package main

import (
  "crypto/rsa"
  "crypto/x509"
  "encoding/pem"
  "errors"
  "io"
  "os"
  "time"

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

type signaturePayload struct {
  Blockchain         string `json:"blockchain"`
  FlowType           string `json:"flow_type"`
  BaseCurrencyCode   string `json:"base_currency_code"`
  BaseCurrencyAmount string `json:"base_currency_amount"`
  QuoteCurrencyCode  string `json:"quote_currency_code"`
  WalletAddress      string `json:"wallet_address,omitempty"`
  RedirectURL        string `json:"redirect_url"`
}

const pathPrivateKey = "jwtRS256.key"

func main() {
  privateKey, err := loadRSAPrivateKey(pathPrivateKey)
  if err != nil {
    panic(err)
  }

  payload := signaturePayload{
    Blockchain:         "ethereum",
    FlowType:           "off-ramp",
    BaseCurrencyCode:   "eth:USDC",
    BaseCurrencyAmount: "15",
    QuoteCurrencyCode:  "iso4217:USD",
    WalletAddress:      "0x807cF9A772d5a3f9CeFBc1192e939D62f0D9bD38",
    RedirectURL:        "http://localhost:8000/callback",
  }

  var signature string
  if signature, err = makeSignature(privateKey, payload); err != nil {
    panic(err)
  }

  println("signature: \n", signature)
}


func loadRSAPrivateKey(path string) (*rsa.PrivateKey, error) {
  privateKeyFile, err := os.Open(path)
  if err != nil {
    return nil, err
  }
  defer privateKeyFile.Close()

  pemBytes, err := io.ReadAll(privateKeyFile)
  if err != nil {
    return nil, err
  }

  block, _ := pem.Decode(pemBytes)
  if block == nil || block.Type != "RSA PRIVATE KEY" {
    return nil, errors.New("failed to decode PEM block containing private key")
  }

  privateKey, err := x509.ParsePKCS1PrivateKey(block.Bytes)
  if err != nil {
    return nil, err
  }

  return privateKey, nil
}


func makeSignature(privateKey *rsa.PrivateKey, payload signaturePayload) (string, error){
  token := jwt.NewWithClaims(jwt.SigningMethodRS256, jwt.MapClaims{
    "payload": payload,
    "exp":     time.Now().Add(time.Minute * 30).Unix(),
  })

  return token.SignedString(privateKey)
}

The result of this command will be the signature - encoded token:

eyJhbGciOiJSUzI1NiIsInR5cCI6IkpXVCJ9.eyJleHAiOjE3MzA4MDcyODMsInBheWxvYWQiOnsiYmxvY2tjaGFpbiI6ImV0aGVyZXVtIiwiZmxvd190eXBlIjoib2ZmLXJhbXAiLCJiYXNlX2N1cnJlbmN5X2NvZGUiOiJldGg6VVNEQyIsImJhc2VfY3VycmVuY3lfYW1vdW50IjoiMTUiLCJxdW90ZV9jdXJyZW5jeV9jb2RlIjoiaXNvNDIxNzpVU0QiLCJ3YWxsZXRfYWRkcmVzcyI6IjB4ODA3Y0Y5QTc3MmQ1YTNmOUNlRkJjMTE5MmU5MzlENjJmMEQ5YkQzOCIsInJlZGlyZWN0X3VybCI6Imh0dHA6Ly9sb2NhbGhvc3Q6ODAwMC9jYWxsYmFjayJ9fQ.VsEjAfOWq6da0iqGS38yguNxxmJ77wwmX7HCXdbc4hfUrHRfSfeIqaLCopWBbWSu5VBhBpleImxosRDfiqpiurwq1Bfb1rUHHSg1um1iIhG77onBgXDPvn99Aghi7-JzF62UPZEsAPqUIrvboxuUMd05KaajEkKqWbVlJR62e5UsoFrpZ8eEfnh5vmjRyra_BfvSvkbmOgMHJ6YP00bFmyXOKXILTPrH0yEsPn6xfWBTSeckUm4BM291OqVlY0Sdjsv5_ieR8ycvRNiVMVA34b5-6dlJl14LIcRwgVXHH0goTaIVL6dXz71ncHBX_YbSnJY3I3Qr23p8o38M8TNLWQMMnZRPFuCLnX70H8JZeFYvWj5RoZWJH9uKtxrsdjx_9cvBv3f2O8NQSJbf04e0avNGvOrVJG7SpYXtFXn7cn8mRXpE-we3AofX88mblS5zOF0yNDhL1w3VsO0rQlZkGNxktk9yW75FxUzECZh2ZGjgtzmT2Xm5R-8uxgRlB_X-uu-iDFjDhTJbYh-d7IKHRHTkuRPWPp433oJI5Iv22i2FIBNQen3fyIFFz69OnPhv4vtVNUXE648KYNK-6JGZGQWv5gXxQEHOmp9KcJojIzobfOMfR3SQzzQsMI5MNxTBLO7MaEWTEhH1Po3LKw8JVxcMy9u4ybJoH4DH4Sd6bfM

that corresponds to a decoded object: 

