OAuth signature with RSA-SHA1 on iOS - iphone

Question

I need help creating an RSA-SHA1 signature to be used in a 3-legged OAuth implementation on iOS.
I was able to do this using HMAC-SHA1 using CommonCrypto.h, but this library doesn't seem to support RSA-SHA1.
Have any of you implemented OAuth signatures with RSA? Could you point me to some resources where I can find more information?
Thanks.

Answer 1

The answer by Erik Villegas was also the solution for me. But there is a bug in the posted code which I encountered when using this solution: secretFile was opened with fopen(), so it must be closed with fclose()
- (NSString *)RSASHA1HashForString:(NSString *)source {
NSLog(#"encrypting %#", source);
if (source == nil) return nil;
OpenSSL_add_all_algorithms();
NSString *signature = nil;
// make a SHA-1 digest of the source string
const char* sourceChars = [source UTF8String];
unsigned char digest[SHA_DIGEST_LENGTH];
SHA1((const unsigned char *)sourceChars, strlen(sourceChars), digest);
NSString *path = [[NSBundle mainBundle] pathForResource:#"privatekey" ofType:#"pem"];
const char *pathCString = [path cStringUsingEncoding:NSUTF8StringEncoding];
FILE *secretFile = fopen(pathCString, "r");
RSA *rsa = NULL;
PEM_read_RSAPrivateKey(secretFile, &rsa, NULL, NULL);
if (rsa != NULL) {
unsigned int sigLen = 0;
unsigned char *sigBuff = malloc(RSA_size(rsa));
int result = RSA_sign(NID_sha1, digest, (unsigned int) sizeof(digest),
sigBuff, &sigLen, rsa);
if (result != 0) {
NSData *sigData = [NSData dataWithBytes:sigBuff length:sigLen];
signature = [self base64forData:sigData];
}
free(sigBuff);
RSA_free(rsa);
}
fclose(secretFile);
NSLog(#"generated signature: %#", signature);
return signature;
}

Answer 2

I finally found the solution. Here is a method that will look for a privatekey.pem file in your bundle and create a RSA-SHA1 signature using the string that is passed in. You will need to add the openssl library. You can use this project as a reference: https://github.com/x2on/OpenSSL-for-iPhone
- (NSString *)RSASHA1HashForString:(NSString *)source {
NSLog(#"encrypting %#", source);
if (source == nil) return nil;
OpenSSL_add_all_algorithms();
NSString *signature = nil;
// make a SHA-1 digest of the source string
const char* sourceChars = [source UTF8String];
unsigned char digest[SHA_DIGEST_LENGTH];
SHA1((const unsigned char *)sourceChars, strlen(sourceChars), digest);
NSString *path = [[NSBundle mainBundle] pathForResource:#"privatekey" ofType:#"pem"];
const char *pathCString = [path cStringUsingEncoding:NSUTF8StringEncoding];
FILE *secretFile = fopen(pathCString, "r");
RSA *rsa = NULL;
PEM_read_RSAPrivateKey(secretFile, &rsa, NULL, NULL);
if (rsa != NULL) {
unsigned int sigLen = 0;
unsigned char *sigBuff = malloc(RSA_size(rsa));
int result = RSA_sign(NID_sha1, digest, (unsigned int) sizeof(digest),
sigBuff, &sigLen, rsa);
if (result != 0) {
NSData *sigData = [NSData dataWithBytes:sigBuff length:sigLen];
signature = [self base64forData:sigData];
}
free(sigBuff);
RSA_free(rsa);
}
NSLog(#"generated signature: %#", signature);
return signature;
}
If you are implementing OAuth you'll need to pass the signature base into this method. More info can be found here: http://oauth.net/core/1.0a/#anchor13

Answer 3

Here is implementation using core security iOS framework.
Key should be converted to pkcs12 format.
Algorythm name is "kSecKeyAlgorithmRSASignatureMessagePKCS1v15SHA1"
- (NSString *)getStringSHA1RSASignature:(NSString *)str
{
NSString *path = [[NSBundle bundleForClass:[self class]]
pathForResource:#"rsaPrivate.pfx" ofType:#"pkcs12"];
NSData *p12data = [NSData dataWithContentsOfFile:path];
NSMutableDictionary * options = [[NSMutableDictionary alloc] init];
[options setObject:#" your password " forKey:(id)kSecImportExportPassphrase];
CFArrayRef items = CFArrayCreate(NULL, 0, 0, NULL);
OSStatus securityError = SecPKCS12Import((CFDataRef)p12data, (CFDictionaryRef)options, &items);
CFDictionaryRef identityDict = CFArrayGetValueAtIndex(items, 0);
SecIdentityRef identityApp =
(SecIdentityRef)CFDictionaryGetValue(identityDict,
kSecImportItemIdentity);
assert(securityError == noErr);
SecKeyRef privateKeyRef;
SecIdentityCopyPrivateKey(identityApp, &privateKeyRef);
/// have a key
NSData *input = [str dataUsingEncoding:NSUTF8StringEncoding];
CFErrorRef error = nil;
///////////////////////////////// "SHA1withRSA" java
CFDataRef signature = SecKeyCreateSignature(privateKeyRef, kSecKeyAlgorithmRSASignatureMessagePKCS1v15SHA1, (CFDataRef)input, &error);
NSAssert(signature != nil, #"");
NSString *result = [(__bridge NSData *)signature base64EncodedStringWithOptions:0];
CFRelease(signature);
NSAssert(result != nil, #"");
NSLog(#"generated signature: %#", result);
return result;
}