看到网上有很多朋友都提出过这个问题，也没有详细的解答，感觉挺坑的，因为我之前也在为这个问题纠结了好久，之后是在技术群有高人相助才解决掉的，废话不多说直接上代码吧！

这是当时后台提供给我的公钥字符串：服务器是java

#defineRSA_PUBLIC_KEY  @"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"

注意：跟后台服务器一定要沟通好，用什么格式 不然会有很多坑等着你去跳，编码格式建议采用（utf-8） 

//获取公钥

static SecKeyRef _public_key=nil;

- (SecKeyRef) getPublicKey{// 从公钥证书文件中获取到公钥的SecKeyRef指针

    if(_public_key == nil){

//由于后台给我的字符串时base过的所以要转回来，没有的请忽视

        NSData *certificateData = [GTMBase64 decodeString:RSA_PUBLIC_KEY];

        SecCertificateRef myCertificate =  SecCertificateCreateWithData(kCFAllocatorDefault, (CFDataRef)certificateData);

        SecPolicyRef myPolicy = SecPolicyCreateBasicX509();

        SecTrustRef myTrust;

        OSStatus status = SecTrustCreateWithCertificates(myCertificate,myPolicy,&myTrust);

        SecTrustResultType trustResult;

        if (status == noErr) {

            status = SecTrustEvaluate(myTrust, &trustResult);

        }

        _public_key = SecTrustCopyPublicKey(myTrust);

        CFRelease(myCertificate);

        CFRelease(myPolicy);

        CFRelease(myTrust);

    }

    return _public_key;

}