Iphone - How to encrypt NSData with public key and decrypt with private key?

Question

I am converting a UIImage to NSData. Now I need to encrypt that NSData using a public key and I need to decrypt using a private key. Please provide a step by step procedure. Which algorithm do I need to use? Is there any good library for encryption and decryption? Also provide some code snippet for encryption and decryption.

Answer 1

I have tried RSA Encryption and Decryption for NSString and you may well modify it and make it work for NSData

Add Security.Framework to your project bundle.

ViewController.h code is as follows:

#import <UIKit/UIKit.h>
#import <Security/Security.h>

@interface ViewController : UIViewController
{
SecKeyRef publicKey;
SecKeyRef privateKey;
    NSData *publicTag;
    NSData *privateTag;
}
- (void)encryptWithPublicKey:(uint8_t *)plainBuffer cipherBuffer:(uint8_t *)cipherBuffer;
- (void)decryptWithPrivateKey:(uint8_t *)cipherBuffer plainBuffer:(uint8_t *)plainBuffer;
- (SecKeyRef)getPublicKeyRef;
- (SecKeyRef)getPrivateKeyRef;
- (void)testAsymmetricEncryptionAndDecryption;
- (void)generateKeyPair:(NSUInteger)keySize;
@end

ViewController.m file code is as follows:

#import "ViewController.h"

const size_t BUFFER_SIZE = 64;
const size_t CIPHER_BUFFER_SIZE = 1024;
const uint32_t PADDING = kSecPaddingNone;
static const UInt8 publicKeyIdentifier[] = "com.apple.sample.publickey";
static const UInt8 privateKeyIdentifier[] = "com.apple.sample.privatekey";

@implementation ViewController

-(SecKeyRef)getPublicKeyRef { 

    OSStatus sanityCheck = noErr; 
    SecKeyRef publicKeyReference = NULL;

    if (publicKeyReference == NULL) { 
        [self generateKeyPair:512];
                NSMutableDictionary *queryPublicKey = [[NSMutableDictionary alloc] init];

        // Set the public key query dictionary.
        [queryPublicKey setObject:(__bridge id)kSecClassKey forKey:(__bridge id)kSecClass];
        [queryPublicKey setObject:publicTag forKey:(__bridge id)kSecAttrApplicationTag];
        [queryPublicKey setObject:(__bridge id)kSecAttrKeyTypeRSA forKey:(__bridge id)kSecAttrKeyType];
        [queryPublicKey setObject:[NSNumber numberWithBool:YES] forKey:(__bridge id)kSecReturnRef];


        // Get the key.
        sanityCheck = SecItemCopyMatching((__bridge CFDictionaryRef)queryPublicKey, (CFTypeRef *)&publicKeyReference);


        if (sanityCheck != noErr)
        {
            publicKeyReference = NULL;
        }


//        [queryPublicKey release];

    } else { publicKeyReference = publicKey; }

    return publicKeyReference; }

- (void)didReceiveMemoryWarning
{
    [super didReceiveMemoryWarning];
    // Release any cached data, images, etc that aren't in use.
}




- (void)testAsymmetricEncryptionAndDecryption {

    uint8_t *plainBuffer;
    uint8_t *cipherBuffer;
    uint8_t *decryptedBuffer;



    const char inputString[] = "How to Encrypt data with public key and Decrypt data with private key";
    int len = strlen(inputString);
    // TODO: this is a hack since i know inputString length will be less than BUFFER_SIZE
    if (len > BUFFER_SIZE) len = BUFFER_SIZE-1;

    plainBuffer = (uint8_t *)calloc(BUFFER_SIZE, sizeof(uint8_t));
    cipherBuffer = (uint8_t *)calloc(CIPHER_BUFFER_SIZE, sizeof(uint8_t));
    decryptedBuffer = (uint8_t *)calloc(BUFFER_SIZE, sizeof(uint8_t));

    strncpy( (char *)plainBuffer, inputString, len);