{
  "exp": 1730807283,
  "payload": {
    "blockchain": "ethereum",
    "flow_type": "off-ramp",
    "base_currency_code": "eth:USDC",
    "base_currency_amount": "15",
    "quote_currency_code": "iso4217:USD",
    "wallet_address": "0x807cF9A772d5a3f9CeFBc1192e939D62f0D9bD38",
    "redirect_url": "http://localhost:8000/callback"
  }
}

const fs = require('fs')
const path = require('path')
const jwt = require('jsonwebtoken');

const privateKey = fs.readFileSync(path.join(__dirname, 'jwtRS256.key'), 'utf8')

const current_time = Math.floor(Date.now() / 1000);
const expiration_time = current_time + (60 * 30); // 30m

const claims = {
    'exp': expiration_time,
    'payload': {
        'blockchain': 'ethereum',
        'flow_type': 'off-ramp',
        'base_currency_code': 'eth:USDC',
        'base_currency_amount': '15',
        'quote_currency_code': 'iso4217:USD',
        'wallet_address': '0x807cF9A772d5a3f9CeFBc1192e939D62f0D9bD38',
        'redirect_url': 'http://localhost:8000/callback',
    }
};

const jwt_token =  jwt.sign(claims, privateKey, { algorithm: 'RS256'});
console.log(jwt_token);

eyJhbGciOiJSUzI1NiIsInR5cCI6IkpXVCJ9.eyJleHAiOjE3MzA4MDc0MTksInBheWxvYWQiOnsiYmxvY2tjaGFpbiI6ImV0aGVyZXVtIiwiZmxvd190eXBlIjoib2ZmLXJhbXAiLCJiYXNlX2N1cnJlbmN5X2NvZGUiOiJldGg6VVNEQyIsImJhc2VfY3VycmVuY3lfYW1vdW50IjoiMTUiLCJxdW90ZV9jdXJyZW5jeV9jb2RlIjoiaXNvNDIxNzpVU0QiLCJ3YWxsZXRfYWRkcmVzcyI6IjB4ODA3Y0Y5QTc3MmQ1YTNmOUNlRkJjMTE5MmU5MzlENjJmMEQ5YkQzOCIsInJlZGlyZWN0X3VybCI6Imh0dHA6Ly9sb2NhbGhvc3Q6ODAwMC9jYWxsYmFjayJ9LCJpYXQiOjE3MzA4MDU2MTl9.wyO7sbsYcnxbh3IvC3QiINONIsGm7ECrlFiWkd3hPWT0bhujBbVyGra20s_IotyC-bE1gewwN2OYaMOX-J_9SD37H4U35azxz0wf7VvNJXe7m_bi7wMud_5umAM-OlUSbS7ORk0_3HPZbADINan7_9cM-UmGwpPvVFpH3RNmh6NTfD7AQH7jkxyLIk_ljbRRbgUGMYwTLjvJVyhjgyzPqspWZuvtZhsRQLazuG_mbDU6cZtJiwinoEHycm7p4mn05DvvBShEKFLlBb-Rp8EySVtzcxBlOYV9Mv-0Q8ze_t_gEAw1WCo9ywFXk67fD8JcAN4CSTndMsaJV6_moweA4zp9AL2e54R1bRgEexqOBhLOd4tVcX_DcPMhZjnxCPBWNGs4OS2KL2g9qNWqk4wZPFXl1rJIMkhaxJ5gX8GU4frrHt3qJjp53UkAEuWhlp6u_T_GalEI56qmtDE2DGCqWK8MmGmhv2jeIjft7cEevi8PbQXUrwLU7btv0l2sN1LDNnYWCm7XvxRgHLhVzf6AGUnOyrOoPOWunJP78WpgV1-qdM1vmC-gJmVWRZTSQthnPI7ARbcGfZfphH4PsBjcKVgXkMrtYF2RfcN7Ym_lYVyNe8sWJURIGvPQsq2AzxmcFaIhaJWQbzOzMTbe5MHvSNTMPb6KWFarwVyMnoWWA80

{
  "exp": 1730807419,
  "payload": {
    "blockchain": "ethereum",
    "flow_type": "off-ramp",
    "base_currency_code": "eth:USDC",
    "base_currency_amount": "15",
    "quote_currency_code": "iso4217:USD",
    "wallet_address": "0x807cF9A772d5a3f9CeFBc1192e939D62f0D9bD38",
    "redirect_url": "http://localhost:8000/callback"
  },
  "iat": 1730805619
}

openssl rsa -in jwtRS256.key -pubout -outform PEM -out jwtRS256.key.pub