在iOS9之后，在默认情况下是不能发送HTTP请求，推荐使用HTTPS，当然你可以在plist里面设置NSAppTransportSecurity的NSAllowsArbitraryLoads为true，App就依然支持HTTP请求

而AFSecurityPolicy主要的作用就是验证HTTPS请求的证书的有效性

AFSecurityPolicy是安全策略类，有三种SSL Pinning模式

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typedef NS_ENUM(NSUInteger, AFSSLPinningMode) {
    AFSSLPinningModeNone,// 在证书列表中校验服务端返回的证书
    AFSSLPinningModePublicKey,// 客户端要有服务端的证书拷贝，只是验证时只验证证书里的公钥
    AFSSLPinningModeCertificate,// 客户端要有服务端的证书拷贝，第一步先验证证书域名/有效期等信息，第二步对服务端返回的证书和客户端返回的是否一致
};

这个是证书集合，泛型里面表示了集合里面是NSData类型，表明这个是用来存证书数据的集合，这些证书根据SSL Pinning模式来和服务器进行校验，默认是没有证书的，我们需要调用+ certificatesInBundle:方法将bundle里面的证书文件转成里面是data类型的集合

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@property (nonatomic, strong, nullable) NSSet <NSData *> *pinnedCertificates;
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+ (NSSet *)certificatesInBundle:(NSBundle *)bundle {
    // 获取证书
  	NSArray *paths = [bundle pathsForResourcesOfType:@"cer" inDirectory:@"."];

    NSMutableSet *certificates = [NSMutableSet setWithCapacity:[paths count]];
    // 将证书文件转成data
  	for (NSString *path in paths) {
        NSData *certificateData = [NSData dataWithContentsOfFile:path];
        [certificates addObject:certificateData];
    }

    return [NSSet setWithSet:certificates];
}

有三种初始化的方法，一种是默认策略，AFSSLPinningModeNone

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+ (instancetype)defaultPolicy {
    AFSecurityPolicy *securityPolicy = [[self alloc] init];
    securityPolicy.SSLPinningMode = AFSSLPinningModeNone;

    return securityPolicy;
}

第二种是自定义一个安全策略，然后读取cer文件放到集合里面

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+ (instancetype)policyWithPinningMode:(AFSSLPinningMode)pinningMode {
    return [self policyWithPinningMode:pinningMode withPinnedCertificates:[self defaultPinnedCertificates]];
}
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+ (NSSet *)defaultPinnedCertificates {
    static NSSet *_defaultPinnedCertificates = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        NSBundle *bundle = [NSBundle bundleForClass:[self class]];
        _defaultPinnedCertificates = [self certificatesInBundle:bundle];
    });

    return _defaultPinnedCertificates;
}

第三种则是需要我们多传入一个证书集合

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+ (instancetype)policyWithPinningMode:(AFSSLPinningMode)pinningMode withPinnedCertificates:(NSSet *)pinnedCertificates {
    AFSecurityPolicy *securityPolicy = [[self alloc] init];
    securityPolicy.SSLPinningMode = pinningMode;

    [securityPolicy setPinnedCertificates:pinnedCertificates];

    return securityPolicy;
}

设置证书的时候，就是把上面初始化时传入的证书取出公钥，再把公钥保存到mutablePinnedPublicKeys集合中

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- (void)setPinnedCertificates:(NSSet *)pinnedCertificates {
    _pinnedCertificates = pinnedCertificates;

    if (self.pinnedCertificates) {
        NSMutableSet *mutablePinnedPublicKeys = [NSMutableSet setWithCapacity:[self.pinnedCertificates count]];
        for (NSData *certificate in self.pinnedCertificates) {
            // 取出公钥
          	id publicKey = AFPublicKeyForCertificate(certificate);
            if (!publicKey) {
                continue;
            }
            // 将公钥存到集合
            [mutablePinnedPublicKeys addObject:publicKey];
        }
        self.pinnedPublicKeys = [NSSet setWithSet:mutablePinnedPublicKeys];
    } else {
        self.pinnedPublicKeys = nil;
    }
}

在AFPublicKeyForCertificate方法里面，做了一系列操作后返回公钥，如下：

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static id AFPublicKeyForCertificate(NSData *certificate) {
    id allowedPublicKey = nil;
    SecCertificateRef allowedCertificate;
    SecCertificateRef allowedCertificates[1];
    CFArrayRef tempCertificates = nil;
    SecPolicyRef policy = nil;
    SecTrustRef allowedTrust = nil;
    SecTrustResultType result;

