在iOS/Swift中创建并导出为base64的RSA公钥

Question

TL;DR:在iOS中生成并存储在密钥链中的RSA公钥，导出为base64并发送到java后端，是不被识别的。

我正在iOS应用程序中实现聊天加密功能，我使用对称+非对称密钥来处理它。

在后端，我使用用户的公钥加密用于加密和解密消息的对称密钥，而不需要过多的细节。

我创建了两个框架，分别用Swift和Java (后端)来处理密钥生成、加密、解密等。我还为它们进行了测试，所以我100%地按照预期的方式工作。

但是，后端似乎无法识别从iOS传递的公钥的格式。使用RSA双方，这是我在Swift中用来生成密钥的代码：

// private key parameters
static let privateKeyParams: [String : Any] = [
        kSecAttrIsPermanent as String: true,
        kSecAttrApplicationTag as String: "..." // I have a proper unique tag here
]

// public  key parameters
static let publicKeyParams: [String : Any] = [
        kSecAttrIsPermanent as String: true,
        kSecAttrApplicationTag as String: "..." // I have a proper unique tag here
]

// global parameters for our key generation
static let keyCreationParameters: [String : Any] = [
        kSecAttrKeyType as String: kSecAttrKeyTypeRSA,
        kSecAttrKeySizeInBits as String: 2048,
        kSecPublicKeyAttrs as String: publicKeyParams,
        kSecPrivateKeyAttrs as String: privateKeyParams
]

...

var publicKey, privateKey: SecKey?
let status = SecKeyGeneratePair(Constants.keyCreationParameters as CFDictionary, &publicKey, &privateKey)

我用镜面密码来读取钥匙链上的钥匙。

这是一段代码，用于将公钥导出为base64字符串：

extension SecKey {
  func asBase64() throws -> String {
    var dataPtr: CFTypeRef?
    let query: [String:Any] = [
      kSecClass as String: kSecClassKey,
      kSecAttrApplicationTag as String: "...", // Same unique tag here
      kSecAttrKeyType as String: kSecAttrKeyTypeRSA,
      kSecReturnData as String: kCFBooleanTrue
    ]
    let result = SecItemCopyMatching(query as CFDictionary, &dataPtr)

    switch (result, dataPtr) {
    case (errSecSuccess, .some(let data)):
      // convert to Base64 string
      let base64PublicKey = data.base64EncodedString(options: [])
      return base64PublicKey
    default:
      throw CryptoError.keyConversionError
    }
  }
}

在后端级别，我使用这段Java代码将base64字符串转换为公钥：

public PublicKey publicKeyFrom(String data) throws NoSuchAlgorithmException, InvalidKeySpecException {
    byte[] publicBytes = Base64.decodeBase64(data);
    X509EncodedKeySpec keySpec = new X509EncodedKeySpec(publicBytes);
    KeyFactory keyFactory = KeyFactory.getInstance("RSA");
    return keyFactory.generatePublic(keySpec);
}

但是，这在最后一行失败了，但有以下例外：

java.security.spec.InvalidKeySpecException: java.security.InvalidKeyException: IOException: algid parse error, not a sequence

在进行一些手动调试时，我注意到公钥的格式是不同的--当我在iOS中生成一个密钥，然后导出为base 64时，如下所示：

MIIBCgKCAQEA4M/bRDdH0f6qFIXxOg13RHka+g4Yv8u9PpPp1IR6pSwrM1aq8B6cyKRwnLe/MOkvODvDfJzvGXGQ01zSTxYWAW1B4uc/NCEemCmZqMosSB/VUJdNxxWtt2hJxpz06hAawqV+6HmweAB2dUn9tDEsQLsNHdwYouOKpyRZGimcF9qRFn1RjR0Q54sUh1tQAj/EwmgY2S2bI5TqtZnZw7X7Waji7wWi6Gz88IkuzLAzB9VBNDeV1cfJFiWsZ/MIixSvhpW3dMNCrJShvBouIG8nS+vykBlbFVRGy3gJr8+OcmIq5vuHVhqrWwHNOs+WR87K/qTFO/CB7MiyiIV1b1x5DQIDAQAB

