I’ve been trying to get my head around cryptography on the iPhone so that I can create a native iPhone app (iPasskeep) that interoperates with my JPasskeep password keeper application. It has taken a while to get my head around CommonCrypto APIs – how to use them, how not to use them and their limitations. It then took a bit of fiddling to find the right incantations in Java to get an interoperable cryptographic transformation.

Caveat: The following Objective-C code is not yet production ready or quality. It passes unit tests, but I haven’t checked it for memory leaks, performance, etc.

Hope this helps.

Listing: Cipher.h

#import <Cocoa/Cocoa.h>

#import <CommonCrypto/CommonDigest.h>
#import <CommonCrypto/CommonCryptor.h>

@interface Cipher : NSObject {
	NSString* cipherKey;
}

@property (retain) NSString* cipherKey;

- (Cipher *) initWithKey:(NSString *) key;

- (NSData *) encrypt:(NSData *) plainText;
- (NSData *) decrypt:(NSData *) cipherText;

- (NSData *) transform:(CCOperation) encryptOrDecrypt data:(NSData *) inputData;

+ (NSData *) md5:(NSString *) stringToHash;

@end

Listing: Cipher.m

#import "Cipher.h"

@implementation Cipher

@synthesize cipherKey;

- (Cipher *) initWithKey:(NSString *) key {
	self = [super init];
	if (self) {
		[self setCipherKey:key];
	}
	return self;
}

- (NSData *) encrypt:(NSData *) plainText {
	return [self transform:kCCEncrypt data:plainText];
}

- (NSData *) decrypt:(NSData *) cipherText {
	return [self transform:kCCDecrypt data:cipherText];
}

- (NSData *) transform:(CCOperation) encryptOrDecrypt data:(NSData *) inputData {

	// kCCKeySizeAES128 = 16 bytes
	// CC_MD5_DIGEST_LENGTH = 16 bytes
	NSData* secretKey = [Cipher md5:cipherKey];

	CCCryptorRef cryptor = NULL;
	CCCryptorStatus status = kCCSuccess;

	uint8_t iv[kCCBlockSizeAES128];
    memset((void *) iv, 0x0, (size_t) sizeof(iv));

	status = CCCryptorCreate(encryptOrDecrypt, kCCAlgorithmAES128, kCCOptionPKCS7Padding,
							 [secretKey bytes], kCCKeySizeAES128, iv, &cryptor);

	if (status != kCCSuccess) {
		return nil;
	}

	size_t bufsize = CCCryptorGetOutputLength(cryptor, (size_t)[inputData length], true);

	void * buf = malloc(bufsize * sizeof(uint8_t));
	memset(buf, 0x0, bufsize);

	size_t bufused = 0;
    size_t bytesTotal = 0;

	status = CCCryptorUpdate(cryptor, [inputData bytes], (size_t)[inputData length],
							 buf, bufsize, &bufused);

	if (status != kCCSuccess) {
		free(buf);
		CCCryptorRelease(cryptor);
		return nil;
	}

	bytesTotal += bufused;

	status = CCCryptorFinal(cryptor, buf + bufused, bufsize - bufused, &bufused);

	if (status != kCCSuccess) {
		free(buf);
		CCCryptorRelease(cryptor);
		return nil;
	}

	bytesTotal += bufused;

	CCCryptorRelease(cryptor);

	return [NSData dataWithBytesNoCopy:buf length:bytesTotal];
}

+ (NSData *) md5:(NSString *) stringToHash {

	const char *src = [stringToHash UTF8String];

	unsigned char result[CC_MD5_DIGEST_LENGTH];

	CC_MD5(src, strlen(src), result);

	return [NSData dataWithBytes:result length:CC_MD5_DIGEST_LENGTH];
}

@end

Listing: Cipher.java

package com.tomczarniecki.iphone;

import org.apache.commons.codec.digest.DigestUtils;
import org.bouncycastle.crypto.BufferedBlockCipher;
import org.bouncycastle.crypto.engines.AESFastEngine;
import org.bouncycastle.crypto.modes.CBCBlockCipher;
import org.bouncycastle.crypto.paddings.PaddedBufferedBlockCipher;
import org.bouncycastle.crypto.params.KeyParameter;

import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;

public class Cipher {

    private final String password;

    public Cipher(String password) {
        this.password = password;
    }

    public byte[] encrypt(byte[] plainText) throws Exception {
        return transform(true, plainText);
    }

    public byte[] decrypt(byte[] cipherText) throws Exception {
        return transform(false, cipherText);
    }

    private byte[] transform(boolean encrypt, byte[] inputBytes) throws Exception {
        byte[] key = DigestUtils.md5(password.getBytes("UTF-8"));

        BufferedBlockCipher cipher = new PaddedBufferedBlockCipher(new CBCBlockCipher(new AESFastEngine()));
        cipher.init(encrypt, new KeyParameter(key));

        ByteArrayInputStream input = new ByteArrayInputStream(inputBytes);
        ByteArrayOutputStream output = new ByteArrayOutputStream();

        int inputLen;
        int outputLen;

        byte[] inputBuffer = new byte[1024];
        byte[] outputBuffer = new byte[cipher.getOutputSize(inputBuffer.length)];

        while ((inputLen = input.read(inputBuffer)) > -1) {
            outputLen = cipher.processBytes(inputBuffer, 0, inputLen, outputBuffer, 0);
            if (outputLen > 0) {
                output.write(outputBuffer, 0, outputLen);
            }
        }

        outputLen = cipher.doFinal(outputBuffer, 0);
        if (outputLen > 0) {
            output.write(outputBuffer, 0, outputLen);
        }

        return output.toByteArray();
    }
}

Tags: aes, bouncycastle, cryptography, iPhone, Java, jpasskeep, objective-c