converting uint8_t to NSString in RSA decryption - iphone

Question

I am trying to implement the RSA algorithm and i followed apple reference.
But i am getting problem to convert uint8_t to NSData to NSString.
I have done this so far..These function defined in reference
-(void)test{
[self generateKeyPairPlease];
NSData *data = [self encryptWithPublicKey]; //All goes well until here
[self decryptWithPrivateKey:data];
}
For encryption i did..
- (NSData *)encryptWithPublicKey
{
OSStatus status = noErr;
size_t cipherBufferSize;
uint8_t *cipherBuffer; // 1
// [cipherBufferSize]
const uint8_t dataToEncrypt[] = "the quick brown fox jumps "
"over the lazy dog\0"; // 2
size_t dataLength = sizeof(dataToEncrypt)/sizeof(dataToEncrypt[0]);
SecKeyRef publicKey = NULL; // 3
NSData * publicTag = [NSData dataWithBytes:publicKeyIdentifier
length:strlen((const char *)publicKeyIdentifier)]; // 4
NSMutableDictionary *queryPublicKey =
[[NSMutableDictionary alloc] init]; // 5
[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];
// 6
status = SecItemCopyMatching
((__bridge CFDictionaryRef)queryPublicKey, (CFTypeRef *)&publicKey); // 7
// Allocate a buffer
cipherBufferSize = SecKeyGetBlockSize(publicKey);
cipherBuffer = malloc(cipherBufferSize);
// Error handling
if (cipherBufferSize < sizeof(dataToEncrypt)) {
// Ordinarily, you would split the data up into blocks
// equal to cipherBufferSize, with the last block being
// shorter. For simplicity, this example assumes that
// the data is short enough to fit.
printf("Could not decrypt. Packet too large.\n");
return NULL;
}
// Encrypt using the public.
status = SecKeyEncrypt( publicKey,
kSecPaddingPKCS1,
dataToEncrypt,
(size_t) dataLength,
cipherBuffer,
&cipherBufferSize
); // 8
// Error handling
// Store or transmit the encrypted text
if (publicKey) CFRelease(publicKey);
NSData *encryptedData = [NSData dataWithBytes:cipherBuffer length:dataLength];
free(cipherBuffer);
return encryptedData;
}
But in decryptWithPrivateKey I can't able to convert uint8_t *plainBuffer (decrypted result) to NSString.First i tried to convert it into NSData on printing with NSLog it show the bytes properly but then NSData is not converting in to string.
- (void)decryptWithPrivateKey: (NSData *)dataToDecrypt
{
OSStatus status = noErr;
size_t cipherBufferSize = [dataToDecrypt length];
uint8_t *cipherBuffer = (uint8_t *)[dataToDecrypt bytes];
size_t plainBufferSize;
uint8_t *plainBuffer;
SecKeyRef privateKey = NULL;
NSData * privateTag = [NSData dataWithBytes:privateKeyIdentifier
length:strlen((const char *)privateKeyIdentifier)];
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];
// 1
status = SecItemCopyMatching
((__bridge CFDictionaryRef)queryPrivateKey, (CFTypeRef *)&privateKey); // 2
// Allocate the buffer
plainBufferSize = SecKeyGetBlockSize(privateKey);
plainBuffer = malloc(plainBufferSize);
if (plainBufferSize < cipherBufferSize) {
// Ordinarily, you would split the data up into blocks
// equal to plainBufferSize, with the last block being
// shorter. For simplicity, this example assumes that
// the data is short enough to fit.
printf("Could not decrypt. Packet too large.\n");
return;
}
// Error handling
status = SecKeyDecrypt( privateKey,
kSecPaddingPKCS1,
cipherBuffer,
cipherBufferSize,
plainBuffer,
&plainBufferSize
); // 3
//*******************************************************************************
// Not able to convert uint8_t *plainBuffer to string
// I also tried to convert it into NSData and then try to convert it into NSString but can't
//What Should i do here to get string back
if(privateKey) CFRelease(privateKey);
}
#end
I want to know how to convert decrypted result uint8_t plainBuffer to NSData to NSString or simply NSString so that i get my string back.For my encryption and key generation code please refer this reference.
Thanks in advance..

Answer 1

i know this question is old and marked as solved but i got here with a similar problem.
what i found out is that there seems to be an error in the encryption method the apple docs in the line
NSData *encryptedData = [NSData dataWithBytes:cipherBuffer length:dataLength];
datalength is the wrong variable here. i replaced it with cipherbuffersize to
NSData *encryptedData = [NSData dataWithBytes:cipherBuffer length:cipherBufferSize];
and everythings works well for me now. i hope it is useful for someone else than me.

Answer 2

The most likely problem is that you're calling +[stringWithCharacters:length:], which expects Unicode characters (more precisely, UTF-16 code points), but you probably have UTF-8 or another 8-bit encoding.
You have to know which encoding you have, so you can call +[stringWithCString:encoding:] (or, if it's UTF-8, +[stringWithUTF8String:] for short).
Note that, unlike stringWithCharacters, these methods don't take a length, and expect your data to be null-terminated. So, unless you're sure the data will always have a terminator, you should either check for it explicitly, or alloc the string and then call -[initWithBytes:length:encoding:].
If you don't know what charset the plaintext was in before it was encrypted, you need to find out. As a human, you can often tell by looking at it, especially if it's mostly ASCII. Create an NSData with the buffer and log that and look at the hex. If it's UTF-16, the ASCII characters will alternate with nulls, so 'Hello' will be 48 00 65 00 6C 00 6C 00 6F 00. But a computer isn't going to do a very good job guessing. (The reason +[stringWithContentsOfFile:usedEncoding:error:] usually gets it right is that it looks at the extended attribute com.apple.TextEncoding, which most Cocoa apps write when they save a file.)

Answer 3

I manage to solve my problem by following this code.It's the same code but encoding of string is different.In encryptWithPublicKey function I have used
uint8_t *dataToEncrypt= (uint8_t *)[#"the quick brown fox jumps over the lazy dog" UTF8String];
instead of
const uint8_t dataToEncrypt[] = "the quick brown fox jumps "
"over the lazy dog\0";
and in decryptWithPrivateKey I have used this to decode
NSData *decryptedData = [NSData dataWithBytes:plainBuffer length:plainBufferSize];
NSString *decryptedString = [[[NSString alloc] initWithData:decryptedData encoding:NSUTF8StringEncoding] autorelease];