(Tcl) 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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load ./chilkat.dll

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

set crypt [new_CkCrypt2]

# Set the encryption algorithm to "AES"	
CkCrypt2_put_CryptAlgorithm $crypt "aes"

# Indicate that the Galois/Counter Mode (GCM) should be used:
CkCrypt2_put_CipherMode $crypt "gcm"

# KeyLength may be 128, 192, 256
CkCrypt2_put_KeyLength $crypt 128

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

# This is the 16-byte initialization vector:
set 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.
set AAD "feedfacedeadbeeffeedfacedeadbeefabaddad2"

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

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

# The expected authenticated tag given the above inputs.
set 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".
CkCrypt2_put_EncodingMode $crypt "hex"

# Set the secret key and IV
CkCrypt2_SetEncodedIV $crypt $IV "hex"
CkCrypt2_SetEncodedKey $crypt $K "hex"

# Set the additional authenticated data (AAD)
set success [CkCrypt2_SetEncodedAad $crypt $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.
set ctResult [CkCrypt2_encryptEncoded $crypt $PT]
if {[CkCrypt2_get_LastMethodSuccess $crypt] != 1} then {
    puts [CkCrypt2_lastErrorText $crypt]
    delete_CkCrypt2 $crypt
    exit
}

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

# Examine the authenticated tag. It should be the same (case insensitive) as our expected authenticated tag:
set tResult [CkCrypt2_getEncodedAuthTag $crypt "hex"]
puts "computed authTag: $tResult"
puts "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.
set success [CkCrypt2_SetEncodedAuthTag $crypt $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.
set ptResult [CkCrypt2_decryptEncoded $crypt $CT]
if {[CkCrypt2_get_LastMethodSuccess $crypt] != 1} then {
    # Failed.  The resultant authenticated tag did not equal the expected authentication tag.
    puts [CkCrypt2_lastErrorText $crypt]
    delete_CkCrypt2 $crypt
    exit
}

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

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

set success [CkCrypt2_SetEncodedAuthTag $crypt $tInvalid "hex"]

set ptResult [CkCrypt2_decryptEncoded $crypt $CT]
if {[CkCrypt2_get_LastMethodSuccess $crypt] != 1} then {
    # Failed.  The resultant authenticated tag did not equal the expected authentication tag.
    puts [CkCrypt2_lastErrorText $crypt]
}


delete_CkCrypt2 $crypt