ECDHE Curve25519 Key Exchange Signature and KeyValue Fields

Question

I have an API where I need to pass the "KeyMaterial" which according to API Docs is for:

Contains the cryptographic parameters that are required to perform End-to-End encryption for sharing the information between the producer and the consumer in a secure manner.

Here is the example they have shared

"KeyMaterial": {
  "cryptoAlg": "ECDHE",
  "curve": "Curve25519",
  "params": "string",
  "DHPublicKey": {
    "expiry": "2018-12-23T11:39:57.153Z",
    "Parameters": "string",
    "KeyValue": "683938505ec529a700fcceab66273d1aa78d494208a4769930f0818872159265"
  },
  "Nonce": "R4s6vNI7I/JfdeA3/6dMMQ==",
  "Signature": "jFJcYCOTVV6iiLPlM7qY+Zz+3PF8oUPFg1byb1GNr+k="
}

They also mention that

"Signature" field : Refers the signature standard for more information: https://www.w3.org/TR/xmldsig-core1/

"KeyValue" : Contains the value of ephemeral public key

However, I confused to how exactly should I be filling in these parameters. I could not find any reference implementations for ECDHE + Curve25519. Can anyone explain how exactly can I fill in these parameters? How do I generate the signature field? and the KeyValue field in "DHPublicKey"

Answer 1

Public Key can be generated like this.

1. Generate a X25519 private key : 32 byte random

2. private key * Generator point = Public Key

Nonce must be random bytes transformed into base64 encoding.

About the signature, I don't know what kind of signature it is.