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，无法识别
我正在iOS应用程序中实现聊天加密功能，并使用对称+非对称密钥进行处理。
不需要过多的细节，在后端，我使用用户的公钥来加密用于加密和解密消息的对称密钥。
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。我也对它们进行了测试，所以我100%都能按预期工作。
然而，后端似乎无法识别从iOS传递的公钥的格式。两边都使用RSA，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)
我使用镜面反射代码来读取钥匙链中的钥匙。
这是我用来将公钥导出为base64字符串的一段代码：
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，我使用此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);
}
但这在最后一行失败了，只有一个例外：
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不同——当我在iOS中生成密钥，然后导出为base64时，它看起来像这样：
MIIBCgKCAQEA4M/bRDdH0f6qFIXxOg13RHka+g4Yv8u9PpPp1IR6pSwrM1aq8B6cyKRwnLe/MOkvODvDfJzvGXGQ01zSTxYWAW1B4uc/NCEemCmZqMosSB/VUJdNxxWtt2hJxpz06hAawqV+6HmweAB2dUn9tDEsQLsNHdwYouOKpyRZGimcF9qRFn1RjR0Q54sUh1tQAj/EwmgY2S2bI5TqtZnZw7X7Waji7wWi6Gz88IkuzLAzB9VBNDeV1cfJFiWsZ/MIixSvhpW3dMNCrJShvBouIG8nS+vykBlbFVRGy3gJr8+OcmIq5vuHVhqrWwHNOs+WR87K/qTFO/CB7MiyiIV1b1x5DQIDAQAB
总共360个字符，而在Java中(仍然使用RSA)执行相同操作就像：
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用不同的密钥处理密钥，并且需要特殊处理才能与其他人交谈，我不会感到惊讶。
知道吗？
我们在将iOS应用程序连接到Java后端时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类中的代码有点复杂，可能很难维护。这是因为在向DER编码添加新组件时使用自上而下的方法。因此，长度字段包含的数字必须提前计算(即，在定义长度适用的内容之前)。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()中使用此类，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
在使用上述类一段时间后，我们偶然发现了另一个问题。有时，从密钥链中提取的公钥似乎是无效的，因为出于某种原因，它的大小有所增加。这种行为与问题中描述的结果相匹配(尽管在我们的案例中，Base64编码的密钥已经增长到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，但我们找到了两种解决方案。第一种解决方案是在定义查询时指定kSecAttrKeySizeInBits和kSecAttrEffectiveKeySize，如下面的代码片段所示：
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密钥之前，始终从密钥链中删除旧密钥(如果有的话)。
UPDATE-替代解决方案
我在GitHub上发布了这个项目，它可以作为上述类的替代品。
参考文献
ASN.1、BER和DER 子集的外行指南
RFC 5280(X.509 v3)
RFC 8017(PKCS#1 v2.2)
在创建lengthField(...)函数时，我在这里找到的一些代码启发了我。
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，需要进行额外的转换(我不知道在最新版本的swift中是否会有所改进)
在这里查看我的答案您可以使用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
}
}