Converting a PrivateKey (SecRefKey) into NSData or Base64

Question

I have a p12 file, from which I extract the PrivateKey, PublicKey both as SecKeyRef and the certificate as SecCertificateRef.

The P12 file have a ECDSA PrivateKey that I need to use to sign data (ECC).

So I'm using a suggested library: https://github.com/ricmoo/GMEllipticCurveCrypto

But I need to feed the library with the keys on Base64 or NSData, I can't feed SecKeyRef. I managed to extract the NSData for the publicKey using the methods provided here , it works.

But I can't find any way to convert a SecRefKey that points to a privateKey to NSData. Any ideas on how to do this, OR, sign and verify data in iOS using ECC with SecKeyRefs.

---

For reference, this method converts the P12 into iOS SecRefs:

- (BOOL)storeDetailsForP12CertificateData:(NSData *)certData password:(NSString*)pass identifier:(NSString*)identifier{    
    SecKeyRef publicKey, privateKey;
    SecCertificateRef certRef;

    //Extract keys and Certificate
    NSMutableDictionary * options = [[NSMutableDictionary alloc] init];
    [options setObject:pass forKey:(id)kSecImportExportPassphrase];

    CFArrayRef items = CFArrayCreate(NULL, 0, 0, NULL);
    OSStatus securityError = SecPKCS12Import((CFDataRef) certData,
                                             (CFDictionaryRef)options, &items);

    CFDictionaryRef identityDict = CFArrayGetValueAtIndex(items, 0);
    SecIdentityRef identityApp =
    (SecIdentityRef)CFDictionaryGetValue(identityDict,
                                         kSecImportItemIdentity);

    assert(securityError == noErr);

    //get private key
    SecIdentityCopyPrivateKey(identityApp, &privateKey);

    //get certificate
    SecIdentityCopyCertificate(identityApp, &certRef);

    //evaluate certificate.
    CFArrayRef certs = CFArrayCreate(kCFAllocatorDefault, (const void **) &certRef, 1, NULL);
    SecPolicyRef policy = SecPolicyCreateBasicX509();
    SecTrustRef trust;
    SecTrustCreateWithCertificates(certs, policy, &trust);
    (CFRelease(certs));
    SecTrustResultType trustResult;
    SecTrustEvaluate(trust, &trustResult);

    //get publickey
    publicKey = SecTrustCopyPublicKey(trust);

    //clean memory
    (CFRelease(trust));
    (CFRelease(policy));

    if (!publicKey || !privateKey || !certRef) {
        return NO;
    } else {
        KeyData *details = [[KeyData alloc] init];
        details.publicKey = publicKey;
        details.privateKey = privateKey;
        details.certificate = certRef;
        details.fileData = certData;

        return YES;
    }
}

Answer 1

Looks like you simply need to use SecKeyCopyExternalRepresentation :

/*!
    @function SecKeyCopyExternalRepresentation
    @abstract Create an external representation for the given key suitable for the key's type.
    @param key The key to be exported.
    @param error On error, will be populated with an error object describing the failure.
    See "Security Error Codes" (SecBase.h).
    @result A CFData representing the key in a format suitable for that key type.
    @discussion This function may fail if the key is not exportable (e.g., bound to a smart card or Secure Enclave).
    The format in which the key will be exported depends on the type of key:
    * kSecAttrKeyTypeRSA                 PKCS#1 format
    * kSecAttrKeyTypeECSECPrimeRandom    SEC1 format (www.secg.org)
 */
CFDataRef _Nullable SecKeyCopyExternalRepresentation(SecKeyRef key, CFErrorRef *error)

Note that CFDataRef is toll-free bridged to NSData , so you can easily convert them.

Answer 2

To export a private key for SEC, you need to retrieve SecKey and convert it to base64 (and format the result with 64 characters max per line. Here the Swift 5 code:

var keyResult: CFTypeRef?
var statusCode = SecItemCopyMatching(attributes as CFDictionary, &keyResult)
if statusCode == noErr && keyResult != nil {
       if let privateKey = SecKeyCopyExternalRepresentation(keyResult as! SecKey, nil) as Data? {
            print("Private Key: \((privateKey.base64EncodedString()))")
       }
       return (keyResult as! SecKey)
}