Visual FoxPro

AES GCM Encrypt and Decrypt a File

See more Encryption Examples

Demonstrates how to AES GCM encrypt and decrypt a file. Visual FoxPro

LOCAL lnSuccess
LOCAL loCrypt
LOCAL K
LOCAL lcIV
LOCAL lcAAD
LOCAL lcInFile
LOCAL lcOutFile
LOCAL lcAuthTag

lnSuccess = 0

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

loCrypt = CreateObject('Chilkat.Crypt2')

* Set the encryption algorithm to "AES"	
loCrypt.CryptAlgorithm = "aes"

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

* KeyLength may be 128, 192, 256
loCrypt.KeyLength = 256

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

* This is the 16-byte initialization vector (in hex format)
lcIV = "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.
lcAAD = "feedfacedeadbeeffeedfacedeadbeefabaddad2"

* Set the secret key and IV
loCrypt.SetEncodedIV(lcIV,"hex")
loCrypt.SetEncodedKey(K,"hex")

* Set the additional authenticated data (AAD)
lnSuccess = loCrypt.SetEncodedAad(lcAAD,"hex")

* Encrypt a file.
lcInFile = "qa_data/hamlet.xml"
lcOutFile = "c:/temp/qa_output/hamlet_aes_gcm.enc"
lnSuccess = loCrypt.CkEncryptFile(lcInFile,lcOutFile)
IF (lnSuccess = 0) THEN
    ? loCrypt.LastErrorText
    RELEASE loCrypt
    CANCEL
ENDIF

* Get the authentication tag in hex format
lcAuthTag = loCrypt.GetEncodedAuthTag("hex")
? "authentication tag = " + lcAuthTag

* 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.
loCrypt.SetEncodedAuthTag(lcAuthTag,"hex")

lcInFile = lcOutFile
lcOutFile = "c:/temp/qa_output/hamlet_restored.xml"
lnSuccess = loCrypt.CkDecryptFile(lcInFile,lcOutFile)
IF (lnSuccess = 0) THEN
    ? loCrypt.LastErrorText
    RELEASE loCrypt
    CANCEL
ENDIF

? "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

RELEASE loCrypt