Get Modulus and Exponent from Public Key iOS - ios

Question

I am looking to get the Modulus and Exponent from a public key in iOS. I have looked at many different sites and looked at Apple, but cannot just get it working.
This is my code so far:
- (void)generateKeyPairPlease
{
OSStatus status = noErr;
NSMutableDictionary *privateKeyAttr = [[NSMutableDictionary alloc] init];
NSMutableDictionary *publicKeyAttr = [[NSMutableDictionary alloc] init];
NSMutableDictionary *keyPairAttr = [[NSMutableDictionary alloc] init];
NSData * publicTag = [NSData dataWithBytes:publicKeyIdentifier
length:strlen((const char *)publicKeyIdentifier)];
NSData * privateTag = [NSData dataWithBytes:privateKeyIdentifier
length:strlen((const char *)privateKeyIdentifier)];
SecKeyRef publicKey = NULL;
SecKeyRef privateKey = NULL;
[keyPairAttr setObject:(__bridge id)kSecAttrKeyTypeRSA
forKey:(__bridge id)kSecAttrKeyType];
[keyPairAttr setObject:[NSNumber numberWithInt:2048]
forKey:(__bridge id)kSecAttrKeySizeInBits];
[privateKeyAttr setObject:[NSNumber numberWithBool:YES]
forKey:(__bridge id)kSecAttrIsPermanent];
[privateKeyAttr setObject:privateTag
forKey:(__bridge id)kSecAttrApplicationTag];
[publicKeyAttr setObject:[NSNumber numberWithBool:YES]
forKey:(__bridge id)kSecAttrIsPermanent];
[publicKeyAttr setObject:publicTag
forKey:(__bridge id)kSecAttrApplicationTag];
[keyPairAttr setObject:privateKeyAttr
forKey:(__bridge id)kSecPrivateKeyAttrs];
[keyPairAttr setObject:publicKeyAttr
forKey:(__bridge id)kSecPublicKeyAttrs];
status = SecKeyGeneratePair((__bridge CFDictionaryRef)keyPairAttr,
&publicKey, &privateKey);
// error handling...
NSData *ppp = [NSData dataWithBytes:publicKey length:strlen((const char *)publicKey)];
NSString *responseString, *responseStringASCII, *responseStringUTF8;
responseStringASCII = [[NSString alloc] initWithData:ppp encoding:NSASCIIStringEncoding];
if (!responseStringASCII)
{
responseString = [[NSString alloc] initWithData:ppp encoding:NSUTF8StringEncoding];
}
else
{
responseStringUTF8 = [[NSString alloc] initWithData:ppp encoding:NSUTF8StringEncoding];
if(responseStringUTF8 != nil && [responseStringUTF8 length] < [responseStringASCII length])
{
responseString = [responseStringUTF8 retain];
}
else
{
responseString = [responseStringASCII retain];
}
[responseStringUTF8 release];
}
publicKeyString = responseString;
if(publicKey) CFRelease(publicKey);
if(privateKey) CFRelease(privateKey);
NSData *exp = [self getPublicKeyExp];
NSData *mod = [self getPublicKeyMod];
NSString *expString = [[NSString alloc] initWithData:exp encoding:NSUTF8StringEncoding];
NSString *modString = [[NSString alloc] initWithData:mod encoding:NSUTF8StringEncoding];
NSLog(#"exponent = %# \n modulus = %#", expString, modString);
}
- (NSData *)getPublicKeyBits: (NSString*) publicKeyIdentifier {
OSStatus sanityCheck = noErr;
NSData * publicKeyBits = nil;
CFTypeRef pk = NULL;
NSMutableDictionary * queryPublicKey = [[NSMutableDictionary alloc] init];
NSData* publicTag = [publicKeyIdentifier dataUsingEncoding:NSUTF8StringEncoding];
// Set the public key query dictionary.
[queryPublicKey setObject:(__bridge_transfer id)kSecClassKey forKey:(__bridge_transfer id)kSecClass];
[queryPublicKey setObject:publicTag forKey:(__bridge_transfer id)kSecAttrApplicationTag];
[queryPublicKey setObject:(__bridge_transfer id)kSecAttrKeyTypeRSA forKey:(__bridge_transfer id)kSecAttrKeyType];
[queryPublicKey setObject:[NSNumber numberWithBool:YES] forKey:(__bridge_transfer id)kSecReturnData];
// Get the key bits.
sanityCheck = SecItemCopyMatching((__bridge CFDictionaryRef)queryPublicKey, &pk);
if (sanityCheck != noErr)
{
publicKeyBits = nil;
}
publicKeyBits = (__bridge id)pk;
NSLog(#"public bits %#",publicKeyBits);
return publicKeyBits;
}
- (NSData *)getPublicKeyExp
{
NSData* pk = [self getPublicKeyBits:publicKeyString];
if (pk == NULL) {
return NULL;
}
int iterator = 0;
iterator++; // TYPE - bit stream - mod + exp
[self derEncodingGetSizeFrom:pk at:&iterator]; // Total size
iterator++; // TYPE - bit stream mod
int mod_size = [self derEncodingGetSizeFrom:pk at:&iterator];
iterator += mod_size;
iterator++; // TYPE - bit stream exp
int exp_size = [self derEncodingGetSizeFrom:pk at:&iterator];
return [pk subdataWithRange:NSMakeRange(iterator, exp_size)];
return pk;
}
- (NSData *)getPublicKeyMod
{
NSData* pk = [self getPublicKeyBits:publicKeyString];
if (pk == NULL) {
return NULL;
}
int iterator = 0;
iterator++; // TYPE - bit stream - mod + exp
[self derEncodingGetSizeFrom:pk at:&iterator]; // Total size
iterator++; // TYPE - bit stream mod
int mod_size = [self derEncodingGetSizeFrom:pk at:&iterator];
return [pk subdataWithRange:NSMakeRange(iterator, mod_size)];
}
- (int)derEncodingGetSizeFrom:(NSData*)buf at:(int*)iterator
{
const uint8_t* data = [buf bytes];
int itr = *iterator;
int num_bytes = 1;
int ret = 0;
if (data[itr] > 0x80) {
num_bytes = data[itr] - 0x80;
itr++;
}
for (int i = 0 ; i < num_bytes; i++) ret = (ret * 0x100) + data[itr + i];
*iterator = itr + num_bytes;
return ret;
}
However, when I get to retrieving the Modulus and Exponent from the relevant methods, they keep returning NULL?