How to pin the Public key of a certificate on iOS

Question

While improving the security of an iOS application that we are developing, we found the need to PIN (the entire or parts of) the SSL certificate of server to prevent man-in-the-middle attacks.

Even though there are various approaches to do this, when you searching for thisI only found examples for pinning the entire certificate. Such practice poses a problem: As soon as the certificate is updated, your application will not be able to connect anymore. If you choose to pin the public key instead of the entire certificate you will find yourself (I believe) in an equally secure situation, while being more resilient to certificate updates in the server.

But how do you do this?

Answer 1

In case you are in need of knowing how to extract this information from the certificate in your iOS code, here you have one way to do it.

First of all add the security framework.

#import <Security/Security.h>

The add the openssl libraries. You can download them from https://github.com/st3fan/ios-openssl

#import <openssl/x509.h>

The NSURLConnectionDelegate Protocol allows you to decide whether the connection should be able to respond to a protection space. In a nutshell, this is when you can have a look at the certificate that is coming from the server, and decide to allow the connection to proceed or to cancel. What you want to do here is compare the certificates public key with the one you've pinned. Now the question is, how do you get such public key? Have a look at the following code:

First get the certificate in X509 format (you will need the ssl libraries for this)

const unsigned char *certificateDataBytes = (const unsigned char *)[serverCertificateData bytes];
X509 *certificateX509 = d2i_X509(NULL, &certificateDataBytes, [serverCertificateData length]);

Now we will prepare to read the public key data

ASN1_BIT_STRING *pubKey2 = X509_get0_pubkey_bitstr(certificateX509);

NSString *publicKeyString = [[NSString alloc] init];    

At this point you can iterate through the pubKey2 string and extract the bytes in HEX format into a string with the following loop

 for (int i = 0; i < pubKey2->length; i++)
{
    NSString *aString = [NSString stringWithFormat:@"%02x", pubKey2->data[i]];
    publicKeyString = [publicKeyString stringByAppendingString:aString];
}

Print the public key to see it

 NSLog(@"%@", publicKeyString);

The complete code

- (BOOL)connection:(NSURLConnection *)connection canAuthenticateAgainstProtectionSpace:(NSURLProtectionSpace *)protectionSpace
{
const unsigned char *certificateDataBytes = (const unsigned char *)[serverCertificateData bytes];
X509 *certificateX509 = d2i_X509(NULL, &certificateDataBytes, [serverCertificateData length]);
ASN1_BIT_STRING *pubKey2 = X509_get0_pubkey_bitstr(certificateX509);

NSString *publicKeyString = [[NSString alloc] init];    

for (int i = 0; i < pubKey2->length; i++)
 {
     NSString *aString = [NSString stringWithFormat:@"%02x", pubKey2->data[i]];
     publicKeyString = [publicKeyString stringByAppendingString:aString];
 }

if ([publicKeyString isEqual:myPinnedPublicKeyString]){
    NSLog(@"YES THEY ARE EQUAL, PROCEED");
    return YES;
}else{
   NSLog(@"Security Breach");
   [connection cancel];
   return NO;
}

}

Answer 2

As far as I can tell you cannot easily create the expected public key directly in iOS, you need to do it via a certificate. So the steps needed are similar to pinning the certificate, but additionally you need to extract the public key from the actual certificate, and from a reference certificate (the expected public key).

What you need to do is:

1. Use a NSURLConnectionDelegate to retrieve the data, and implement willSendRequestForAuthenticationChallenge.
2. Include a reference certificate in DER format. In the example I've used a simple resource file.
3. Extract the public key presented by the server
4. Extract the public key from your reference certificate
5. Compare the two
6. If they match, continue with the regular checks (hostname, certificate signing, etc)
7. If they don't match, fail.

Some example code:

 (void)connection:(NSURLConnection *)connection willSendRequestForAuthenticationChallenge:(NSURLAuthenticationChallenge *)challenge {
    // get the public key offered by the server
    SecTrustRef serverTrust = challenge.protectionSpace.serverTrust;
    SecKeyRef actualKey = SecTrustCopyPublicKey(serverTrust);

    // load the reference certificate
    NSString *certFile = [[NSBundle mainBundle] pathForResource:@"ref-cert" ofType:@"der"];
    NSData* certData = [NSData dataWithContentsOfFile:certFile];
    SecCertificateRef expectedCertificate = SecCertificateCreateWithData(NULL, (__bridge CFDataRef)certData);

    // extract the expected public key
    SecKeyRef expectedKey = NULL;
    SecCertificateRef certRefs[1] = { expectedCertificate };
    CFArrayRef certArray = CFArrayCreate(kCFAllocatorDefault, (void *) certRefs, 1, NULL);
    SecPolicyRef policy = SecPolicyCreateBasicX509();
    SecTrustRef expTrust = NULL;
    OSStatus status = SecTrustCreateWithCertificates(certArray, policy, &expTrust);
    if (status == errSecSuccess) {
      expectedKey = SecTrustCopyPublicKey(expTrust);
    }
    CFRelease(expTrust);
    CFRelease(policy);
    CFRelease(certArray);

    // check a match
    if (actualKey != NULL && expectedKey != NULL && [(__bridge id) actualKey isEqual:(__bridge id)expectedKey]) {
      // public keys match, continue with other checks
      [challenge.sender performDefaultHandlingForAuthenticationChallenge:challenge];
    } else {
      // public keys do not match
      [challenge.sender cancelAuthenticationChallenge:challenge];
    }
    if(actualKey) {
      CFRelease(actualKey);
    }
    if(expectedKey) {
      CFRelease(expectedKey);
    }
 }

Disclaimer: this is example code only, and not thoroughly tested. For a full implementation start with the certificate pinning example by OWASP.

And remember that certificate pinning can always be avoided using SSL Kill Switch and similar tools.