package main

import (
  "crypto/ecdsa"
  "crypto/sha256"
  "crypto/x509"
  "encoding/base64"
  "encoding/pem"
  "log"
)

var (
  // example pubkey, you should receive one at https://api.monobank.ua/api/merchant/pubkey
  pubKeyBase64 = `LS0tLS1CRUdJTiBQVUJMSUMgS0VZLS0tLS0KTUZrd0V3WUhLb1pJemowQ0FRWUlLb1pJemowREFRY0RRZ0FFQUc1LzZ3NnZubGJZb0ZmRHlYWE4vS29CbVVjTgo3NWJSUWg4MFBhaEdldnJoanFCQnI3OXNSS0JSbnpHODFUZVQ5OEFOakU1c0R3RmZ5Znhub0ZJcmZBPT0KLS0tLS1FTkQgUFVCTElDIEtFWS0tLS0tCg==`

  // value from X-Sign header in webhook request
  xSignBase64 = `MEUCIQC/mVKhi8FKoayul2Mim3E2oaIOCNJk5dEXxTqbkeJSOQIgOM0hsW0qcP2H8iXy1aQYpmY0SJWEaWur7nQXlKDCFxA=`

  // webhook request body bytes
  bodyBytes = []byte(`{
  "invoiceId": "p2_9ZgpZVsl3",
  "status": "created",
  "failureReason": "string",
  "amount": 4200,
  "ccy": 980,
  "finalAmount": 4200,
  "createdDate": "2019-08-24T14:15:22Z",
  "modifiedDate": "2019-08-24T14:15:22Z",
  "reference": "84d0070ee4e44667b31371d8f8813947",
  "cancelList": [
    {
      "status": "processing",
      "amount": 4200,
      "ccy": 980,
      "createdDate": "2019-08-24T14:15:22Z",
      "modifiedDate": "2019-08-24T14:15:22Z",
      "approvalCode": "662476",
      "rrn": "060189181768",
      "extRef": "635ace02599849e981b2cd7a65f417fe"
    }
  ]
}`)
)

func main() {
  pubKeyBytes, err := base64.StdEncoding.DecodeString(pubKeyBase64)
  if err != nil {
      panic(err)

  }

  signatureBytes, err := base64.StdEncoding.DecodeString(xSignBase64)
  if err != nil {
      panic(err)
  }

  block, _ := pem.Decode(pubKeyBytes)
  if block == nil {
      panic("invalid pem")
  }

  genericPubKey, err := x509.ParsePKIXPublicKey(block.Bytes)
  if err != nil {
      panic(err)
  }

  pubKey, ok := genericPubKey.(*ecdsa.PublicKey)
  if !ok {
      panic("invalid key")
  }

  hash := sha256.Sum256(bodyBytes)

  ok = ecdsa.VerifyASN1(pubKey, hash[:], signatureBytes)
  if !ok {
      panic("invalid X-Sign")
  }

  log.Println("OK")
}

import base64
import hashlib

import ecdsa

# example pubkey, you should receive one at https://api.monobank.ua/api/merchant/pubkey
pub_key_base64 = "LS0tLS1CRUdJTiBQVUJMSUMgS0VZLS0tLS0KTUZrd0V3WUhLb1pJemowQ0FRWUlLb1pJemowREFRY0RRZ0FFQUc1LzZ3NnZubGJZb0ZmRHlYWE4vS29CbVVjTgo3NWJSUWg4MFBhaEdldnJoanFCQnI3OXNSS0JSbnpHODFUZVQ5OEFOakU1c0R3RmZ5Znhub0ZJcmZBPT0KLS0tLS1FTkQgUFVCTElDIEtFWS0tLS0tCg=="

# value from X-Sign header in webhook request
x_sign_base64 = "MEUCIQC/mVKhi8FKoayul2Mim3E2oaIOCNJk5dEXxTqbkeJSOQIgOM0hsW0qcP2H8iXy1aQYpmY0SJWEaWur7nQXlKDCFxA="

# webhook request body bytes
body_bytes = b'''{
  "invoiceId": "p2_9ZgpZVsl3",
  "status": "created",
  "failureReason": "string",
  "amount": 4200,
  "ccy": 980,
  "finalAmount": 4200,
  "createdDate": "2019-08-24T14:15:22Z",
  "modifiedDate": "2019-08-24T14:15:22Z",
  "reference": "84d0070ee4e44667b31371d8f8813947",
  "cancelList": [
    {
      "status": "processing",
      "amount": 4200,
      "ccy": 980,
      "createdDate": "2019-08-24T14:15:22Z",
      "modifiedDate": "2019-08-24T14:15:22Z",
      "approvalCode": "662476",
      "rrn": "060189181768",
      "extRef": "635ace02599849e981b2cd7a65f417fe"
    }
  ]
}'''

if __name__ == '__main__':
    pub_key_bytes = base64.b64decode(pub_key_base64)
    signature_bytes = base64.b64decode(x_sign_base64)
    pub_key = ecdsa.VerifyingKey.from_pem(pub_key_bytes.decode())

    ok = pub_key.verify(signature_bytes, body_bytes, sigdecode=ecdsa.util.sigdecode_der, hashfunc=hashlib.sha256)
    if ok:
        print("OK")
    else:
        print("NOT OK")

