ios - 使用 EC key 签名时 SecKeyRawSign 返回 -50

Question

我正在尝试编写一个原型(prototype)来生成一组 Ecliptic Curve key (256 位)，然后使用私钥签署一条消息。我有生成和管理运行良好的 key 的代码，但是当我尝试调用 SecKeyRawSign 时，出现 -50 errSecParam 错误。生成 key 的代码如下所示:

private func generateKeyPair() throws {
    var error: Unmanaged<CFError>? = nil
    let acl = SecAccessControlCreateWithFlags(nil, kSecAttrAccessibleWhenPasscodeSetThisDeviceOnly,
                                              [.touchIDAny, .privateKeyUsage], &error)

    guard error == nil else {
        throw MessageError(message: "Could not create ACL: \(error)")
    }

    // We don't want the public key stored in the ecure enclave, so we create it as
    // non permament and add it manually to the keychain later
    let publicKeyParameters: [CFString: Any] = [
        kSecAttrIsPermanent: false,
        kSecAttrApplicationTag: ViewController.KeyTag,
        kSecAttrLabel: ViewController.PublicLabel
    ]

    let privateKeyParameters: [CFString: Any] = [
        kSecAttrIsPermanent: true,
        kSecAttrApplicationTag: ViewController.KeyTag,
        kSecAttrLabel: ViewController.PrivateLabel,
        kSecAttrAccessControl: acl!
    ]

    var parameters: [CFString: Any] = [
        kSecAttrKeyType: kSecAttrKeyTypeEC,
        kSecAttrKeySizeInBits: NSNumber(value: 256),
        kSecPublicKeyAttrs: publicKeyParameters,
        kSecPrivateKeyAttrs: privateKeyParameters
    ]

    // On the simulator we can't use the Secure Enclave
    if hasSecureEnclave() {
        parameters[kSecAttrTokenID] = kSecAttrTokenIDSecureEnclave
    }

    var pubKeyRef, privKeyRef: SecKey?
    var result = SecKeyGeneratePair(parameters as CFDictionary, &pubKeyRef, &privKeyRef)
    guard result == noErr else {
        throw MessageError(message: "Could not create key pair: \(result)")
    }

    parameters = [
        kSecClass: kSecClassKey,
        kSecAttrKeyType: kSecAttrKeyTypeEC,
        kSecAttrApplicationTag: ViewController.KeyTag,
        kSecAttrLabel: ViewController.PublicLabel,
        kSecAttrKeyClass: kSecAttrKeyClassPublic,
        kSecValueRef: pubKeyRef!
    ]

    result = SecItemAdd(parameters as CFDictionary, nil)
    guard result == noErr else {
        throw MessageError(message: "Could not add public key to keychain: \(result)")
    }
}

签名代码如下所示:

private func signWithPrivateKey(_ text: String, _ key: SecKey) throws -> String? {
    var digest = Data(count: Int(CC_SHA256_DIGEST_LENGTH))
    let data = text.data(using: .utf8)!

    let _ = digest.withUnsafeMutableBytes { digestBytes in
        data.withUnsafeBytes { dataBytes in
            CC_SHA256(dataBytes, CC_LONG(data.count), digestBytes)
        }
    }

    var signature = Data(count: SecKeyGetBlockSize(key))
    var signatureLength = signature.count

    let result = signature.withUnsafeMutableBytes { signatureBytes in
        digest.withUnsafeBytes { digestBytes in
            SecKeyRawSign(key,
                          SecPadding.PKCS1SHA256,
                          digestBytes,
                          digest.count,
                          signatureBytes,
                          &signatureLength)
        }
    }

    guard result == noErr else {
        throw MessageError(message: "Could not sign data: \(result)")
    }

    return signature.base64EncodedString()
}

很明显，sign 函数中的最后一个守卫被触发了，它返回了 errSecParam。

有人使用 EC key 在 iOS 中成功完成数据签名吗？如果是这样，你在这里看到任何明显的东西吗？切线地有没有办法获得有关错误本身的更多信息。

编辑:要添加一个重要的细节，如果我只更改此代码以生成 2048 位 RSA key ，则代码可以正常工作。生成 key 并签署消息。只有 256 位 EC key 才会失败。有没有其他方法可以在 iOS 中执行 ECDSA？

Answer 1

我解决了这个问题。我为保存签名而创建的缓冲区太小。我改成使用 SecKeyGetBlockSize() * 4，然后在调用后将缓冲区减少到 signatureLenght。此时我唯一的问题是是否有更好的方法计算长度(除了调用 SecKeyRawSign，让它失败，然后将缓冲区大小调整为返回的大小)。

新的标志代码如下所示:

private func signWithPrivateKey(_ text: String, _ key: SecKey) throws -> String? {
    var digest = Data(count: Int(CC_SHA256_DIGEST_LENGTH))
    let data = text.data(using: .utf8)!

    let _ = digest.withUnsafeMutableBytes { digestBytes in
        data.withUnsafeBytes { dataBytes in
            CC_SHA256(dataBytes, CC_LONG(data.count), digestBytes)
        }
    }

    var signature = Data(count: SecKeyGetBlockSize(key) * 4)
    var signatureLength = signature.count

    let result = signature.withUnsafeMutableBytes { signatureBytes in
        digest.withUnsafeBytes { digestBytes in
            SecKeyRawSign(key,
                          SecPadding.PKCS1SHA256,
                          digestBytes,
                          digest.count,
                          signatureBytes,
                          &signatureLength)
        }
    }

    let count = signature.count - signatureLength
    signature.removeLast(count)

    guard result == noErr else {
        throw MessageError(message: "Could not sign data: \(result)")
    }

    return signature.base64EncodedString()
}