P2P Message Proof: Digital Signatures

TL;DR

Yes, a client can prove they sent a message in a P2P network using digital signatures. Each message is signed with the sender’s private key, and anyone can verify it using their public key. This provides non-repudiation – the sender can’t deny sending the message.

How to Prove Message Sending in a P2P Network

1. Understand Digital Signatures: A digital signature is like a handwritten signature, but for electronic data. It uses cryptography (maths) to ensure authenticity and integrity.
   • Private Key: This is secret – only the sender knows it.
   • Public Key: This can be shared with anyone.
2. Signing the Message: Before sending, the client uses their private key to create a signature for the message.

   # Example using Python and cryptography library (simplified)

   from cryptography.hash import SHA256

   from cryptography.signature import PKCS1v15

   from cryptography.publickey import RSAKey

   def sign_message(private_key, message):
     hash_object = SHA256(message.encode('utf-8'))
     signer = PKCS1v15.new(private_key)
     signature = signer.sign(hash_object)
     return signature

3. Sending the Message and Signature: The client sends both the message *and* the digital signature to the recipient.
   • The signature is usually sent alongside the message as a separate data field.
4. Verification by Third Party (or Recipient): A third party (or even the recipient) can verify that the message came from the claimed sender.

   # Example verification code (simplified)
   from cryptography.publickey import RSAKey
   from cryptography.signature import PKCS1v15
   from cryptography.hash import SHA256
   
   def verify_message(public_key, message, signature):
     hash_object = SHA256(message.encode('utf-8'))
     verifier = PKCS1v15.new(public_key)
     try:
       verifier.verify(hash_object, signature)
       return True # Signature is valid
     except Exception as e:
       return False # Signature is invalid

5. Key Distribution: The third party needs the sender’s public key to verify the signature.
   • Public Key Infrastructure (PKI): A trusted system for managing and distributing public keys. This often involves Certificate Authorities (CAs).
   • Web of Trust: Users vouch for each other’s public keys.
   • Direct Exchange: The sender directly provides their public key to the third party. This is less secure if the key isn’t verified independently.
6. Timestamping (Optional but Recommended): To prevent replay attacks, include a timestamp in the message and sign that too.
   • A trusted timestamp server can provide proof of when the message was signed.
7. Considerations:
   • Key Security: The sender *must* keep their private key secure. If compromised, anyone could forge signatures.
   • Algorithm Choice: Use strong cryptographic algorithms (e.g., RSA with a large key size, ECDSA).
   • Hashing Function: Use a secure hashing function like SHA-256 or SHA-3.