go - Marshal AppleID 的公钥到 rsa.PublicKey

Question

我正在尝试在 Go 中验证来自 Apple 的 JWT，我需要他们的公钥作为 Go 的 rsa.PublicKey 这样做。我从以下指定的端点检索了他们的 key :

https://developer.apple.com/documentation/sign_in_with_apple/fetch_apple_s_public_key_for_verifying_token_signature

并得到以下信息:

{
  "keys": [
    {
      "kty": "RSA",
      "kid": "86D88Kf",
      "use": "sig",
      "alg": "RS256",
      "n": "iGaLqP6y-SJCCBq5Hv6pGDbG_SQ11MNjH7rWHcCFYz4hGwHC4lcSurTlV8u3avoVNM8jXevG1Iu1SY11qInqUvjJur--hghr1b56OPJu6H1iKulSxGjEIyDP6c5BdE1uwprYyr4IO9th8fOwCPygjLFrh44XEGbDIFeImwvBAGOhmMB2AD1n1KviyNsH0bEB7phQtiLk-ILjv1bORSRl8AK677-1T8isGfHKXGZ_ZGtStDe7Lu0Ihp8zoUt59kx2o9uWpROkzF56ypresiIl4WprClRCjz8x6cPZXU2qNWhu71TQvUFwvIvbkE1oYaJMb0jcOTmBRZA2QuYw-zHLwQ",
      "e": "AQAB"
    },
    {
      "kty": "RSA",
      "kid": "eXaunmL",
      "use": "sig",
      "alg": "RS256",
      "n": "4dGQ7bQK8LgILOdLsYzfZjkEAoQeVC_aqyc8GC6RX7dq_KvRAQAWPvkam8VQv4GK5T4ogklEKEvj5ISBamdDNq1n52TpxQwI2EqxSk7I9fKPKhRt4F8-2yETlYvye-2s6NeWJim0KBtOVrk0gWvEDgd6WOqJl_yt5WBISvILNyVg1qAAM8JeX6dRPosahRVDjA52G2X-Tip84wqwyRpUlq2ybzcLh3zyhCitBOebiRWDQfG26EH9lTlJhll-p_Dg8vAXxJLIJ4SNLcqgFeZe4OfHLgdzMvxXZJnPp_VgmkcpUdRotazKZumj6dBPcXI_XID4Z4Z3OM1KrZPJNdUhxw",
      "e": "AQAB"
    }
  ]
}

我试过 x509.ParsePKCS1PublicKey(...)通过解析单个键和使用 asn1.Marshall(...) 返回的整个 JSON 来实现函数功能。我在尝试解析 der 时遇到错误.
然后我注意到 key 包含 "n"和 "e"字符串对，并且定义为 here所以我尝试直接创建公钥。但是， N和 E在 rsa.PublicKey是 *big.Int和 int分别。
我似乎找不到 "n" 的编码和 "e" ，所以我无法准确地将值转换为 rsa.PublicKey .我试过base64，但是没有用。
谁能告诉我如何将 Apple 的公钥转换为合适的 rsa.PublicKey请？

Answer 1

"n" 的值和 "e"在你的 JSON 中只是 base64 编码的大端二进制整数，所以一旦你解码它们，你可以将它们转换为类型 *big.Int与 big.Int.SetBytes ，然后使用这些来填充 *rsa.PublicKey .
您提到您尝试过 base64，但它不起作用，但您需要确保使用正确的编码和填充选项 - - 的存在和 _编码字符串中的字符表明您正在处理 RFC 4648 URL 安全编码，并且字符串的长度不能被 4 整除的事实表明不存在填充字符，因此 base64.URLEncoding.WithPadding(base64.NoPadding)是你需要使用的。
您可以直接解码和转换的类型的综合示例:

package main

import (
        "crypto/rsa"
        "encoding/base64"
        "encoding/json"
        "log"
        "math/big"
)

const keyJSON = `{
    "kty": "RSA",
    "kid": "86D88Kf",
    "use": "sig",
    "alg": "RS256",
    "n": "4dGQ7bQK8LgILOdLsYzfZjkEAoQeVC_aqyc8GC6RX7dq_KvRAQAWPvkam8VQv4GK5T4ogklEKEvj5ISBamdDNq1n52TpxQwI2EqxSk7I9fKPKhRt4F8-2yETlYvye-2s6NeWJim0KBtOVrk0gWvEDgd6WOqJl_yt5WBISvILNyVg1qAAM8JeX6dRPosahRVDjA52G2X-Tip84wqwyRpUlq2ybzcLh3zyhCitBOebiRWDQfG26EH9lTlJhll-p_Dg8vAXxJLIJ4SNLcqgFeZe4OfHLgdzMvxXZJnPp_VgmkcpUdRotazKZumj6dBPcXI_XID4Z4Z3OM1KrZPJNdUhxw",
    "e": "AQAB"
}`

// decodeBase64BigInt decodes a base64-encoded larger integer from Apple's key format.
func decodeBase64BigInt(s string) *big.Int {
        buffer, err := base64.URLEncoding.WithPadding(base64.NoPadding).DecodeString(s)
        if err != nil {
                log.Fatalf("failed to decode base64: %v", err)
        }

        return big.NewInt(0).SetBytes(buffer)
}

// appleKey is a type of public key.
type appleKey struct {
        KTY string
        KID string
        Use string
        Alg string
        N   *big.Int
        E   int
}

// UnmarshalJSON parses a JSON-encoded value and stores the result in the object.
func (k *appleKey) UnmarshalJSON(b []byte) error {
        var tmp struct {
                KTY string `json:"kty"`
                KID string `json:"kid"`
                Use string `json:"use"`
                Alg string `json:"alg"`
                N   string `json:"n"`
                E   string `json:"e"`
        }

        if err := json.Unmarshal(b, &tmp); err != nil {
                return err
        }

        *k = appleKey{
                KTY: tmp.KTY,
                KID: tmp.KID,
                Use: tmp.Use,
                Alg: tmp.Alg,
                N:   decodeBase64BigInt(tmp.N),
                E:   int(decodeBase64BigInt(tmp.E).Int64()),
        }

        return nil
}

// RSA returns a corresponding *rsa.PublicKey
func (k appleKey) RSA() *rsa.PublicKey {
        return &rsa.PublicKey{
                N: k.N,
                E: k.E,
        }
}

func main() {
        // Decode the Apple key.
        var ak appleKey
        if err := json.Unmarshal([]byte(keyJSON), &ak); err != nil {
                log.Fatalf("failed to unmarshal JSON: %v", err)
        }

        // Convert it to a normal *rsa.PublicKey.
        rk := ak.RSA()

        if rk.Size() != 256 {
                log.Fatalf("unexpected key size: %d", rk.Size())
        }

        // Do what you like with the RSA key now.
}