This document covers the internal protocol and cryptographic details of KeePassRPC CLI.

KeePassRPC uses a two-stage authentication process:

1. Client sends SRP identifyToServer with a random username and public ephemeral A
2. Server responds with salt s and public ephemeral B
3. Client computes SRP proof M and sends proofToServer
4. Server verifies M and sends proof M2
5. Both sides derive session key K = SHA-256(S) where S is the SRP shared secret
6. Client saves the session key locally

After the initial SRP handshake, the client reconnects and uses the stored session key:

1. Client sends stored username
2. Server sends a random challenge sc
3. Client generates a random challenge cc and computes response cr = SHA-256("1" + key + sc + cc)
4. Server verifies cr and sends its response sr = SHA-256("0" + key + sc + cc)
5. Client verifies sr

All subsequent communication uses encrypted JSON-RPC:

• Algorithm: AES-256-CBC
• HMAC: SHA-1
• Key: Session key derived from SRP (K = SHA-256(S))
• IV: Random 16 bytes per message

• Prime (N): 512-bit safe prime
• Generator (g): 2
• Key derivation constant (k): 0xb7867f1299da8cc24ab93e08986ebc4d6a478ad0
• Hash function: SHA-256

All messages are JSON with the following envelope:

{
  "protocol": "setup|jsonrpc",
  "srp": { ... },
  "key": { ... },
  "error": { ... },
  "jsonrpc": { ... },
  "version": 131072
}

• The WebSocket connection is unencrypted (runs over localhost only)
• Session keys are stored locally in keepassrpc-cli.auth and should be protected
• The connection password is randomly generated by KeePass and displayed in a pop-up during SRP negotiation