如何在没有 x5c 的情况下从 jwks 验证 JWT 的签名

Posted 2023-03-08

【中文标题】如何在没有 x5c 的情况下从 jwks 验证 JWT 的签名

【英文标题】：How to validate signature of JWT from jwks without x5c

【发布时间】：2020-08-07 06:03:59

【问题描述】：

我有一个 JWT 安全令牌，我需要通过 jwks 端点进行验证。 jwks 中的数据如下所示：

  "keys": [
    
      "kty": "RSA",
      "e": "AQAB",
      "use": "sig",
      "alg": "RS256",
      "n": "......",
      "kid": "2132132-b1e6-47e7-a30f-1831942f74bd"
    ,
    
      "kty": "RSA",
      "e": "AQAB",
      "use": "sig",
      "alg": "RS256",
      "n": "......",
      "kid": "tsp-app-a"
    ,
    
      "kty": "RSA",
      "e": "AQAB",
      "use": "sig",
      "alg": "RS256",
      "n": ".....",
      "kid": "tsp-app-b"
    
  ]

我尝试了一个第三方 api，但它看起来依赖于 x5c 密钥，这在我的案例中不存在。

我的代码是：

public static bool Validate(JwtSecurityToken jsonToken)
        
            bool result = false;
            try
            
                var headers = Jose.JWT.Headers<JWTHeader>(jsonToken.RawData);
                var payload = Jose.JWT.Payload<JWTPayload>(jsonToken.RawData);

                string jwk = "";
                using (HttpClient cli = new HttpClient())
                
                    jwk = cli.GetStringAsync(MyclientUrlforWellknownjson).Result;
                

                var jwkinfo = JsonSerializer.Deserialize<JWKS>(jwk);
                //Find right key. Match kid and alg,  (To be changed later. It is possible that there are multiple x5c elements in key)
                var jwkkey = (from item in jwkinfo.keys where item.kid == headers.kid && item.alg == headers.alg select item).SingleOrDefault();

                //If key was found then load its public key
                System.Security.Cryptography.X509Certificates.X509Certificate2 cert = null;
                if (jwkkey != null)
                
                    //Get public key from well known information
                    byte[] key = System.Text.Encoding.ASCII.GetBytes(jwkkey.x5c[0]); //??todo 
                    //Create cert                   
                    cert = new System.Security.Cryptography.X509Certificates.X509Certificate2(key);
                

                var o = Jose.JWT.Decode(jsonToken.RawData, cert.PublicKey.Key);


            
            catch (Exception ex)
            

            
            return result;
        

如何在没有 x5c 的情况下通过 jwks 验证 JWT？

【问题讨论】：

我创建了一个示例类来根据公钥 URL 验证令牌。我希望它可以帮助某人前进。 github.com/danielnord/node-azureTokenValidation/blob/danielnord/…

【参考方案1】：

使用x5c 只是一种方法，但您也可以使用参数e（公共指数）和n（模数）检索公钥，这也记录在jose-jwt github page：

//If kid was found then load public key
if (jwkkey != null)

    RSACryptoServiceProvider key = new RSACryptoServiceProvider();
    key.ImportParameters(new RSAParameters
    
        Modulus = Base64Url.Decode(jwkkey.n),
        Exponent = Base64Url.Decode(jwkkey.e)
    );


// get the public key in PEM format, e.g. to use it on jwt.io
var pubkey = Convert.ToBase64String(key.ExportSubjectPublicKeyInfo());
const string pemHeader = "-----BEGIN PUBLIC KEY-----";
const string pemFooter = "-----END PUBLIC KEY-----";
var publicKeyPem = pemHeader + Environment.NewLine + pubkey + Environment.NewLine + pemFooter;

var o = Jose.JWT.Decode(jsonToken.RawData, key);

您还可以再次以 PEM 格式导出公钥，如上面的代码所示，如​​下所示：

-----BEGIN PUBLIC KEY-----
MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAgIdJV4qWKyt3wkS66yBG5Ii9ew+eofuPU49TjlRIU5Iu5jX2mRMoHdcI7V78iKYSQHKYxz17cqzQyERxKnEiDgy/gwouStRgvPdm3H4rq//7p0t15SunsG2T1rEVf0sZEDnQ5qRkm7iqs6ZG1NqqIUtnOTd1Pd1MhbEqeENFtaPHvN37eZL82WmsQlJviFH4I9iZQVR/QT4GREQlRro8IjJTaloUyeDQTOQ+4ll1+4+g/ug2tZ+s9xleLzl5L9ZKSVJFhtmln8WGaVldagarwa7kMLfuiVe8B5Lr7poQa4NCAR54ECPWoOHrABdPZKrkkxjVypTXUzL5cPzmzFC2xwIDAQAB
-----END PUBLIC KEY-----

然后使用该密钥在https://jwt.io 上手动验证您的令牌

（在来自@Topaco 的hint 之后更正了密钥导出）

【讨论】：

非常感谢@jps