最近在項(xiàng)目中網(wǎng)絡(luò)請(qǐng)求項(xiàng)目需要用到ECDH算法（橢圓曲線選擇P-384），計(jì)算出協(xié)商秘鑰后導(dǎo)出密碼使用HKDF進(jìn)行密鑰擴(kuò)展，這里將算法遇到的幾個(gè)問題記錄下來

1、生成公鑰和私鑰

iOS 10.0之后有個(gè)類SecKey，專門用于生成公鑰和私鑰。生成代碼如下圖

var error: Unmanaged<CFError>?
        let attributes: [String: Any] = [kSecAttrKeySizeInBits as String: 384,
                                         kSecAttrKeyType as String: kSecAttrKeyTypeECSECPrimeRandom,
                                         kSecAttrKeyClass as String: kSecAttrKeyClassPrivate,
                                         kSecPrivateKeyAttrs as String: [kSecAttrIsPermanent as String: false],
                                         kSecPublicKeyAttrs as String:[kSecAttrIsPermanent as String: false]]
        
        self.privateKey = SecKeyCreateRandomKey(attributes as CFDictionary, &error)
        
        if let privateKey = self.privateKey {
            publicKey = SecKeyCopyPublicKey(privateKey)
        }

其中384表示的是 P-384橢圓曲線。另外還有P-224, P-256, P-521這些橢圓曲線

2、密鑰字符串和SecKey的互相轉(zhuǎn)換

使用ECDH加密，必然涉及到服務(wù)端傳給APP公鑰，APP傳給服務(wù)端公鑰，就這需要密鑰字符串和SecKey的互相轉(zhuǎn)換。在這里就不能不說到一個(gè)

SecKey的一個(gè)坑：在將密鑰字符串轉(zhuǎn)化為SecKey時(shí)，會(huì)自動(dòng)的去掉密鑰的ASN.1，所以在轉(zhuǎn)化時(shí)得先加上

// MARK: -
public extension SecKey {
    func publicSeckeyToString() -> String? {
        var error:Unmanaged<CFError>?
        if let cfdata = SecKeyCopyExternalRepresentation(self, &error) {
            // 添加secp384r1的asn.1
            let pemPrefixBuffer :[UInt8] = SecKey.getPKCS1SHA384ASN1()
            var finalPemData = Data(bytes: pemPrefixBuffer as [UInt8], count: pemPrefixBuffer.count)
            finalPemData.append(cfdata as Data)
            let finalPemString = finalPemData.base64EncodedString(options: .lineLength64Characters)
            return finalPemString
        }
        return nil
    }
   
    
    // 根據(jù)公鑰字符串生成seckey
    class func initPubkeyString(pubkeyString: String) -> SecKey {
        let pubKeyByte:Array<UInt8> = Data.init(base64Encoded: pubkeyString)!.bytes
        let asn1Byte:Array<UInt8> = SecKey.getPKCS1SHA384ASN1()
        let array = pubKeyByte[asn1Byte.count...]
        
        var error: Unmanaged<CFError>?
        let attributes = [kSecAttrKeySizeInBits as String: 384,
                          kSecAttrKeyType: kSecAttrKeyTypeECSECPrimeRandom,
                        kSecAttrKeyClass: kSecAttrKeyClassPublic] as NSDictionary
        let pubkey = SecKeyCreateWithData(Data(array) as NSData, attributes, &error)
        return pubkey!
    }
    
    class func getPKCS1SHA384ASN1() -> Array<UInt8> {
        return [
            0x30, 0x76,
            0x30, 0x10,
            0x06, 0x07, 0x2a, 0x86, 0x48, 0xce, 0x3d, 0x02, 0x01, 0x06, 0x05,
            0x2b, 0x81, 0x04,
            0x00, 0x22, 0x03, 0x62, 0x00
        ]
    }

}

這里我偷了懶，沒有繼續(xù)翻蘋果的文檔看有沒有關(guān)于asn.1的類，我是直接用的ASN.1 JavaScript decoder 這個(gè)網(wǎng)址去查看了我這所需要的P-384曲線的ASN.1

3、之后就是計(jì)算出協(xié)商秘鑰了，這里代碼很簡(jiǎn)單

let exchangeData = exchangeKey(privateKey: privateKey, pubkey: pubkey)!

后面用HKDF算法對(duì)協(xié)商密鑰擴(kuò)展我這用的第三方庫(kù)CryptoSwift，代碼

let key = try! HKDF(password: [UInt8](exchangeData), variant: .sha256).calculate()