iOS SecKeyRef to NSData

• use of undeclared identifier ‘publicTag’

The publicTag is just some unique identifier added to the Keychain items. In the CryptoExercise sample project it is defined as

#define kPublicKeyTag "com.apple.sample.publickey"
static const uint8_t publicKeyIdentifier[] = kPublicKeyTag;
NSData *publicTag = [[NSData alloc] initWithBytes:publicKeyIdentifier length:sizeof(publicKeyIdentifier)];

• Cast of an indirect pointer to an Objective-C pointer to ‘CFTypeRef ‘ (aka ‘const void *’) is disallowed with ARC

This can be solved by using a temporary CFTypeRef variable:

CFTypeRef result;
sanityCheck = SecItemCopyMatching((__bridge CFDictionaryRef)queryPublicKey, &result);
if (sanityCheck == errSecSuccess) {
    publicKeyBits = CFBridgingRelease(result);
}

• I do not want to build a query or whatnot to extract the key from the keychain. I have it in a variable and I wish to extract it from there …

As far as I know, you have to store the SecKeyRef to the Keychain temporarily. SecItemAdd
has the option to return the added item as data. From the documentation:

To obtain the data of the added item as an object of type CFDataRef,
specify the return type key kSecReturnData with a value of
kCFBooleanTrue.

Putting all that together, the following code should do what you want:

- (NSData *)getPublicKeyBitsFromKey:(SecKeyRef)givenKey {

    static const uint8_t publicKeyIdentifier[] = "com.your.company.publickey";
    NSData *publicTag = [[NSData alloc] initWithBytes:publicKeyIdentifier length:sizeof(publicKeyIdentifier)];

    OSStatus sanityCheck = noErr;
    NSData * publicKeyBits = nil;

    NSMutableDictionary * queryPublicKey = [[NSMutableDictionary alloc] init];
    [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];

    // Temporarily add key to the Keychain, return as data:
    NSMutableDictionary * attributes = [queryPublicKey mutableCopy];
    [attributes setObject:(__bridge id)givenKey forKey:(__bridge id)kSecValueRef];
    [attributes setObject:@YES forKey:(__bridge id)kSecReturnData];
    CFTypeRef result;
    sanityCheck = SecItemAdd((__bridge CFDictionaryRef) attributes, &result);
    if (sanityCheck == errSecSuccess) {
        publicKeyBits = CFBridgingRelease(result);

        // Remove from Keychain again:
        (void)SecItemDelete((__bridge CFDictionaryRef) queryPublicKey);
    }

    return publicKeyBits;
}

I hope that this works, I cannot test it at the moment.

• Follow-up: How can I export a private key to NSData, since I’ve read several time that the function I’m trying to work with only works for public keys.

I don’t know.