Objective-C RC4 Decryption - objective-c

Question

I am new to Objective-C, but am an experienced developer (C#), but I can't figure this out:
I have a string which is RC4 encrypted, and I need to decrypt it using Objective-C on the iPad (iOS 5.0). I have looked all over the net for a working example, but have had no luck finding an example that works end-to-end. Not only does the code below not return the decrypted string correctly, it returns something different every time it executes, which makes me thing a pointer is being released someplace.
Note: I do not know if it matters, but the string was encrypted using http://archive.plugins.jquery.com/project/RC4 and then stored as text in a Sqlite database, which I am now accessing from Objective-C (I know, the architecture sounds messy, but I can't change that at this point.)
The code I am using is (taken from RC4 encryption - CommonCrypto (Objective-C) vs PHP):
+ (NSString*)decryptData:(NSData*) dataToDecrypt
{
const void *vplainText;
size_t plainTextBufferSize;
plainTextBufferSize = [dataToDecrypt length];
vplainText = [dataToDecrypt bytes];
CCCryptorStatus ccStatus;
uint8_t *bufferPtr = NULL;
size_t bufferPtrSize = 0;
size_t movedBytes = 0;
bufferPtrSize = (plainTextBufferSize + kCCBlockSize3DES) & ~(kCCBlockSize3DES - 1);
bufferPtr = malloc( bufferPtrSize * sizeof(uint8_t));
memset((void *)bufferPtr, 0x0, bufferPtrSize);
NSString *key = #"theKeyIUsedtoEncryptInTheFirstPlace";
const void *vkey = (const void *) [key UTF8String];
size_t keyLength = [[key dataUsingEncoding:NSUTF8StringEncoding] length];
ccStatus = CCCrypt(kCCDecrypt,
kCCAlgorithmRC4,
0,
vkey,
kCCKeySizeDES,
nil,
vplainText,
plainTextBufferSize,
(void *)bufferPtr,
bufferPtrSize,
&movedBytes);
if (ccStatus == kCCSuccess) NSLog(#"SUCCESS");
/*else*/ if (ccStatus == kCCParamError) return #"PARAM ERROR";
else if (ccStatus == kCCBufferTooSmall) return #"BUFFER TOO SMALL";
else if (ccStatus == kCCMemoryFailure) return #"MEMORY FAILURE";
else if (ccStatus == kCCAlignmentError) return #"ALIGNMENT";
else if (ccStatus == kCCDecodeError) return #"DECODE ERROR";
else if (ccStatus == kCCUnimplemented) return #"UNIMPLEMENTED";
NSString *result = [[ NSString alloc ] initWithData: [NSData dataWithBytes:(const void *)bufferPtr length:(NSUInteger)movedBytes] encoding:NSASCIIStringEncoding];
NSLog(#"%#", result);
return result;
}

Answer 1

Use this function for encryption and decryption. (Just put in the encoded string with same key again to decode it).
-(NSString*) rc4Key:(NSString*) key str:(NSString*) str
{
int j = 0;
unichar res[str.length];
const unichar* buffer = res;
unsigned char s[256];
for (int i = 0; i < 256; i++)
{
s[i] = i;
}
for (int i = 0; i < 256; i++)
{
j = (j + s[i] + [key characterAtIndex:(i % key.length)]) % 256;
swap(s[i], s[j]);
}
int i = j = 0;
for (int y = 0; y < str.length; y++)
{
i = (i + 1) % 256;
j = (j + s[i]) % 256;
swap(s[i], s[j]);
unsigned char f = [str characterAtIndex:y] ^ s[ (s[i] + s[j]) % 256];
res[y] = f;
}
return [NSString stringWithCharacters:buffer length:str.length];
}

Answer 2

I see a couple of references to DES in your code (kCCKeySizeDES, kCCBlockSize3DES). That doesn't seem right -- at a minimum, kCCKeySizeDES should probably be replaced with keyLength.
If that doesn't solve it, I'd look next at possible text encoding issues. The data in SQLite might be UTF8-encoded binary data, in which case you'll probably have to "decode" it by converting from UTF8 to ISO8859-1.

Answer 3

RC4 implementation translated from .net:
+(NSString*)RC4:(NSString *)data key:(NSString *)key
{
id x;
int y = 0;
int i = 0;
int j = 0;
NSMutableArray *box = [[NSMutableArray alloc] initWithCapacity:256];
NSString *result = #"";
for (i = 0; i < 256; i++) {
[box addObject:[NSNumber numberWithInt:i]];
}
for (i = 0; i < 256; i++) {
j = ((int)[key characterAtIndex:(i % key.length)] + [[box objectAtIndex:i] intValue] + j) % 256;
x = [box objectAtIndex:i];
[box setObject:[box objectAtIndex:j] atIndexedSubscript:i];
[box setObject:x atIndexedSubscript:j];
}
for (i = 0; i < data.length; i++) {
y = i % 256;
j = ([[box objectAtIndex:y] intValue] + j) % 256;
x = [box objectAtIndex:y];
[box setObject:[box objectAtIndex:j] atIndexedSubscript:y];
[box setObject:x atIndexedSubscript:j];
NSString *c = [NSString stringWithFormat:#"%c", ([data characterAtIndex:i] ^ (char)[[box objectAtIndex:([[box objectAtIndex:y] intValue] + [[box objectAtIndex:j] intValue]) % 256] intValue])];
result = [result stringByAppendingString:c];
}
return result;
}