Objective-C: How to verify SecCertificateRef with signer's public key?

I've got a certificate B which is signed with A. I have an A's public key and I want to verify the certificate if it's valid signed by A. How do I do 开发者_如何学Pythonin Objective-C?

In Java, there's a verify() method in X509Certificate class

http://download.oracle.com/javase/1.4.2/docs/api/java/security/cert/Certificate.html#verify%28java.security.PublicKey%29