Signing the Payload

{
  "request": {
    "key2": "value2",
    "key1": "value1",   
    "key3": "value3"
  },
  "signature": "$$REPLACE$$"
}

{
  "key1": "value1",   
  "key2": "value2",
  "key3": "value3"
}

import os, base64, json
from cryptography.hazmat.backends import default_backend
from cryptography.hazmat.primitives import hashes, serialization
from cryptography.hazmat.primitives.asymmetric import ec

def stringify_transaction_details():
    transaction_details = {
    "key1": "value1",
    "key2": "value2",
    "key3": "value3"
  }

    return json.dumps(transaction_details, sort_keys=True, separators=(',', ':'))

def sign_transaction_details(details, user):
    with open(os.path.join("keys", user + ".private.pem"), "rb") as private_key_in:
        ec_private_key = serialization.load_pem_private_key(private_key_in.read(),
                                                            password=None,
                                                            backend=default_backend())
        signed_payload = ec_private_key.sign(str.encode(details), ec.ECDSA(hashes.SHA256()))
    b64_signed_payload = base64.b64encode(signed_payload).decode('UTF-8')
    return b64_signed_payload

def create_transaction_payload(details, signature):
    payload = {
            "request": json.loads(details),
            "signature": signature  # this is signature of the json i.e. transaction_details
        }
    }
    return json.dumps(payload, sort_keys=True, separators=(',', ':'))

if __name__ == "__main__":
    stringified_details = stringify_transaction_details()
    user_signature = sign_transaction_details(stringified_details, "api-maker@zodia.io")
    transaction_payload = create_transaction_payload(stringified_details, user_signature)

    print(transaction_payload)
    

{
  "request": {
    "key1": "value1",
    "key2": "value2",
    "key3": "value3"
  },
  "signature": "MEQCIHuLPOFmbnjWnJ0l6QpYRLMh/6LTwIV6LNlGXRiChProAiBjDQCsvbsGxb6pw9Il/m8zIFmZ+Wh3QFwBmIZJCOk8kA=="
}

[
  {
    "request": {
      "key1": "value1a",
      "key2": "value2a"
    },
    "signature": "$$REPLACE$$"
  },
  {
    "request": {
      "key1": "value1b",
      "key2": "value2b"
    },
    "signature": "$$REPLACE$$"
  }
]

{
  "requestId": "SERV-REQ-XHGFJUUG",
  "request": [
    {
      "request": {
        "key1": "value1a",
        "key2": "value2a"
      },
      "signature": "MEUCIC3VIuw4pfk+BLnZrk1qklGS9phAlQFSQoAnlhw59x7cAiEAm5nq8ANlHcRNcONj5FXXl1v0EK5U8gZyQ22geFSsFL8="
    },
    {
      "request": {
        "key1": "value1b",
        "key2": "value2b"
      },
      "signature": "MEUCIC3VIuw4pfk+BLnZrk1qklGS9phAlQFSQoAnlhw59x7cAiEAm5nq8ANlHcRNcONj5FXXl1v0EK5U8gZyQ22geFSsFL8="
    }
  ]
}