Chilkat2-Python

Example: Crypt2.RandomizeIV method

Demonstrates using a random initialization vector for AES GCM encryption. Chilkat2-Python

import sys
import chilkat2

success = False

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

crypt = chilkat2.Crypt2()

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

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

# Generate a random IV.
crypt.RandomizeIV()
IV = 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"
cipherText = crypt.EncryptStringENC(PT)
if (crypt.LastMethodSuccess != True):
    print(crypt.LastErrorText)
    sys.exit()

# Get the GCM authenticated tag computed when encrypting.
authTag = crypt.GetEncodedAuthTag("base64")

print("Cipher Text: " + cipherText)
print("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.
bdEncrypted = chilkat2.BinData()
bdEncrypted.AppendEncoded(IV,"hex")
bdEncrypted.AppendEncoded(cipherText,"base64")
bdEncrypted.AppendEncoded(authTag,"base64")

concatenatedGcmOutput = bdEncrypted.GetEncoded("base64")
print("Concatenated GCM Output: " + concatenatedGcmOutput)

# Sample output so far:

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

decrypt = chilkat2.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")

bdFromEncryptor = chilkat2.BinData()
bdFromEncryptor.AppendEncoded(concatenatedGcmOutput,"base64")

sz = bdFromEncryptor.NumBytes

# Extract the parts.
extractedIV = bdFromEncryptor.GetEncodedChunk(0,16,"hex")
extractedCipherText = bdFromEncryptor.GetEncodedChunk(16,sz - 32,"base64")
expectedAuthTag = 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"
decryptedText = decrypt.DecryptStringENC(extractedCipherText)
if (decrypt.LastMethodSuccess != True):
    # Failed.  The resultant authenticated tag did not equal the expected authentication tag.
    print(decrypt.LastErrorText)
    sys.exit()

print("Decrypted: " + decryptedText)