    NSLog(@"init() plainBuffer: %s", plainBuffer);
    //NSLog(@"init(): sizeof(plainBuffer): %d", sizeof(plainBuffer));
    [self encryptWithPublicKey:(UInt8 *)plainBuffer cipherBuffer:cipherBuffer];
    NSLog(@"encrypted data: %s", cipherBuffer);
    //NSLog(@"init(): sizeof(cipherBuffer): %d", sizeof(cipherBuffer));
    [self decryptWithPrivateKey:cipherBuffer plainBuffer:decryptedBuffer];
    NSLog(@"decrypted data: %s", decryptedBuffer);
    //NSLog(@"init(): sizeof(decryptedBuffer): %d", sizeof(decryptedBuffer));
    NSLog(@"====== /second test =======================================");

    free(plainBuffer);
    free(cipherBuffer);
    free(decryptedBuffer);
}

/* Borrowed from:
 * https://developer.apple.com/library/mac/#documentation/security/conceptual/CertKeyTrustProgGuide/iPhone_Tasks/iPhone_Tasks.html
 */
- (void)encryptWithPublicKey:(uint8_t *)plainBuffer cipherBuffer:(uint8_t *)cipherBuffer
{

    NSLog(@"== encryptWithPublicKey()");

    OSStatus status = noErr;

    NSLog(@"** original plain text 0: %s", plainBuffer);

    size_t plainBufferSize = strlen((char *)plainBuffer);
    size_t cipherBufferSize = CIPHER_BUFFER_SIZE;

    NSLog(@"SecKeyGetBlockSize() public = %lu", SecKeyGetBlockSize([self getPublicKeyRef]));
    //  Error handling
    // Encrypt using the public.
    status = SecKeyEncrypt([self getPublicKeyRef],
                           PADDING,
                           plainBuffer,
                           plainBufferSize,
                           &cipherBuffer[0],
                           &cipherBufferSize
                           );
    NSLog(@"encryption result code: %ld (size: %lu)", status, cipherBufferSize);
    NSLog(@"encrypted text: %s", cipherBuffer);
}

- (void)decryptWithPrivateKey:(uint8_t *)cipherBuffer plainBuffer:(uint8_t *)plainBuffer
{
    OSStatus status = noErr;

    size_t cipherBufferSize = strlen((char *)cipherBuffer);

    NSLog(@"decryptWithPrivateKey: length of buffer: %lu", BUFFER_SIZE);
    NSLog(@"decryptWithPrivateKey: length of input: %lu", cipherBufferSize);

    // DECRYPTION
    size_t plainBufferSize = BUFFER_SIZE;

    //  Error handling
    status = SecKeyDecrypt([self getPrivateKeyRef],
                           PADDING,
                           &cipherBuffer[0],
                           cipherBufferSize,
                           &plainBuffer[0],
                           &plainBufferSize
                           );
    NSLog(@"decryption result code: %ld (size: %lu)", status, plainBufferSize);
    NSLog(@"FINAL decrypted text: %s", plainBuffer);

}



- (SecKeyRef)getPrivateKeyRef {
    OSStatus resultCode = noErr;
    SecKeyRef privateKeyReference = NULL;
//    NSData *privateTag = [NSData dataWithBytes:@"ABCD" length:strlen((const char *)@"ABCD")];
//    if(privateKey == NULL) {
        [self generateKeyPair:512];
        NSMutableDictionary * queryPrivateKey = [[NSMutableDictionary alloc] init];

        // Set the private key query dictionary.
        [queryPrivateKey setObject:(__bridge id)kSecClassKey forKey:(__bridge id)kSecClass];
        [queryPrivateKey setObject:privateTag forKey:(__bridge id)kSecAttrApplicationTag];
        [queryPrivateKey setObject:(__bridge id)kSecAttrKeyTypeRSA forKey:(__bridge id)kSecAttrKeyType];
        [queryPrivateKey setObject:[NSNumber numberWithBool:YES] forKey:(__bridge id)kSecReturnRef];

        // Get the key.
        resultCode = SecItemCopyMatching((__bridge CFDictionaryRef)queryPrivateKey, (CFTypeRef *)&privateKeyReference);
        NSLog(@"getPrivateKey: result code: %ld", resultCode);

        if(resultCode != noErr)
        {
            privateKeyReference = NULL;
        }

