(Objective-C) RSA Encrypt/Decrypt AES Key

Demonstrates how to use RSA to protect a key for AES encryption. It can be used in this scenario: You will provide your RSA public key to any number of counterparts. Your counterpart will generate an AES key, encrypt data (or a file) using it, then encrypt the AES key using your RSA public key. Your counterpart sends you both the encrypted data and the encrypted key. Since you are the only one with access to the RSA private key, only you can decrypt the AES key. You decrypt the key, then decrypt the data using the AES key.

This example will show the entire process. (1) Generate an RSA key and save both private and public parts to PEM files. (2) Encrypt a file using a randomly generated AES encryption key. (3) RSA encrypt the AES key. (4) RSA decrypt the AES key. (5) Use it to AES decrypt the file or data.

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#import <CkoRsa.h>
#import <NSString.h>
#import <CkoPublicKey.h>
#import <CkoPrivateKey.h>
#import <CkoCrypt2.h>

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

// ------------------
// Step 1.  Generate an RSA key and save to PEM files.

CkoRsa *rsa = [[CkoRsa alloc] init];

// Generate a 1024-bit key. 
BOOL success = [rsa GenerateKey: [NSNumber numberWithInt: 1024]];
if (success != YES) {
    NSLog(@"%@",rsa.LastErrorText);
    return;
}

// Keys are exported in XML format:
NSString *pubKeyXml = [rsa ExportPublicKey];

CkoPublicKey *pubKey = [[CkoPublicKey alloc] init];
success = [pubKey LoadFromString: pubKeyXml];
// For brevity, we are not checking the return value...
success = [pubKey SavePemFile: NO path: @"qa_temp/pubKey.pem"];

NSString *privKeyXml = [rsa ExportPrivateKey];

CkoPrivateKey *privKey = [[CkoPrivateKey alloc] init];
success = [privKey LoadXml: privKeyXml];
success = [privKey SavePemFile: @"qa_temp/privKey.pem"];

// Other methods exist for saving the private key in PKCS8 format,
// both encrypted and un-encrypted..

// ------------------
// Step 2.   This is the code your counterpart will run to 
// AES encrypt a file.  It generates a random AES key
// to use for the encryption.

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

crypt.CryptAlgorithm = @"aes";
crypt.CipherMode = @"cbc";
crypt.KeyLength = [NSNumber numberWithInt:256];

// Generate random bytes and return the random bytes
// in a hex string:
crypt.EncodingMode = @"hex";
NSString *randomKey = [crypt GenRandomBytesENC: [NSNumber numberWithInt: 32]];
NSLog(@"%@%@",@"AES key = ",randomKey);

// Set the key.
[crypt SetEncodedKey: randomKey encoding: @"hex"];

// Set the IV to a known value that will be used on both sides.
// (If desired, you could generate a random IV and protect it in the same
// way as the key...)
// The length of the IV for AES is always 16 bytes, regardless of the key size.
[crypt SetEncodedIV: @"000102030405060708090A0B0C0D0E0F" encoding: @"hex"];

// AES Encrypt the file (the file may be any size because it will
// stream the file in/out.
success = [crypt CkEncryptFile: @"qa_data/hamlet.xml" destFile: @"qa_temp/aesEncrypted.dat"];
if (success != YES) {
    NSLog(@"%@",crypt.LastErrorText);
    return;
}

// ------------------
// Step 3.
// ------------------
// RSA encrypt the AES key.
// We'll pretend  your counter part has the public-key PEM file and needs
// to load it.  Therefore, we'll start with a new RSA object and load
// the public key from the PEM..
CkoRsa *rsa2 = [[CkoRsa alloc] init];
CkoPublicKey *pubKey2 = [[CkoPublicKey alloc] init];

// For brevity, don't check the success..  
success = [pubKey2 LoadFromFile: @"qa_temp/pubKey.pem"];
pubKeyXml = [pubKey2 GetXml];
success = [rsa2 ImportPublicKey: pubKeyXml];

// RSA encrypt the AES key and return it as a base64 encoded string.
rsa2.EncodingMode = @"base64";
BOOL bUsePrivateKey = NO;
NSString *encryptedAesKey = [rsa2 EncryptStringENC: randomKey bUsePrivateKey: bUsePrivateKey];

// At this point, your counterpart sends you the encryptedAesKey,
// and the encrypted file.

// ------------------
// Step 4 - RSA decrypt the AES key.
// ------------------

// Start with a new RSA object and load the private key from the PEM.
CkoRsa *rsa3 = [[CkoRsa alloc] init];
CkoPrivateKey *privKey2 = [[CkoPrivateKey alloc] init];

success = [privKey2 LoadPemFile: @"qa_temp/privKey.pem"];
privKeyXml = [privKey2 GetXml];
success = [rsa3 ImportPrivateKey: privKeyXml];

// The encrypted AES key is encoded using base64, so set
// our EncodingMode to "base64".
rsa3.EncodingMode = @"base64";

bUsePrivateKey = YES;
NSString *decryptedAesKey = [rsa3 DecryptStringENC: encryptedAesKey bUsePrivateKey: bUsePrivateKey];
NSLog(@"%@%@",@"decryptedAesKey = ",decryptedAesKey);

// ------------------
// Step 5 - AES decrypt the file.
// ------------------

CkoCrypt2 *crypt2 = [[CkoCrypt2 alloc] init];

crypt2.CryptAlgorithm = @"aes";
crypt2.CipherMode = @"cbc";
crypt2.KeyLength = [NSNumber numberWithInt:256];

// Set the key.
[crypt2 SetEncodedKey: decryptedAesKey encoding: @"hex"];

// Set the IV 
[crypt2 SetEncodedIV: @"000102030405060708090A0B0C0D0E0F" encoding: @"hex"];

// AES Decrypt the file (the file may be any size because it will
// stream the file in/out.
success = [crypt2 CkDecryptFile: @"qa_temp/aesEncrypted.dat" destFile: @"qa_temp/decrypted.xml"];
if (success != YES) {
    NSLog(@"%@",crypt2.LastErrorText);
    return;
}