sign data using rsa private key - ios

Question

i know is christmas but i have a huge problem that i need to solve, and i'm looking for my christmas miracle here...
I have read apples documentation, and there were only guides how to create RSA public and private keys from certificates. In my case, i have only RSA private key in .pem file. So my question is his: how should i sign data, using that key? I dont want to use openssl. i have tried it with no luck, and i think it's possible to sign data with RSA, by using apples API's.
This is how my key looks like:
-----BEGIN RSA PRIVATE KEY-----
..............................
-----END RSA PRIVATE KEY-----
This is what i have done so far:
-(NSString *)signing:(NSString *)dataString {
NSString *filePath = [[NSBundle mainBundle] pathForResource:#"PrestaMobilekey" ofType:#"pem"];
NSData *data = [[NSData alloc]initWithContentsOfFile:filePath];
SecKeyRef privateKey = (__bridge SecKeyRef)(data);
uint8_t *signedHashBytes = NULL;
// calculate private key size
size_t signedHashBytesSize = SecKeyGetBlockSize(privateKey);
// create space to put signature
signedHashBytes = (uint8_t *)malloc(signedHashBytesSize * sizeof(uint8_t));
memset((void *)signedHashBytes, 0x0, signedHashBytesSize);
OSStatus status = NULL;
// sign data
status = SecKeyRawSign(privateKey,
kSecPaddingPKCS1SHA1,
[[[dataString dataUsingEncoding:NSUTF8StringEncoding] SHA1] bytes],
CC_SHA1_DIGEST_LENGTH,
signedHashBytes,
&signedHashBytesSize);
if (privateKey) {
CFRelease(privateKey);
}
// get signature hash
NSData *signedHash = [NSData dataWithBytes:(const void *)signedHashBytes length:(NSUInteger)signedHashBytesSize];
// release created space
if (signedHashBytes) {
free(signedHashBytes);
}
if (status != errSecSuccess) {
return #"";
}
// return Base64 encoded signature string
return [Base64 encode:signedHash];
}
I really hope that someone will help me, with some good information and answer.
Thank you.

Answer 1

You don't need to use OpenSSL. You can sign your data using your method with a few tweaks. I don't think you can simply bridge and cast an NSData object to a SecKeyRef. You most likely need to save it to the keychain first.
You can do so with this method:
- (SecKeyRef)saveKeyToKeychain:(NSData *)key keySize:(NSUInteger)keySize private:(BOOL)isPrivate {
OSStatus sanityCheck = noErr;
NSData *tag;
id keyClass;
if (isPrivate) {
tag = privateTag;
keyClass = (__bridge id) kSecAttrKeyClassPrivate;
}
else {
tag = publicTag;
keyClass = (__bridge id) kSecAttrKeyClassPublic;
}
NSDictionary *saveDict = #{
(__bridge id) kSecClass : (__bridge id) kSecClassKey,
(__bridge id) kSecAttrKeyType : (__bridge id) kSecAttrKeyTypeRSA,
(__bridge id) kSecAttrApplicationTag : tag,
(__bridge id) kSecAttrKeyClass : keyClass,
(__bridge id) kSecValueData : key,
(__bridge id) kSecAttrKeySizeInBits : [NSNumber numberWithUnsignedInteger:keySize],
(__bridge id) kSecAttrEffectiveKeySize : [NSNumber numberWithUnsignedInteger:keySize],
(__bridge id) kSecAttrCanDerive : (__bridge id) kCFBooleanFalse,
(__bridge id) kSecAttrCanEncrypt : (__bridge id) kCFBooleanTrue,
(__bridge id) kSecAttrCanDecrypt : (__bridge id) kCFBooleanFalse,
(__bridge id) kSecAttrCanVerify : (__bridge id) kCFBooleanTrue,
(__bridge id) kSecAttrCanSign : (__bridge id) kCFBooleanFalse,
(__bridge id) kSecAttrCanWrap : (__bridge id) kCFBooleanTrue,
(__bridge id) kSecAttrCanUnwrap : (__bridge id) kCFBooleanFalse
};
SecKeyRef savedKey = NULL;
sanityCheck = SecItemAdd((__bridge CFDictionaryRef) saveDict, (CFTypeRef *)&savedKey);
if (sanityCheck != errSecSuccess) {
LOGGING_FACILITY1(sanityCheck != noErr, #"Problem saving the key to keychain, OSStatus == %d.", sanityCheck);
}
return savedKey;
}
If you don't want to get the reference immediately, you can change the method type to void and remove the return statement. Change (CFTypeRef *)&savedKey to NULL.
You can then retrieve the saved key like so:
- (SecKeyRef)getKeyRef:(BOOL)isPrivate {
OSStatus sanityCheck = noErr;
NSData *tag;
id keyClass;
if (isPrivate) {
if (privateKeyRef != NULL) {
// already exists in memory, return
return privateKeyRef;
}
tag = privateTag;
keyClass = (__bridge id) kSecAttrKeyClassPrivate;
}
else {
if (publicKeyRef != NULL) {
// already exists in memory, return
return publicKeyRef;
}
tag = publicTag;
keyClass = (__bridge id) kSecAttrKeyClassPublic;
}
NSDictionary *queryDict = #{
(__bridge id) kSecClass : (__bridge id) kSecClassKey,
(__bridge id) kSecAttrKeyType : (__bridge id) kSecAttrKeyTypeRSA,
(__bridge id) kSecAttrApplicationTag : tag,
(__bridge id) kSecAttrKeyClass : keyClass,
(__bridge id) kSecReturnRef : (__bridge id) kCFBooleanTrue
};
SecKeyRef keyReference = NULL;
sanityCheck = SecItemCopyMatching((__bridge CFDictionaryRef) queryDict, (CFTypeRef *) &keyReference);
if (sanityCheck != errSecSuccess) {
NSLog(#"Error trying to retrieve key from server. isPrivate: %d. sanityCheck: %li", isPrivate, sanityCheck);
}
if (isPrivate) {
privateKeyRef = keyReference;
}
else {
publicKeyRef = keyReference;
}
return keyReference;
}
Also, an easier way to return a base64 encoded string is to do this:
NSString *signatureString = [signedHash base64EncodedStringWithOptions:nil];
About privateTag and publicTag
privateTag and publicTag are used to mark the kSecAttrApplicationTag which defines the application that uses this key. You want to have a separate privateTag and publicTag to differentiate between your private key and public key.
It's a bit convoluted because I followed the sample code, but I defined my privateTag and publicTag this way:
SecKeyWrapper.h
#define kPublicKeyTag "com.sample.app.publickey"
#define kPrivateKeyTag "com.sample.app.privatekey"
SecKeyWrapper.m
// just under #implementation or #synthesize lines
static const uint8_t publicKeyIdentifier[] = kPublicKeyTag;
static const uint8_t privateKeyIdentifier[] = kPrivateKeyTag;
- (id)init {
if (self = [super init]) {
// Tag data to search for keys.
privateTag = [[NSData alloc] initWithBytes:privateKeyIdentifier length:sizeof(privateKeyIdentifier)];
publicTag = [[NSData alloc] initWithBytes:publicKeyIdentifier length:sizeof(publicKeyIdentifier)];
}
return self;
}
Then use the privateTag and publicTag as you would in the code samples I provided above.

Answer 2

Ok, o found a solution for this problem my self. I hope that this will help to others... this is what helped me. I thought that i could do this without openssl, but i was wrong. But still, by doing like in that post, you won't need an extra library in you project. Use terminal