无法验证JWT Token：使用ECDSA密钥时提示密钥类型无效

我用Go写了生成ECDSA密钥、创建JWT Token并验证的代码，但运行时出现错误提示Token is invalid: key is of invalid type，代码如下：

package main

import (
    "crypto/ecdsa"
    "crypto/elliptic"
    "crypto/rand"
    "crypto/x509"
    "encoding/base64"
    "fmt"
    "log"
    "time"

    "github.com/golang-jwt/jwt/v4"
)

func main() {
    key, err := ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
    if err != nil {
        log.Fatalln(err)
        return
    }

    privateK, err := x509.MarshalECPrivateKey(key)
    if err != nil {
        log.Fatalln(err)
        return
    }

    claims := jwt.MapClaims{}
    claims["authorized"] = true
    claims["user_id"] = 10
    claims["exp"] = time.Now().Add(time.Hour * time.Duration(1)).Unix()
    t := jwt.NewWithClaims(jwt.SigningMethodES256, claims)

    tokenStr, err := t.SignedString(key)
    if err != nil {
        log.Fatalln(err)
        return
    }

    fmt.Printf("Secret: %s\n", base64.StdEncoding.EncodeToString(privateK))
    fmt.Printf("Token: %s\n", tokenStr)

    // Validate token

    _, err = jwt.Parse(tokenStr, func(token *jwt.Token) (interface{}, error) {
        if _, ok := token.Method.(*jwt.SigningMethodECDSA); !ok {
            return nil, fmt.Errorf("unexpected signing method %v", token.Header["alg"])
        }
        return key, nil
    })

    if err != nil {
        log.Fatalf("Token is invalid %v", err)
    } else {
        fmt.Println("Token is valid")
    }
}

错误原因 #

ECDSA算法的JWT验证需要使用公钥，而你在验证回调里返回的是私钥(key，类型为*ecdsa.PrivateKey)，这不符合jwt库的验证要求，因此抛出"key is of invalid type"错误。

修正方案 #

从私钥中提取公钥（key.Public()，类型为*ecdsa.PublicKey），在验证时返回这个公钥即可。修正后的验证代码片段：

// Validate token
_, err = jwt.Parse(tokenStr, func(token *jwt.Token) (interface{}, error) {
    if _, ok := token.Method.(*jwt.SigningMethodECDSA); !ok {
        return nil, fmt.Errorf("unexpected signing method %v", token.Header["alg"])
    }
    // 返回公钥而非私钥
    return key.Public(), nil
})

完整修正代码 #

package main

import (
    "crypto/ecdsa"
    "crypto/elliptic"
    "crypto/rand"
    "crypto/x509"
    "encoding/base64"
    "fmt"
    "log"
    "time"

    "github.com/golang-jwt/jwt/v4"
)

func main() {
    key, err := ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
    if err != nil {
        log.Fatalln(err)
        return
    }

    privateK, err := x509.MarshalECPrivateKey(key)
    if err != nil {
        log.Fatalln(err)
        return
    }

    claims := jwt.MapClaims{}
    claims["authorized"] = true
    claims["user_id"] = 10
    claims["exp"] = time.Now().Add(time.Hour * time.Duration(1)).Unix()
    t := jwt.NewWithClaims(jwt.SigningMethodES256, claims)

    tokenStr, err := t.SignedString(key)
    if err != nil {
        log.Fatalln(err)
        return
    }

    fmt.Printf("Secret: %s\n", base64.StdEncoding.EncodeToString(privateK))
    fmt.Printf("Token: %s\n", tokenStr)

    // Validate token
    _, err = jwt.Parse(tokenStr, func(token *jwt.Token) (interface{}, error) {
        if _, ok := token.Method.(*jwt.SigningMethodECDSA); !ok {
            return nil, fmt.Errorf("unexpected signing method %v", token.Header["alg"])
        }
        // 使用公钥验证
        return key.Public(), nil
    })

    if err != nil {
        log.Fatalf("Token is invalid %v", err)
    } else {
        fmt.Println("Token is valid")
    }
}

内容的提问来源于stack exchange，提问作者Don Tomato