B4X

Example: Crypt2.RandomizeIV method

Demonstrates using a random initialization vector for AES GCM encryption. B4X

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 ChilkatCrypt2
crypt.Initialize("crypt")

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

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

'  Generate a random IV.
crypt.RandomizeIV
Dim IV As String = crypt.GetEncodedIV("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
    Log(crypt.LastErrorText)
    Return
End If


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

Log("Cipher Text: " & cipherText)
Log("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 ChilkatBinData
bdEncrypted.Initialize
bdEncrypted.AppendEncoded(IV, "hex")
bdEncrypted.AppendEncoded(cipherText, "base64")
bdEncrypted.AppendEncoded(authTag, "base64")

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

'  Sample output so far:

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

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

'  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 ChilkatBinData
bdFromEncryptor.Initialize
bdFromEncryptor.AppendEncoded(concatenatedGcmOutput, "base64")

Dim sz As Int = 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.
    Log(decrypt.LastErrorText)
    Return
End If


Log("Decrypted: " & decryptedText)