前言

前文iOS如何保證下載資源的可靠性介紹了基于RSA的下載資源驗(yàn)證方案鉴未，這次詳細(xì)介紹開(kāi)發(fā)過(guò)程中的問(wèn)題。

iOS接入步驟

• 后臺(tái)上傳資源文件，配置平臺(tái)對(duì)文件進(jìn)行hash并用私鑰進(jìn)行簽名得到簽名串signature胯究；
• 把文件和signature打包成zip包，下發(fā)到客戶端岸蜗；
• 客戶端解壓zip埃难，得到文件和簽名串signature，對(duì)文件進(jìn)行hash蟋恬，加載本地公鑰翁潘，把hash值、signature歼争、公鑰傳給Security.framework拜马；
• 用Security.framework提供的SecKeyRawVerify方法對(duì)hash值渗勘、signature、公鑰進(jìn)行驗(yàn)證俩莽，如果通過(guò)則表示文件未修改旺坠。

1、zip解壓

iOS平臺(tái)上可以使用MiniZipArchive進(jìn)行解壓扮超。

- (BOOL)unzipFile:(NSString *)file toFilePath:(NSString *)unZipFilePath overWrite:(BOOL)overWrite
{
    MiniZipArchive *za = [[MiniZipArchive alloc] init];
    BOOL success = NO;
    if ([za UnzipOpenFile:file]) {
        success = [za UnzipFileTo:unZipFilePath overWrite:overWrite];
        [za UnzipCloseFile];

    }
    return success;
}

2取刃、公鑰和私鑰的加載

.der格式和.pem格式：.der格式表示二進(jìn)制編碼，.pem格式表示Base64編碼出刷。
iOS的公鑰需要用.der格式璧疗，私鑰需要用.p12格式，這個(gè)可以用openssl的指令來(lái)轉(zhuǎn)換馁龟。（指令見(jiàn)末尾）
加載的時(shí)候先用NSData加載密鑰崩侠，再用下面的：
getPrivateKeyRefWithContentsOfFile: password:方法加載密鑰；
getPublicKeyRefrenceFromeData:方法加載公鑰坷檩；

//獲取私鑰
- (SecKeyRef)getPrivateKeyRefWithContentsOfFile:(NSData *)p12Data password:(NSString*)password {
    if (!p12Data) {
        return nil;
    }
    SecKeyRef privateKeyRef = NULL;
    NSMutableDictionary * options = [[NSMutableDictionary alloc] init];
    [options setObject: password forKey:(__bridge id)kSecImportExportPassphrase];
    CFArrayRef items = CFArrayCreate(NULL, 0, 0, NULL);
    OSStatus securityError = SecPKCS12Import((__bridge CFDataRef) p12Data, (__bridge CFDictionaryRef)options, &items);
    if (securityError == noErr && CFArrayGetCount(items) > 0) {
        CFDictionaryRef identityDict = CFArrayGetValueAtIndex(items, 0);
        SecIdentityRef identityApp = (SecIdentityRef)CFDictionaryGetValue(identityDict, kSecImportItemIdentity);
        securityError = SecIdentityCopyPrivateKey(identityApp, &privateKeyRef);
        if (securityError != noErr) {
            privateKeyRef = NULL;
        }
    }
    CFRelease(items);
    
    return privateKeyRef;
}

- (SecKeyRef)getPublicKeyRefrenceFromeData:(NSData *)certData {
    SecKeyRef publicKeyRef = NULL;
    CFDataRef myCertData = (__bridge CFDataRef)certData;
    SecCertificateRef cert = SecCertificateCreateWithData(NULL, (CFDataRef)myCertData);
    if (cert == nil) {
        NSLog(@"Can not read certificate ");
        return nil;
    }
    SecPolicyRef policy = SecPolicyCreateBasicX509();
    SecCertificateRef certArray[1] = {cert};
    CFArrayRef myCerts = CFArrayCreate(NULL, (void *)(void *)certArray, 1, NULL);
    SecTrustRef trust;
    OSStatus status = SecTrustCreateWithCertificates(myCerts, policy, &trust);
    if (status != noErr) {
        NSLog(@"SecTrustCreateWithCertificates fail. Error Code: %d", (int)status);
        CFRelease(cert);
        CFRelease(policy);
        CFRelease(myCerts);
        return nil;
    }
    SecTrustResultType trustResult;
    status = SecTrustEvaluate(trust, &trustResult);
    if (status != noErr) {
        NSLog(@"SecTrustEvaluate fail. Error Code: %d", (int)status);
        CFRelease(cert);
        CFRelease(policy);
        CFRelease(trust);
        CFRelease(myCerts);
        return nil;
    }
    publicKeyRef = SecTrustCopyPublicKey(trust);
    
    CFRelease(cert);
    CFRelease(policy);
    CFRelease(trust);
    CFRelease(myCerts);
    
    return publicKeyRef;
}

3却音、私鑰簽名和公鑰驗(yàn)證

加載完公鑰和私鑰之后，用私鑰可以對(duì)原始數(shù)據(jù)進(jìn)行簽名矢炼，詳見(jiàn)PKCSSignBytesSHA256withRSA方法系瓢，返回的是簽名串；
在用zip解壓出來(lái)的簽名串進(jìn)行驗(yàn)證的時(shí)候裸删，需要用本地的公鑰八拱、原始數(shù)據(jù)和簽名串進(jìn)行驗(yàn)簽，詳見(jiàn)PKCSVerifyBytesSHA256withRSA方法涯塔；
注意的是肌稻，因?yàn)檫x擇的算法是kSecPaddingPKCS1SHA256，需要對(duì)原始數(shù)據(jù)進(jìn)行一次SHA256的hash匕荸。（kSecPaddingPKCS1SHA256只能用于SecKeyRawSign/SecKeyRawVerify）