  	// 取出证书SecCertificateRef
    allowedCertificate = SecCertificateCreateWithData(NULL, (__bridge CFDataRef)certificate);
    __Require_Quiet(allowedCertificate != NULL, _out);
	
  	// 生成证书数组
    allowedCertificates[0] = allowedCertificate;
    tempCertificates = CFArrayCreate(NULL, (const void **)allowedCertificates, 1, NULL);

  	// 生成SecPolicyRef
    policy = SecPolicyCreateBasicX509();
    __Require_noErr_Quiet(SecTrustCreateWithCertificates(tempCertificates, policy, &allowedTrust), _out);
    __Require_noErr_Quiet(SecTrustEvaluate(allowedTrust, &result), _out);
	
  	// 从SecPolicyRef中取出公钥
    allowedPublicKey = (__bridge_transfer id)SecTrustCopyPublicKey(allowedTrust);

_out:
    // 一些资源的释放

    return allowedPublicKey;
}

-[evaluateServerTrust:forDomain:]方法是AFSecurityPolicy类最长也是最重要的方法，它用来验证服务端是否是受信的

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- (BOOL)evaluateServerTrust:(SecTrustRef)serverTrust
                  forDomain:(NSString *)domain
{
    // 苹果文档中表示不要隐式地信任自己签名的证书，取而代之的是应该增加自己的CA证书到受信列表里
    if (domain && self.allowInvalidCertificates && self.validatesDomainName && (self.SSLPinningMode == AFSSLPinningModeNone || [self.pinnedCertificates count] == 0)) {
        NSLog(@"In order to validate a domain name for self signed certificates, you MUST use pinning.");
        return NO;
    }

    // policies增加SecPolicyRef
    NSMutableArray *policies = [NSMutableArray array];
    if (self.validatesDomainName) {
        [policies addObject:(__bridge_transfer id)SecPolicyCreateSSL(true, (__bridge CFStringRef)domain)];
    } else {
        [policies addObject:(__bridge_transfer id)SecPolicyCreateBasicX509()];
    }
	
    // 设置信任的policies应当被验证
    SecTrustSetPolicies(serverTrust, (__bridge CFArrayRef)policies);
	
    // 向系统内置的根证书验证服务端返回的证书是否合法
    if (self.SSLPinningMode == AFSSLPinningModeNone) {
        return self.allowInvalidCertificates || AFServerTrustIsValid(serverTrust);
    } else if (!AFServerTrustIsValid(serverTrust) && !self.allowInvalidCertificates) {
        return NO;
    }
	
    // 根据SSLPinningMode对服务端是否受信进行校验
    switch (self.SSLPinningMode) {
        case AFSSLPinningModeNone:
        default:
            return NO;
        case AFSSLPinningModeCertificate: {
            NSMutableArray *pinnedCertificates = [NSMutableArray array];
            for (NSData *certificateData in self.pinnedCertificates) {
                [pinnedCertificates addObject:(__bridge_transfer id)SecCertificateCreateWithData(NULL, (__bridge CFDataRef)certificateData)];
            }
            SecTrustSetAnchorCertificates(serverTrust, (__bridge CFArrayRef)pinnedCertificates);

            if (!AFServerTrustIsValid(serverTrust)) {
                return NO;
            }

            // obtain the chain after being validated, which *should* contain the pinned certificate in the last position (if it's the Root CA)
            NSArray *serverCertificates = AFCertificateTrustChainForServerTrust(serverTrust);
            
            for (NSData *trustChainCertificate in [serverCertificates reverseObjectEnumerator]) {
                if ([self.pinnedCertificates containsObject:trustChainCertificate]) {
                    return YES;
                }
            }
            
            return NO;
        }
        case AFSSLPinningModePublicKey: {
            NSUInteger trustedPublicKeyCount = 0;
            NSArray *publicKeys = AFPublicKeyTrustChainForServerTrust(serverTrust);

            for (id trustChainPublicKey in publicKeys) {
                for (id pinnedPublicKey in self.pinnedPublicKeys) {
                    if (AFSecKeyIsEqualToKey((__bridge SecKeyRef)trustChainPublicKey, (__bridge SecKeyRef)pinnedPublicKey)) {
                        trustedPublicKeyCount += 1;
                    }
                }
            }
            return trustedPublicKeyCount > 0;
        }
    }
    
    return NO;
}