对于总共360个字符，而在Java (仍在使用RSA)中进行相同操作时，如下所示：

MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQCAAnWO4BXUGP0qM3Op36YXkWNxb4I2pPZuZ7jJtfUO7v+IO1mq43WzNaxLqqLPkTnMrv2ACRDK55vin+leQlL1z0LzVxjtZ9F6pajQo1r7PqBlL5N8bzBFKpagEf0QfyHPw0/0kG9DMnvQ+Im881QyN2zdl33wp5Fi+jRT7cunFQIDAQAB

长度为216个字符。

我不知道出了什么问题--显然，如果iOS在另一个键中处理密钥，并且需要特殊处理才能与其他人交谈，我就不会感到惊讶了。

有什么想法吗？

Answer 1

在将iOS应用程序连接到Java后端时，我们遇到了完全相同的问题。pedrofb提到的CryptoExportImportManager也帮助了我们，这太棒了。但是，CryptoExportImportManager类中的代码有点详细，可能很难维护。这是因为在向DER编码中添加新组件时使用的是自顶向下的方法。因此，必须预先计算长度字段所包含的数字(即在定义适用长度的内容之前)。因此，我创建了一个新的类，我们现在使用它来转换RSA公钥的DER编码：

class RSAKeyEncoding: NSObject {

  // ASN.1 identifiers
  private let bitStringIdentifier: UInt8 = 0x03
  private let sequenceIdentifier: UInt8 = 0x30

  // ASN.1 AlgorithmIdentfier for RSA encryption: OID 1 2 840 113549 1 1 1 and NULL
  private let algorithmIdentifierForRSAEncryption: [UInt8] = [0x30, 0x0d, 0x06,
    0x09, 0x2a, 0x86, 0x48, 0x86, 0xf7, 0x0d, 0x01, 0x01, 0x01, 0x05, 0x00]

  /// Converts the DER encoding of an RSA public key that is either fetched from the
  /// keychain (e.g. by using `SecItemCopyMatching(_:_:)`) or retrieved in another way
  /// (e.g. by using `SecKeyCopyExternalRepresentation(_:_:)`), to a format typically
  /// used by tools and programming languages outside the Apple ecosystem (such as
  /// OpenSSL, Java, PHP and Perl). The DER encoding of an RSA public key created by
  /// iOS is represented with the ASN.1 RSAPublicKey type as defined by PKCS #1.
  /// However, many systems outside the Apple ecosystem expect the DER encoding of a
  /// key to be represented with the ASN.1 SubjectPublicKeyInfo type as defined by
  /// X.509. The two types are related in a way that if the SubjectPublicKeyInfo’s
  /// algorithm field contains the rsaEncryption object identifier as defined by
  /// PKCS #1, the subjectPublicKey field shall contain the DER encoding of an
  /// RSAPublicKey type.
  ///
  /// - Parameter rsaPublicKeyData: A data object containing the DER encoding of an
  ///     RSA public key, which is represented with the ASN.1 RSAPublicKey type.
  /// - Returns: A data object containing the DER encoding of an RSA public key, which
  ///     is represented with the ASN.1 SubjectPublicKeyInfo type.
  func convertToX509EncodedKey(_ rsaPublicKeyData: Data) -> Data {
    var derEncodedKeyBytes = [UInt8](rsaPublicKeyData)

    // Insert ASN.1 BIT STRING bytes at the beginning of the array
    derEncodedKeyBytes.insert(0x00, at: 0)
    derEncodedKeyBytes.insert(contentsOf: lengthField(of: derEncodedKeyBytes), at: 0)
    derEncodedKeyBytes.insert(bitStringIdentifier, at: 0)

    // Insert ASN.1 AlgorithmIdentifier bytes at the beginning of the array
    derEncodedKeyBytes.insert(contentsOf: algorithmIdentifierForRSAEncryption, at: 0)

