Objective-C

AES GCM Encrypt and Decrypt a File

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Demonstrates how to AES GCM encrypt and decrypt a file. Objective-C

#import <CkoCrypt2.h>
#import <NSString.h>

BOOL success = NO;

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

CkoCrypt2 *crypt = [[CkoCrypt2 alloc] init];

//  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 = [NSNumber numberWithInt:256];

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

//  This is the 16-byte initialization vector (in hex format)
NSString *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.
NSString *AAD = @"feedfacedeadbeeffeedfacedeadbeefabaddad2";

//  Set the secret key and IV
[crypt SetEncodedIV: IV encoding: @"hex"];
[crypt SetEncodedKey: K encoding: @"hex"];

//  Set the additional authenticated data (AAD)
success = [crypt SetEncodedAad: AAD encoding: @"hex"];

//  Encrypt a file.
NSString *inFile = @"qa_data/hamlet.xml";
NSString *outFile = @"c:/temp/qa_output/hamlet_aes_gcm.enc";
success = [crypt CkEncryptFile: inFile destFile: outFile];
if (success == NO) {
    NSLog(@"%@",crypt.LastErrorText);
    return;
}

//  Get the authentication tag in hex format
NSString *authTag = [crypt GetEncodedAuthTag: @"hex"];
NSLog(@"%@%@",@"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.
[crypt SetEncodedAuthTag: authTag encoding: @"hex"];

inFile = outFile;
outFile = @"c:/temp/qa_output/hamlet_restored.xml";
success = [crypt CkDecryptFile: inFile destFile: outFile];
if (success == NO) {
    NSLog(@"%@",crypt.LastErrorText);
    return;
}

NSLog(@"%@",@"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.