B4X

Decrypt 256-bit AES GCM Produced by Something Unknown

See more Encryption Examples

Demonstrates how to decrypt something produced elsewhere (unknown) with 256-bit AES GCM. B4X

Dim success As Boolean = False

'  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 = "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"

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

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

Dim crypt As ChilkatCrypt2
crypt.Initialize("crypt")

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

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 ChilkatBinData
bdEncrypted.Initialize
Dim bdAuthTag As ChilkatBinData
bdAuthTag.Initialize
success = bdEncrypted.AppendEncoded(cipherBase64Url, "base64url")

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

Log("Auth tag in hex: " & authTagHex)

success = bdAuthTag.AppendEncoded(authTagHex, "hex")
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 = crypt.DecryptStringENC(bdEncrypted.GetEncoded("base64"))
If crypt.LastMethodSuccess = False Then
    Log(crypt.LastErrorText)
Else
    Log(originalText)
    Log("Success.")
End If


'  Decrypted text