前言
最近项目里一部分接口用到了服务端的自建证书, 之前一直是用AFNetwork来处理相关的逻辑, 第一次接触到https证书认证相关逻辑, 写篇小笔记记录下
https在iOS的具体体现 (单向验证)
在整个https通讯流程中, 对于app来说, 关键点其实就是公钥的认证. 确保当前通讯的服务端所下发的公钥与公司所在host的服务端是同一个证书所导出的公钥, 才开始使用该公钥来加密传输数据.
AFNetworking要使用自建证书的代码逻辑
先上处理代码, 要使用自建证书时, 把证书的数据放入AFHTTPSessionManager单例
//cerPath为证书在bundle里的路径
NSData *certData = [NSData dataWithContentsOfFile:cerPath];
SecCertificateRef httpBinCertificate = SecCertificateCreateWithData(NULL, (__bridge CFDataRef)(certData));
NSSet *certSet = [[NSSet alloc] initWithObjects:(__bridge_transfer NSData *)SecCertificateCopyData(httpBinCertificate), nil];
AFSecurityPolicy *securityPolicy = [AFSecurityPolicy policyWithPinningMode:AFSSLPinningModePublicKey];
// 是否允许,NO-- 不允许无效的证书
[securityPolicy setAllowInvalidCertificates:NO];
// 设置证书
[securityPolicy setPinnedCertificates:certSet];
[AFHTTPSessionManager manager].securityPolicy = securityPolicy;
核心方法
如果是用的自建证书是无法通过系统的默认认证规则的, 这时https连接就会被中断, 如果要令自建证书也可以建立https连接, 就需要实现以下这个代理方法
- (void)URLSession:(NSURLSession *)session
didReceiveChallenge:(NSURLAuthenticationChallenge *)challenge
completionHandler:(void (^)(NSURLSessionAuthChallengeDisposition disposition, NSURLCredential *credential))completionHandler
如果你响应这个方法, 系统会将本次TCP链接中的证书数据通过这个代理方法中的challenge参数抛给你, 当你验证完成后, 通过调用completionHandler回调告知系统你的验证结果
AFNetworking处理自建证书的逻辑
- (void)URLSession:(NSURLSession *)session
didReceiveChallenge:(NSURLAuthenticationChallenge *)challenge
completionHandler:(void (^)(NSURLSessionAuthChallengeDisposition disposition, NSURLCredential *credential))completionHandler
{
NSURLSessionAuthChallengeDisposition disposition = NSURLSessionAuthChallengePerformDefaultHandling;
__block NSURLCredential *credential = nil;
if (self.sessionDidReceiveAuthenticationChallenge) {
disposition = self.sessionDidReceiveAuthenticationChallenge(session, challenge, &credential);
} else {
if ([challenge.protectionSpace.authenticationMethod isEqualToString:NSURLAuthenticationMethodServerTrust]) {
if ([self.securityPolicy evaluateServerTrust:challenge.protectionSpace.serverTrust forDomain:challenge.protectionSpace.host]) {
credential = [NSURLCredential credentialForTrust:challenge.protectionSpace.serverTrust];
if (credential) {
disposition = NSURLSessionAuthChallengeUseCredential;
} else {
disposition = NSURLSessionAuthChallengePerformDefaultHandling;
}
} else {
disposition = NSURLSessionAuthChallengeCancelAuthenticationChallenge;
}
} else {
disposition = NSURLSessionAuthChallengePerformDefaultHandling;
}
}
if (completionHandler) {
completionHandler(disposition, credential);
}
}
这里的关键是你验证证书后通过completionHandler告知系统你的验证结果, 该block回调第一个参数就是你的验证结果
typedef NS_ENUM(NSInteger, NSURLSessionAuthChallengeDisposition) {
NSURLSessionAuthChallengeUseCredential = 0, /* Use the specified credential, which may be nil */
NSURLSessionAuthChallengePerformDefaultHandling = 1, /* Default handling for the challenge - as if this delegate were not implemented; the credential parameter is ignored. */
NSURLSessionAuthChallengeCancelAuthenticationChallenge = 2, /* The entire request will be canceled; the credential parameter is ignored. */
NSURLSessionAuthChallengeRejectProtectionSpace = 3, /* This challenge is rejected and the next authentication protection space should be tried; the credential parameter is ignored. */
}
这些结果枚举基本能望文生义就不多做解释了, completionHandler第二个参数一般就是把challenge里的证书对象回传进去, 这一步的关键是如何验证这个证书对象是合法的, 在afn中, 验证的逻辑放在这个方法里
- (BOOL)evaluateServerTrust:(SecTrustRef)serverTrust
forDomain:(NSString *)domain
AFNetworking处理https验证的几种方式
证书里面包含里公钥, SHA-256指纹等多种信息, 根据实际需要, 验证策略可以有所不同, 下面
二进制数据比对
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]) {
//直接比对NSData是否一致
if ([self.pinnedCertificates containsObject:trustChainCertificate]) {
return YES;
}
}
return NO;
}
最简单粗暴的方式, 直接将证书转成NSData进行比对, 完全一致就可以确定是同一张证书
公钥比对
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;
}
只比对证书的公钥, 毕竟只要公钥没被修改, 即使传输了数据, 对方也没有对应的私钥可以解密你的数据, 相对没那么严谨的验证方式, 由于证书链里会有个多证书, 注意afn里面的逻辑是主要证书链上有任意一个节点的证书公钥与self.pinnedPublicKeys这个受信任的公钥是相同时, 就会认为这次认证是成功的
疑问
看完之后还是有几点小疑问
第一是如果证书里面带有该证书的SHA-256签名, 那是不是等于不需要在客户端预埋证书呢? 只需要写死SHA-256的值, 比对服务端下发的证书的SHA-256是否一致就可以提取公钥了.
第二个是锚点证书的问题, 在afn里如果使用自建证书, afn会帮你将该自建证书设置为锚点证书. 但其实注释了锚点证书的设置也一样能建立起https连接, 那设置锚点证书的意义在哪呢?
待续...
