iOS Generate RSA non Random key pair? - ios

Question

i want to generate same asymmetric key pair every time i apply same seed.
i have used iOS RSA crypto exercise to generarte RSA Asymmetric key pair.
i also apply same seed every time. (public and private tags)
However, i receive different keys each time i generate.
- (void)generateKeyPair:(NSUInteger)keySize {
OSStatus sanityCheck = noErr;
publicKeyRef = NULL;
privateKeyRef = NULL;
LOGGING_FACILITY1( keySize == 512 || keySize == 1024 || keySize == 2048, #"%d is an invalid and unsupported key size.", keySize );
// First delete current keys.
[self deleteAsymmetricKeys];
// Container dictionaries.
NSMutableDictionary * privateKeyAttr = [[NSMutableDictionary alloc] init];
NSMutableDictionary * publicKeyAttr = [[NSMutableDictionary alloc] init];
NSMutableDictionary * keyPairAttr = [[NSMutableDictionary alloc] init];
// Set top level dictionary for the keypair.
[keyPairAttr setObject:(id)kSecAttrKeyTypeRSA forKey:(id)kSecAttrKeyType];
[keyPairAttr setObject:[NSNumber numberWithUnsignedInteger:keySize] forKey:(id)kSecAttrKeySizeInBits];
// Set the private key dictionary.
[privateKeyAttr setObject:[NSNumber numberWithBool:YES] forKey:(id)kSecAttrIsPermanent];
[privateKeyAttr setObject:[NSNumber numberWithBool:YES] forKey:(id)kSecAttrCanEncrypt];
[privateKeyAttr setObject:[NSNumber numberWithBool:YES] forKey:(id)kSecAttrCanDecrypt];
[privateKeyAttr setObject:privateTag forKey:(id)kSecAttrApplicationTag];
// See SecKey.h to set other flag values.
// Set the public key dictionary.
[publicKeyAttr setObject:[NSNumber numberWithBool:YES] forKey:(id)kSecAttrIsPermanent];
[publicKeyAttr setObject:publicTag forKey:(id)kSecAttrApplicationTag];
// See SecKey.h to set other flag values.
// Set attributes to top level dictionary.
[keyPairAttr setObject:privateKeyAttr forKey:(id)kSecPrivateKeyAttrs];
[keyPairAttr setObject:publicKeyAttr forKey:(id)kSecPublicKeyAttrs];
// SecKeyGeneratePair returns the SecKeyRefs just for educational purposes.
sanityCheck = SecKeyGeneratePair((CFDictionaryRef)keyPairAttr, &publicKeyRef, &privateKeyRef);
LOGGING_FACILITY( sanityCheck == noErr && publicKeyRef != NULL && privateKeyRef != NULL, #"Something really bad went wrong with generating the key pair." );
NSLog(#"getPublicKeyBits: %#", [self getPublicKeyBits]);
NSLog(#"getPublicKeyExp: %#", [self getPublicKeyExp]);
NSLog(#"getPublicKeyMod: %#", [self getPublicKeyMod]);
// NSLog(#"keyPairAttr: %#" , keyPairAttr);
[privateKeyAttr release];
[publicKeyAttr release];
[keyPairAttr release];
}

Answer 1

The "public and private tags" that you are setting are simply identifiers that you can search for later using SecItemCopyMatching if you store the key pair in the key chain.
Unfortunately, you cannot set the "seed" value for assymetric key pairs using SecKeyGeneratePair or SecKeyGeneratePairAsync. You will always get "randomly generated" key pairs.
If you must do this, you will have to look at other libraries that provide that functionality.