    // Insert ASN.1 SEQUENCE bytes at the beginning of the array
    derEncodedKeyBytes.insert(contentsOf: lengthField(of: derEncodedKeyBytes), at: 0)
    derEncodedKeyBytes.insert(sequenceIdentifier, at: 0)

    return Data(derEncodedKeyBytes)
  }

  private func lengthField(of valueField: [UInt8]) -> [UInt8] {
    var length = valueField.count

    if length < 128 {
      return [ UInt8(length) ]
    }

    // Number of bytes needed to encode the length
    let lengthBytesCount = Int((log2(Double(length)) / 8) + 1)

    // First byte encodes the number of remaining bytes in this field
    let firstLengthFieldByte = UInt8(128 + lengthBytesCount)

    var lengthField: [UInt8] = []
    for _ in 0..<lengthBytesCount {
      // Take the last 8 bits of length
      let lengthByte = UInt8(length & 0xff)
      // Insert them at the beginning of the array
      lengthField.insert(lengthByte, at: 0)
      // Delete the last 8 bits of length
      length = length >> 8
    }

    // Insert firstLengthFieldByte at the beginning of the array
    lengthField.insert(firstLengthFieldByte, at: 0)

    return lengthField
  }
}

用法

您可以在函数asBase64()中使用这个类，如下所示：

extension SecKey {
  func asBase64() throws -> String {
    var dataPtr: CFTypeRef?
    let query: [String:Any] = [
      kSecClass as String: kSecClassKey,
      kSecAttrApplicationTag as String: "...", // Same unique tag here
      kSecAttrKeyType as String: kSecAttrKeyTypeRSA,
      kSecReturnData as String: kCFBooleanTrue
    ]
    let result = SecItemCopyMatching(query as CFDictionary, &dataPtr)

    switch (result, dataPtr) {
    case (errSecSuccess, .some(let data)):

      // convert to X509 encoded key
      let convertedData = RSAKeyEncoding().convertToX509EncodedKey(data)

      // convert to Base64 string
      let base64PublicKey = convertedData.base64EncodedString(options: [])
      return base64PublicKey
    default:
      throw CryptoError.keyConversionError
    }
  }
}

更新-其他问题

在使用上述课程一段时间后，我们偶然发现了另一个问题。有时候，从密钥链中获取的公钥似乎是无效的，因为由于某种原因，它的大小增加了。这种行为与问题中描述的结果相匹配(尽管在我们的例子中，Base64编码密钥已经增长到392个字符，而不是360个字符)。不幸的是，我们没有找到这种奇怪行为的确切原因，但我们找到了两个解决方案。第一个解决方案是在定义查询时指定kSecAttrKeySizeInBits和kSecAttrEffectiveKeySize，如下代码段所示：

let keySize = ... // Key size specified when storing the key, for example: 2048

let query: [String: Any] = [
    kSecAttrKeySizeInBits as String: keySize,
    kSecAttrEffectiveKeySize as String: keySize,
    ... // More attributes
]

var dataPtr: CFTypeRef?

let result = SecItemCopyMatching(query as CFDictionary, &dataPtr)

第二种解决方案是，在添加具有相同标记的新键之前，始终从密钥链(如果有的话)中删除旧键。

更新-替代解决方案

我在GitHub上发布了GitHub，它可以作为上述类的替代。

参考文献

ASN.1、BER和DER子集的Layman指南

RFC 5280 (X.509 v3)

RFC 8017 (PKCS #1 v2.2)

我发现的一些代码在创建这里函数时给了我灵感。

Answer 2

Java需要一个以DER格式编码的公钥。不幸的是，iOS不支持这种标准格式，而且还需要额外的转换(我不知道这在最新版本的needed中是否会有所改进)

参见我的答案这里您可以使用CryptoExportImportManager转换密钥

func exportPublicKeyToDER(keyId:String) -> NSData?{

    let publicKey = loadKeyStringFromKeyChainAsNSData(PUBLIC_KEY + keyId)
    let keyType = kSecAttrKeyTypeRSA
    let keySize = 2048
    let exportImportManager = CryptoExportImportManager()
    if let exportableDERKey = exportImportManager.exportPublicKeyToDER(publicKey, keyType: keyType as String, keySize: keySize) {
        return exportableDERKey
    } else {
        return nil
    }
}