The protocol is a convention.
The product, you build it.

THE PROTOCOL IN 5 MINUTES

Three things to know.

1. 01

   The object: a canonical JSON-LD signature.

   An Alethea signature is a JSON-LD object containing a verified identity hash, content fingerprints (SHA-256, pHash, Chromaprint), a role (author, actor, witness, broadcaster, reclaim), a timestamp, and an ECDSA or Ed25519 cryptographic signature. Nothing else.

2. 02

   The record: on-chain, chain-agnostic.

   The signature is projected and recorded on a public blockchain (Polygon, Base, Ethereum, Solana, or any registry meeting the eligibility criteria). The protocol publishes a profile per chain to normalize the projection.

3. 03

   The verification: by anyone, without permission.

   Anyone can query the blockchain, retrieve a signature, recompute the fingerprints on a content they have at hand, and verify the match. No need to request permission from a builder, no need to pay for a service.

FULL EXAMPLE

One signature, from end to end.

Here is what a complete Alethea signature looks like, as a builder serializes it before writing it on-chain.

{
  "@context": "https://aletheaprotocol.com/context/v1",
  "@type": "AletheaSignature",
  "version": "0.1",

  "identityHash":      "0x9f86d081884c7d659a2feaa0c55ad015...",
  "identityProvider":  "fr-franceconnect-plus",
  "signerPublicKey":   "0x04a1b2c3d4e5f6...",

  "contentHashes": {
    "sha256":  "0xe3b0c44298fc1c149afbf4c8996fb924...",
    "phash":   "0xd4a8c7f2"
  },

  "role":      "author",
  "timestamp": "2026-05-25T20:53:14Z",

  "contextHash":         "0x...",
  "signature":           "0x...",
  "signatureAlgorithm":  "ECDSA-secp256k1"
}

MINIMAL SOLIDITY PROJECTION

A minimal contract on Polygon or Base.

Here is the minimal EVM projection of an Alethea signature. Target transaction cost: less than 1 cent on Polygon. A builder can deploy this contract or use a shared registry.

// SPDX-License-Identifier: AGPL-3.0-or-later
pragma solidity ^0.8.20;

contract AletheaRegistry {
    struct Signature {
        bytes32 identityHash;
        address signerAddress;
        bytes32 identityProviderId;  // keccak256 of provider id
        bytes32 contentSha256;
        bytes32 contentPhash;
        uint8   role;
        uint64  timestamp;
        bytes32 contextHash;
    }

    event SignaturePosted(
        bytes32 indexed contentSha256,
        address indexed signer,
        uint8 role,
        bytes32 identityHash
    );

    mapping(bytes32 => Signature[]) public signaturesByContent;

    function post(Signature calldata sig) external {
        require(msg.sender == sig.signerAddress, "signer mismatch");
        signaturesByContent[sig.contentSha256].push(sig);
        emit SignaturePosted(sig.contentSha256, sig.signerAddress, sig.role, sig.identityHash);
    }

    function get(bytes32 contentSha256) external view returns (Signature[] memory) {
        return signaturesByContent[contentSha256];
    }
}

QUICKSTART

Get started in 3 steps.

1. 1Read the spec. /protocol/specifications describes the canonical JSON-LD format, crypto schemes, and on-chain projection per target chain.
2. 2Implement client-side fingerprint computation. SHA-256 via Web Crypto API or libsodium, pHash via imagehash (Python) or phash-image (JS). See the minimal example /try for the full pattern.
3. 3Wire an identity provider and a chain. Identity provider compliant with eIDAS / NIST 800-63 / ISO 29115 (FranceConnect+, Onfido, Sumsub, etc.) for KYC. Target chain depending on your model (Polygon for cost, Base for Ethereum L2, Solana for perf). Normative profile per chain coming in spec v0.2.