(Tcl) AES GCM Encrypt and Decrypt a File

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Demonstrates how to AES GCM encrypt and decrypt a file.

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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 256

# This is the 256-bit AES secret key (in hex format)
set K "000102030405060708090A0B0C0D0E0F000102030405060708090A0B0C0D0E0F"

# This is the 16-byte initialization vector (in hex format)
set IV "000102030405060708090A0B0C0D0E0F"

# This is the OPTIONAL additional data (in hex format) 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"

# 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"]

# Encrypt a file.
set inFile "qa_data/hamlet.xml"
set outFile "c:/temp/qa_output/hamlet_aes_gcm.enc"
set success [CkCrypt2_CkEncryptFile $crypt $inFile $outFile]
if {$success == 0} then {
    puts [CkCrypt2_lastErrorText $crypt]
    delete_CkCrypt2 $crypt
    exit
}

# Get the authentication tag in hex format
set authTag [CkCrypt2_getEncodedAuthTag $crypt "hex"]
puts "authentication tag = $authTag"

# Decrypt..

# Before decrypting, you must provide the expected authentication tag.
# The decrypt will fail if the resulting authentication tag computed while decrypting is not equal to the
# expected authentication tag.
CkCrypt2_SetEncodedAuthTag $crypt $authTag "hex"

set inFile $outFile
set outFile "c:/temp/qa_output/hamlet_restored.xml"
set success [CkCrypt2_CkDecryptFile $crypt $inFile $outFile]
if {$success == 0} then {
    puts [CkCrypt2_lastErrorText $crypt]
    delete_CkCrypt2 $crypt
    exit
}

puts "Success."

# --------------------------------------------------------------------------------------------
# About AES-GCM:

# AES-GCM (Advanced Encryption Standard - Galois/Counter Mode) is a widely-used
# encryption mode that provides both confidentiality (encryption) and
# integrity/authentication (data integrity verification) in one operation. It is
# commonly used in secure communications due to its efficiency and strong security
# properties.
# 
# Key Concepts:
# 
#     AES (Advanced Encryption Standard):
# 
#         AES is a symmetric encryption algorithm, meaning the same key is used
#         for both encryption and decryption.
# 
#         It operates on fixed-size blocks of data (128 bits) using key sizes of
#         128, 192, or 256 bits.
# 
#         In AES-GCM, AES is used to perform the actual data encryption.
# 
#     GCM (Galois/Counter Mode):
# 
#         Counter Mode (CTR): GCM uses counter mode for encryption. In this mode,
#         a nonce (or initialization vector, IV) and a counter are combined and encrypted
#         with AES. The result is XORed with the plaintext to produce the ciphertext. 
# 
#         Galois Mode (GMAC): GCM also includes an authentication mechanism based
#         on a Galois field. It generates an authentication tag, which ensures the
#         integrity of both the ciphertext and any additional data (called AAD -
#         Additional Authenticated Data). This tag is verified during decryption to ensure
#         that the data hasn't been tampered with.
# 
# Key Features:
# 
#     Confidentiality (Encryption):
# 
#         The plaintext is encrypted using AES in counter mode. Each block of
#         plaintext is XORed with the output of AES applied to a combination of the IV and
#         an incremented counter.
# 
#     Integrity (Authentication):
# 
#         In addition to encryption, GCM provides authentication for both the
#         encrypted data (ciphertext) and any Additional Authenticated Data (AAD), such as
#         headers or metadata that need to be protected but not encrypted.
# 
#         The authentication tag is generated using a Galois field multiplication
#         of the ciphertext and AAD. This ensures that any changes to the encrypted
#         message or the AAD will be detected during decryption.
# 
# Key Components:
# 
#     - Plaintext: The data you want to encrypt.
#     - Ciphertext: The encrypted data.
#     - Key: A symmetric key used for both encryption and decryption.
#     - Nonce/IV: A unique value used for each encryption to ensure security. It is not secret but should never be reused with the same key.
#     - AAD (Additional Authenticated Data): Optional data that is not encrypted but needs to be authenticated (e.g., headers).
#     - Authentication Tag: A tag generated to verify the integrity and authenticity of the ciphertext and AAD

delete_CkCrypt2 $crypt