<?php

// example pubkey, you should receive one at https://api.monobank.ua/api/merchant/pubkey
$pubKeyBase64 = "LS0tLS1CRUdJTiBQVUJMSUMgS0VZLS0tLS0KTUZrd0V3WUhLb1pJemowQ0FRWUlLb1pJemowREFRY0RRZ0FFQUc1LzZ3NnZubGJZb0ZmRHlYWE4vS29CbVVjTgo3NWJSUWg4MFBhaEdldnJoanFCQnI3OXNSS0JSbnpHODFUZVQ5OEFOakU1c0R3RmZ5Znhub0ZJcmZBPT0KLS0tLS1FTkQgUFVCTElDIEtFWS0tLS0tCg==";

// value from X-Sign header in webhook request
$xSignBase64 = "MEUCIQC/mVKhi8FKoayul2Mim3E2oaIOCNJk5dEXxTqbkeJSOQIgOM0hsW0qcP2H8iXy1aQYpmY0SJWEaWur7nQXlKDCFxA=";

$message = '{
  "invoiceId": "p2_9ZgpZVsl3",
  "status": "created",
  "failureReason": "string",
  "amount": 4200,
  "ccy": 980,
  "finalAmount": 4200,
  "createdDate": "2019-08-24T14:15:22Z",
  "modifiedDate": "2019-08-24T14:15:22Z",
  "reference": "84d0070ee4e44667b31371d8f8813947",
  "cancelList": [
    {
      "status": "processing",
      "amount": 4200,
      "ccy": 980,
      "createdDate": "2019-08-24T14:15:22Z",
      "modifiedDate": "2019-08-24T14:15:22Z",
      "approvalCode": "662476",
      "rrn": "060189181768",
      "extRef": "635ace02599849e981b2cd7a65f417fe"
    }
  ]
}';

$signature = base64_decode($xSignBase64);
$publicKey = openssl_get_publickey(base64_decode($pubKeyBase64));

$result = openssl_verify($message, $signature, $publicKey, OPENSSL_ALGO_SHA256);

echo $result === 1 ? "OK" : "NOT OK";

const crypto = require('crypto');

// example pubkey, you should receive one at https://api.monobank.ua/api/merchant/pubkey
let pubKeyBase64 = "LS0tLS1CRUdJTiBQVUJMSUMgS0VZLS0tLS0KTUZrd0V3WUhLb1pJemowQ0FRWUlLb1pJemowREFRY0RRZ0FFQUc1LzZ3NnZubGJZb0ZmRHlYWE4vS29CbVVjTgo3NWJSUWg4MFBhaEdldnJoanFCQnI3OXNSS0JSbnpHODFUZVQ5OEFOakU1c0R3RmZ5Znhub0ZJcmZBPT0KLS0tLS1FTkQgUFVCTElDIEtFWS0tLS0tCg==";

// value from X-Sign header in webhook request
let xSignBase64 = "MEUCIQC/mVKhi8FKoayul2Mim3E2oaIOCNJk5dEXxTqbkeJSOQIgOM0hsW0qcP2H8iXy1aQYpmY0SJWEaWur7nQXlKDCFxA=";

let message = `{
  "invoiceId": "p2_9ZgpZVsl3",
  "status": "created",
  "failureReason": "string",
  "amount": 4200,
  "ccy": 980,
  "finalAmount": 4200,
  "createdDate": "2019-08-24T14:15:22Z",
  "modifiedDate": "2019-08-24T14:15:22Z",
  "reference": "84d0070ee4e44667b31371d8f8813947",
  "cancelList": [
    {
      "status": "processing",
      "amount": 4200,
      "ccy": 980,
      "createdDate": "2019-08-24T14:15:22Z",
      "modifiedDate": "2019-08-24T14:15:22Z",
      "approvalCode": "662476",
      "rrn": "060189181768",
      "extRef": "635ace02599849e981b2cd7a65f417fe"
    }
  ]
}`

let signatureBuf = Buffer.from(xSignBase64, 'base64');
let publicKeyBuf = Buffer.from(pubKeyBase64, 'base64');

let verify = crypto.createVerify('SHA256');

verify.write(message);
verify.end();

let result = verify.verify(publicKeyBuf, signatureBuf);

console.log(result === true ? "OK" : "NOT OK");