VB.NET

AES-GCM Encryption / Decryption

See more Encryption Examples

Demonstrates AES-GCM encryption. Afterwards, the encrypted bytes, the authentication tag, and the IV are concatenated into one byte array and encoded to base64. For decryption, we decode the base64, extract the IV, authentication tag, and encrypted bytes, and then perform AES-GCM decryption. VB.NET

Dim success As Boolean = False

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

Dim crypt As New Chilkat.Crypt2

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

Dim K As String = "000102030405060708090A0B0C0D0E0F000102030405060708090A0B0C0D0E0F"
Dim IV As String = "000102030405060708090A0B0C0D0E0F"
Dim AAD As String = "feedfacedeadbeeffeedfacedeadbeefabaddad2"
Dim PT As String = "This is the text to be AES-GCM encrypted."

crypt.SetEncodedIV(IV,"hex")
crypt.SetEncodedKey(K,"hex")

success = crypt.SetEncodedAad(AAD,"hex")

' Return the encrypted bytes as base64
crypt.EncodingMode = "base64"
crypt.Charset = "utf-8"
Dim cipherText As String = crypt.EncryptStringENC(PT)
If (crypt.LastMethodSuccess <> True) Then
    Debug.WriteLine(crypt.LastErrorText)
    Exit Sub
End If


' Get the GCM authenticated tag computed when encrypting.
Dim authTag As String = crypt.GetEncodedAuthTag("base64")

Debug.WriteLine("Cipher Text: " & cipherText)
Debug.WriteLine("Auth Tag: " & authTag)

' Let's send the IV, CipherText, and AuthTag to the decrypting party.
' We'll send them concatenated like this: [IV || Ciphertext || AuthTag]
' In base64 format.
Dim bdEncrypted As New Chilkat.BinData
bdEncrypted.AppendEncoded(IV,"hex")
bdEncrypted.AppendEncoded(cipherText,"base64")
bdEncrypted.AppendEncoded(authTag,"base64")

Dim concatenatedGcmOutput As String = bdEncrypted.GetEncoded("base64")
Debug.WriteLine("Concatenated GCM Output: " & concatenatedGcmOutput)

' Sample output so far:

' -------------------------------------------------------------------------------------
' Now let's GCM decrypt...
' -------------------------------------------------------------------------------------

Dim decrypt As New Chilkat.Crypt2

' The values shared and agreed upon by both sides beforehand are: algorithm, cipher mode, secret key, and AAD.
' Sometimes the IV can be a value already known and agreed upon, but in this case the encryptor sends the IV to the decryptor.
decrypt.CryptAlgorithm = "aes"
decrypt.CipherMode = "gcm"
decrypt.KeyLength = 256
decrypt.SetEncodedKey(K,"hex")
decrypt.SetEncodedAad(AAD,"hex")

Dim bdFromEncryptor As New Chilkat.BinData
bdFromEncryptor.AppendEncoded(concatenatedGcmOutput,"base64")

Dim sz As Integer = bdFromEncryptor.NumBytes

' Extract the parts.
Dim extractedIV As String = bdFromEncryptor.GetEncodedChunk(0,16,"hex")
Dim extractedCipherText As String = bdFromEncryptor.GetEncodedChunk(16,sz - 32,"base64")
Dim expectedAuthTag As String = bdFromEncryptor.GetEncodedChunk(sz - 16,16,"base64")

' 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 to the expected result.
success = decrypt.SetEncodedAuthTag(expectedAuthTag,"base64")

' Also set the IV.
decrypt.SetEncodedIV(extractedIV,"hex")

' Decrypt..
decrypt.EncodingMode = "base64"
decrypt.Charset = "utf-8"
Dim decryptedText As String = decrypt.DecryptStringENC(extractedCipherText)
If (decrypt.LastMethodSuccess <> True) Then
    ' Failed.  The resultant authenticated tag did not equal the expected authentication tag.
    Debug.WriteLine(decrypt.LastErrorText)
    Exit Sub
End If


Debug.WriteLine("Decrypted: " & decryptedText)

' Sample output:

' Cipher Text: cYspSW4GuSj0Msho4OgZZ0AwspDEpTF5Br8NlA+qT3f+g3nQo+xalmU=
' Auth Tag: z/N82vdj/ZsM0WnHNCnPPw==
' Concatenated GCM Output: AAECAwQFBgcICQoLDA0OD3GLKUluBrko9DLIaODoGWdAMLKQxKUxeQa/DZQPqk93/oN50KPsWpZlz/N82vdj/ZsM0WnHNCnPPw==
' Decrypted: This is the text to be AES-GCM encrypted.