如何从文件中恢复RSA？

Question

我想从文件中恢复公钥。这是有效的Java代码：

PublicKey readPubKeyFromFile(AssetFileDescriptor cle) throws IOException {
    // read RSA public key
    byte[] encodedKey = new byte[(int) cle.getDeclaredLength()];
    cle.createInputStream().read(encodedKey);

    // create public key
    X509EncodedKeySpec publicKeySpec = new X509EncodedKeySpec(encodedKey);
    PublicKey pk = null;
    try {
        KeyFactory kf = KeyFactory.getInstance("RSA");
        pk = kf.generatePublic(publicKeySpec);
    } catch(Exception e) {
        Logger.getInstance().logError("KeyUtils", e.toString());
    }
    return pk;
}

这是不起作用的iOS代码：

-(SecKeyRef)readPublicKeyFromFile:(NSString*)filename andExtension:(NSString*)extension {

NSString*   filePath = [[NSBundle mainBundle] pathForResource:filename ofType:extension];
NSData*     encodedKey = [NSData dataWithContentsOfFile:filePath];

CFDataRef myCertData = (CFDataRef)encodedKey;

SecCertificateRef cert = SecCertificateCreateWithData (kCFAllocatorSystemDefault, myCertData);
CFArrayRef certs = CFArrayCreate(kCFAllocatorDefault, (const void **) &cert, 1, NULL);
SecPolicyRef policy = SecPolicyCreateBasicX509();

SecTrustRef trust;

OSStatus check =  SecTrustCreateWithCertificates(certs, policy, &trust);

if (check != noErr)
{
    NSLog(@"Problem extracting public key from file: %@", filename);
    return nil;
}

SecTrustResultType trustResult;
SecTrustEvaluate(trust, &trustResult);
SecKeyRef pub_key_leaf = SecTrustCopyPublicKey(trust);

return pub_key_leaf;
}

我的iOS代码有什么错误的想法吗？

Answer 1

我测试了你的代码，没有任何问题。该问题似乎与您尝试获取公钥的证书的格式有关。

函数 SecCertificateCreateWithData() 假定您提供的证书是 DER 格式。您找到的大多数证书都是以 Base64 编码的，就像著名的 .pem 格式一样。我已经使用格式正确的 DER 证书测试了您的代码（使用 openssl 将developer.apple.com 形式的证书转换为 DER），并且公钥已正确提取。

要将 .pem 证书转换为 DER，只需在终端中使用 openssl：

openssl x509 -in developer.apple.com.pem  -outform der -out cert.der

之后，输出证书文件应该可以正常使用您的代码。

但是您可以在应用程序本身上转换证书，您只需获取 x509 base64 编码的证书（假设您使用的是 .pem 编码的证书）并将其转换为二进制。

有一个示例，您可以如何做到这一点：

此代码将假定证书按照以下标准进行编码：

-----BEGIN CERTIFICATE-----
< your base64 encoded certificate goes here >
-----END CERTIFICATE-----

将此证书转换为二进制 DER 的代码是：

-(NSData *)getBinaryCertificateFromPemEncodedFile:(NSString *)filename andExtension:(NSString *)extension
{
    NSString* filePath = [[NSBundle mainBundle] pathForResource:filename ofType:extension];
    NSString *pemCert = [NSString stringWithContentsOfFile:filePath encoding:NSUTF8StringEncoding error:nil];
    //The header and footer conforms to .pem specificatio
    NSString *header = @"-----BEGIN CERTIFICATE-----";
    NSString *footer = @"-----END CERTIFICATE-----";

    NSString *base64Cert;
    NSScanner *scanner = [NSScanner scannerWithString:pemCert];
    //First we ignore the header part
    [scanner scanString:header intoString:nil];
    //Then we copy the base64 string excluding the footer
    [scanner scanUpToString:footer intoString:&base64Cert];

    //The reason I'm using NSDataBase64DecodingIgnoreUnknownCharacters is to exclude possible line breaks in the encoding
    NSData *binaryCertificate = [[NSData alloc] initWithBase64EncodedString:base64Cert options:NSDataBase64DecodingIgnoreUnknownCharacters];

    return binaryCertificate;
}

然后，对功能完美的代码进行一些小修改即可达到目的：

-(SecKeyRef)readPublicKeyFromCertificate:(NSData *)binaryCertificate {

    NSData *encodedKey = binaryCertificate;

    CFDataRef myCertData = (CFDataRef)CFBridgingRetain(encodedKey);

    SecCertificateRef cert = SecCertificateCreateWithData(kCFAllocatorSystemDefault, myCertData);
    SecPolicyRef policy = SecPolicyCreateBasicX509();

    SecTrustRef trust;
    //If you only have one certificate you don't need to put it inside an array
    OSStatus check =  SecTrustCreateWithCertificates(cert, policy, &trust);

    if (check != noErr)
    {
        NSLog(@"Problem extracting public key from certificate");
        return nil;
    }

    SecTrustResultType trustResult;
    SecTrustEvaluate(trust, &trustResult);
    SecKeyRef pub_key_leaf = SecTrustCopyPublicKey(trust);

    return pub_key_leaf;
}

然后只需调用它：

NSData *data = [self getBinaryCertificateFromPemEncodedFile:@"developer" andExtension:@"pem"];
SecKeyRef key = [self readPublicKeyFromCertificate:data];
NSLog(@"%@", key);

如果您的证书“有效”，您应该看到：

2014-09-15 21:52:13.275 cert[15813:60b] <SecKeyRef algorithm id: 1,
key type: RSAPublicKey, version: 2, block size: 2048 bits, exponent: {hex: 10001, decimal: 65537},
modulus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
addr: 0x9a48200>

为了进行测试，我使用了来自developer.apple.com的证书，您可以检查日志中的公钥并进行比较。