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Session Signer

The Session Signer is a server-side Ed25519 keypair that signs every inference receipt, providing cryptographic proof that the response was generated through Lucid’s infrastructure.

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How It Works

Client Request → TrustGate → LLM Provider → Response
                     ↓
              Generate Receipt
                     ↓
              Sign with Ed25519
                     ↓
          Return response + receipt_id
  1. When an inference completes, Lucid generates a receipt containing hashes of the input and output
  2. The session signer creates an Ed25519 signature over: receipt_id || input_hash || output_hash || timestamp
  3. The signature is stored alongside the receipt
  4. Anyone can verify the signature using the signer’s public key

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Get the Public Key

The signer’s public key is always available via the API: 
const { publicKey } = await lucid.signer.pubkey();
console.log("Public key:", publicKey); // hex-encoded Ed25519 public key

curl https://api.lucid.foundation/v1/signer/pubkey \
  -H "Authorization: Bearer $LUCID_API_KEY"

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Verify a Signature

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TypeScript

import { verify } from "@noble/ed25519";

const receipt = await lucid.receipts.get(receiptId);
const { publicKey } = await lucid.signer.pubkey();

const message = Buffer.concat([
  Buffer.from(receipt.id),
  Buffer.from(receipt.inputHash),
  Buffer.from(receipt.outputHash),
  Buffer.from(receipt.timestamp),
]);

const valid = await verify(
  Buffer.from(receipt.signature, "hex"),
  message,
  Buffer.from(publicKey, "hex")
);

console.log("Signature valid:", valid);

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Python

from nacl.signing import VerifyKey

verify_key = VerifyKey(bytes.fromhex(public_key))
message = (receipt_id + input_hash + output_hash + timestamp).encode()
signature = bytes.fromhex(receipt_signature)

try:
    verify_key.verify(message, signature)
    print("Valid")
except:
    print("Invalid")

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Security Model

PropertyDescription
AlgorithmEd25519 (EdDSA over Curve25519)
Key storageServer-side only, never exposed to clients
Key rotationSupported — old signatures remain valid with the old key
Public key accessAlways available via /v1/signer/pubkey
Signature scopeCovers receipt ID, input hash, output hash, and timestamp

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Why Ed25519?

  • Fast verification (important for batch receipt checking)
  • Small signatures (64 bytes)
  • Deterministic — same input always produces the same signature
  • Widely supported across languages and blockchains (including Solana)

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Relationship to Epochs

Individual receipt signatures prove authenticity. Epochs batch multiple receipts into a Merkle Mountain Range and anchor the root hash on Solana, providing blockchain-level immutability on top of the cryptographic signatures.