AppendEncoded(cipherBase64Url,"base64url")

Dim numBytes As Long
numBytes = bdEncrypted.NumBytes
Dim authTagHex As String
authTagHex = bdEncrypted.GetEncodedChunk(numBytes - 16,16,"hex")

Debug.Print "Auth tag in hex: " & authTagHex

success = bdAuthTag.AppendEncoded(authTagHex,"hex")
success = bdEncrypted.RemoveChunk(numBytes - 16,16)

' Use this special value to tell Chilkat to ignore the auth tag.
success = crypt.SetEncodedAuthTag("FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF","hex")

' Decrypt
crypt.EncodingMode = "base64"
Dim originalText As String
originalText = crypt.DecryptStringENC(bdEncrypted.GetEncoded("base64"))
If (crypt.LastMethodSuccess = 0) Then
    Debug.Print crypt.LastErrorText
Else
    Debug.Print originalText
    Debug.Print "Success."
End If

' Decrypted text