BOOL PKCSVerifyBytesSHA256withRSA(NSData* plainData, NSData* signature, SecKeyRef publicKey)
{
    if (!plainData || !signature) { // 保護(hù)
        return NO;
    }
    size_t signedHashBytesSize = SecKeyGetBlockSize(publicKey);
    const void* signedHashBytes = [signature bytes];
    
    size_t hashBytesSize = CC_SHA256_DIGEST_LENGTH;
    uint8_t* hashBytes = malloc(hashBytesSize);
    if (!CC_SHA256([plainData bytes], (CC_LONG)[plainData length], hashBytes)) {
        return NO;
    }
    
    OSStatus status = SecKeyRawVerify(publicKey,
                                      kSecPaddingPKCS1SHA256,
                                      hashBytes,
                                      hashBytesSize,
                                      signedHashBytes,
                                      signedHashBytesSize);
    
    return status == errSecSuccess;
}

NSData* PKCSSignBytesSHA256withRSA(NSData* plainData, SecKeyRef privateKey)
{
    size_t signedHashBytesSize = SecKeyGetBlockSize(privateKey);
    uint8_t* signedHashBytes = malloc(signedHashBytesSize);
    memset(signedHashBytes, 0x0, signedHashBytesSize);
    
    size_t hashBytesSize = CC_SHA256_DIGEST_LENGTH;
    uint8_t* hashBytes = malloc(hashBytesSize);
    if (!CC_SHA256([plainData bytes], (CC_LONG)[plainData length], hashBytes)) {
        return nil;
    }
    
    SecKeyRawSign(privateKey,
                  kSecPaddingPKCS1SHA256,
                  hashBytes,
                  hashBytesSize,
                  signedHashBytes,
                  &signedHashBytesSize);
    
    NSData* signedHash = [NSData dataWithBytes:signedHashBytes
                                        length:(NSUInteger)signedHashBytesSize];
    
    if (hashBytes)
        free(hashBytes);
    if (signedHashBytes)
        free(signedHashBytes);
    
    return signedHash;
}

4爹谭、簽名串的保存

簽名串可以使用setxattrf寫(xiě)入文件的擴(kuò)展屬性，保證簽名串和資源的一一對(duì)應(yīng)榛搔。

-(BOOL)setExtendValueWithPath:(NSString *)path key:(NSString *)key value:(NSData *)value {
    ssize_t writelen = setxattr([path fileSystemRepresentation],
                                [key UTF8String],
                                [value bytes],
                                [value length],
                                0,
                                0);
    return writelen == 0;
}

比較奇怪的是诺凡，比較寫(xiě)入擴(kuò)展屬性之后的文件大小，并沒(méi)有發(fā)生較大變化践惑。在特意查詢文檔之后腹泌，發(fā)現(xiàn)下面一句話：

Space consumed for extended attributes is counted towards the disk quotasof the file owner and file group
原來(lái)擴(kuò)展屬性并不是寫(xiě)入文件，而是由文件系統(tǒng)來(lái)保存尔觉。

遇到的問(wèn)題

1凉袱、驗(yàn)證失敗，SecKeyRawVerify返回-9809

經(jīng)常遇到的問(wèn)題是，配置平臺(tái)的簽名在iOS客戶端驗(yàn)證不通過(guò)专甩，可以按照下面的流程檢測(cè)：

• 首先是確保兩端的公鑰和私鑰是一對(duì)钟鸵；
• 配置平臺(tái)簽名完之后，用iOS客戶端的公鑰在本地驗(yàn)證涤躲；
• 確認(rèn)兩邊使用的簽名算法設(shè)置參數(shù)一致棺耍；
• iOS客戶端用配置平臺(tái)的私鑰進(jìn)行簽名，再用公鑰進(jìn)行驗(yàn)證种樱；
• 對(duì)比配置平臺(tái)的簽名串和iOS的簽名串蒙袍；

openssl的驗(yàn)證命令
openssl dgst -sign private_key.pem -sha256 -out sign source
openssl dgst -verify rsa_public_key.pem -sha256 -signature sign source
如果驗(yàn)證通過(guò)會(huì)有文字提示：Verified OK

2、生成證書(shū)失敗缸托，openssl X509: 出現(xiàn) Expecting: TRUSTED CERTIFICATE的錯(cuò)誤

參考這些公鑰和密鑰的openssl生成命令
openssl genrsa -out private_key.pem 1024
openssl req -new -key private_key.pem -out rsaCertReq.csr
openssl x509 -req -days 3650 -in rsaCertReq.csr -signkey private_key.pem -out rsaCert.crt
openssl x509 -outform der -in rsaCert.crt -out public_key.der
openssl pkcs12 -export -out private_key.p12 -inkey private_key.pem -in rsaCert.crt

參考自GithubGist

附錄

Signing and Verifying on iOS using RSA
xattr manpages
demo地址