//        [queryPrivateKey release];
//    } else {
//        privateKeyReference = privateKey;
//    }

    return privateKeyReference;
}


#pragma mark - View lifecycle



- (void)viewDidLoad
{
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
}

- (void)viewDidUnload
{
    [super viewDidUnload];
    // Release any retained subviews of the main view.
    // e.g. self.myOutlet = nil;
}

- (void)viewWillAppear:(BOOL)animated
{
    [super viewWillAppear:animated];
    privateTag = [[NSData alloc] initWithBytes:privateKeyIdentifier length:sizeof(privateKeyIdentifier)];
    publicTag = [[NSData alloc] initWithBytes:publicKeyIdentifier length:sizeof(publicKeyIdentifier)];
    [self testAsymmetricEncryptionAndDecryption];

}

- (void)viewDidAppear:(BOOL)animated
{
    [super viewDidAppear:animated];
}

- (void)viewWillDisappear:(BOOL)animated
{
    [super viewWillDisappear:animated];
}

- (void)viewDidDisappear:(BOOL)animated
{
    [super viewDidDisappear:animated];
}

- (BOOL)shouldAutorotateToInterfaceOrientation:(UIInterfaceOrientation)interfaceOrientation
{
    // Return YES for supported orientations
    if ([[UIDevice currentDevice] userInterfaceIdiom] == UIUserInterfaceIdiomPhone) {
        return (interfaceOrientation != UIInterfaceOrientationPortraitUpsideDown);
    } else {
        return YES;
    }
}

- (void)generateKeyPair:(NSUInteger)keySize {
    OSStatus sanityCheck = noErr;
    publicKey = NULL;
    privateKey = NULL;

//  LOGGING_FACILITY1( keySize == 512 || keySize == 1024 || keySize == 2048, @"%d is an invalid and unsupported key size.", keySize );

    // First delete current keys.
//  [self deleteAsymmetricKeys];

    // Container dictionaries.
    NSMutableDictionary * privateKeyAttr = [[NSMutableDictionary alloc] init];
    NSMutableDictionary * publicKeyAttr = [[NSMutableDictionary alloc] init];
    NSMutableDictionary * keyPairAttr = [[NSMutableDictionary alloc] init];

    // Set top level dictionary for the keypair.
    [keyPairAttr setObject:(__bridge id)kSecAttrKeyTypeRSA forKey:(__bridge id)kSecAttrKeyType];
    [keyPairAttr setObject:[NSNumber numberWithUnsignedInteger:keySize] forKey:(__bridge id)kSecAttrKeySizeInBits];

    // Set the private key dictionary.
    [privateKeyAttr setObject:[NSNumber numberWithBool:YES] forKey:(__bridge id)kSecAttrIsPermanent];
    [privateKeyAttr setObject:privateTag forKey:(__bridge id)kSecAttrApplicationTag];
    // See SecKey.h to set other flag values.

    // Set the public key dictionary.
    [publicKeyAttr setObject:[NSNumber numberWithBool:YES] forKey:(__bridge id)kSecAttrIsPermanent];
    [publicKeyAttr setObject:publicTag forKey:(__bridge id)kSecAttrApplicationTag];
    // See SecKey.h to set other flag values.

    // Set attributes to top level dictionary.
    [keyPairAttr setObject:privateKeyAttr forKey:(__bridge id)kSecPrivateKeyAttrs];
    [keyPairAttr setObject:publicKeyAttr forKey:(__bridge id)kSecPublicKeyAttrs];

    // SecKeyGeneratePair returns the SecKeyRefs just for educational purposes.
    sanityCheck = SecKeyGeneratePair((__bridge CFDictionaryRef)keyPairAttr, &publicKey, &privateKey);
//  LOGGING_FACILITY( sanityCheck == noErr && publicKey != NULL && privateKey != NULL, @"Something really bad went wrong with generating the key pair." );
    if(sanityCheck == noErr  && publicKey != NULL && privateKey != NULL)
    {
        NSLog(@"Successful");
    }
//  [privateKeyAttr release];
//  [publicKeyAttr release];
//  [keyPairAttr release];
}


@end

Let me know if you need more help.

Hope this helps.