(Chilkat2-Python) AEAD AES 128-bit GCM

Demonstrates AES encryption using the Galois/Counter Mode (GCM). GCM is an authenticated encryption mode with "additional data" (often referred to as AEAD). GCM is a cipher mode that can be applied to any symmetric encryption algorithm with a 16-byte block size, such as AES and Twofish. In GCM mode, the block encryption algorithm is transformed into a stream encryption algorithm, and therefore no padding occurs (and the PaddingScheme property does not apply). The "additional data" (known as the AAD) does not get encrypted but plays a role in the computation of the resulting "authenticated tag".

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import sys
import chilkat2

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

crypt = chilkat2.Crypt2()

# Set the encryption algorithm to "AES"	
crypt.CryptAlgorithm = "aes"

# Indicate that the Galois/Counter Mode (GCM) should be used:
crypt.CipherMode = "gcm"

# KeyLength may be 128, 192, 256
crypt.KeyLength = 128

# This is the 128-bit AES secret key (in hex format)
K = "feffe9928665731c6d6a8f9467308308"

# This is the 16-byte initialization vector:
IV = "cafebabefacedbaddecaf888"

# This is the additional data to be used as input to the GCM AEAD algorithm,
# but is not included in the output.  It plays a role in the computation of the
# resulting authenticated tag.
AAD = "feedfacedeadbeeffeedfacedeadbeefabaddad2"

# The plain-text bytes (in hex format) to be encrypted.
PT = "d9313225f88406e5a55909c5aff5269a86a7a9531534f7da2e4c303d8a318a721c3c0c95956809532fcf0e2449a6b525b16aedf5aa0de657ba637b39"

# The expected cipher text (in hex format)
CT = "42831ec2217774244b7221b784d0d49ce3aa212f2c02a4e035c17e2329aca12e21d514b25466931c7d8f6a5aac84aa051ba30b396a0aac973d58e091"

# The expected authenticated tag given the above inputs.
T = "5bc94fbc3221a5db94fae95ae7121a47"

# Note: The above data are the values for test vector #4 from 
# the PDF document at: http://csrc.nist.gov/groups/ST/toolkit/BCM/documents/proposedmodes/gcm/gcm-spec.pdf

# EncodingMode specifies the encoding of the output for
# encryption, and the input for decryption.
# It may be "hex", "url", "base64", or "quoted-printable".
crypt.EncodingMode = "hex"

# Set the secret key and IV
crypt.SetEncodedIV(IV,"hex")
crypt.SetEncodedKey(K,"hex")

# Set the additional authenticated data (AAD)
success = crypt.SetEncodedAad(AAD,"hex")

# For the purpose of duplicating the test vectors, we are using the EncryptEncoded method.
# This method decodes the input string according to the encoding specified by the EncodingMode
# property, which in this case is "hex".  The decoded bytes are encrypted using the mode specified
# by the CipherMode property.  The resulting
# encrypted bytes are encoded (again using the encoding mode specified by EncodingMode),
# and the result is returned.
# <b>Note:</b> The CipherMode property sets the block mode of operation (gcm, cfb, cbc, ofb, ecb, etc.) 
# for any of the Chilkat encryption/decryption methods (such as EncryptBytes, EncryptString, 
# CkEncryptFile, etc.)   Just because GCM mode is demonstrated with EncryptEncoded/DecryptEncoded,
# does not imply that GCM mode is specific to only these methods.
ctResult = crypt.EncryptEncoded(PT)
if (crypt.LastMethodSuccess != True):
    print(crypt.LastErrorText)
    sys.exit()

# Examine the result.  It should be the same (case insensitive) as our expected result:
print("computed result: " + ctResult)
print("expected result: " + CT)

# Examine the authenticated tag. It should be the same (case insensitive) as our expected authenticated tag:
tResult = crypt.GetEncodedAuthTag("hex")
print("computed authTag: " + tResult)
print("expected authTag: " + T)

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

# 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 (case insensitive) to 
# the expected result.
# Note: The return value of SetEncodedAuthTag indicates whether the string passed was a valid
# representation of the encoding specified in the 2nd arg.
success = crypt.SetEncodedAuthTag(T,"hex")

# All of our properties (IV, secret key, cipher mode, and AAD) are already set from the code above...

# So let's decrypt CT to and check to see if we get PT.
ptResult = crypt.DecryptEncoded(CT)
if (crypt.LastMethodSuccess != True):
    # Failed.  The resultant authenticated tag did not equal the expected authentication tag.
    print(crypt.LastErrorText)
    sys.exit()

# Examine the decrypted result.  It should be the same as our expected plaintext (case insensitive)
print("plaintext decrypted: " + ptResult)
print("plaintext expected:  " + PT)

# Let's intentionally set the expected authenticated tag to an incorrect value. 
# The decrypt operation should fail:
tInvalid = "ffaabbbc3221a5db94fae95ae7121a47"

success = crypt.SetEncodedAuthTag(tInvalid,"hex")

ptResult = crypt.DecryptEncoded(CT)
if (crypt.LastMethodSuccess != True):
    # Failed.  The resultant authenticated tag did not equal the expected authentication tag.
    print(crypt.LastErrorText)