Introduction

SignChain is a document signing platform that anchors signatures on a blockchain, providing tamper-evident proof that a specific person signed a specific document at a specific time.

What Problem Does SignChain Solve?

Traditional digital signatures rely on certificate authorities (CAs) and PKI infrastructure. If a CA is compromised, revoked, or simply ceases to exist, the trust chain breaks. SignChain takes a different approach:

• Blockchain anchoring provides an immutable, publicly verifiable record of every signature
• Cryptographic hashing binds the signature to the exact document content
• Client-side encryption ensures signer privacy while maintaining verifiability
• QR codes embedded in the PDF enable instant verification by anyone with a phone

How It Works (30-second version)

1. Sign -- The signer opens a PDF in the SignChain desktop app, places their signature, and confirms
2. Hash -- The app computes a cryptographic hash of the signed document
3. Anchor -- The hash is recorded on a blockchain via a smart contract
4. Embed -- A QR code containing the verification URL is embedded into the PDF
5. Verify -- Anyone can scan the QR code to verify the signature against the blockchain record

Key Properties

Property	How
Tamper evidence	Any change to the PDF invalidates the hash stored on-chain
Non-repudiation	Blockchain record is immutable and timestamped
Privacy	Signer data is encrypted; only the QR holder can decrypt
Offline signing	PDF operations happen locally; only the hash goes to the network
No vendor lock-in	Blockchain record is public; verification is open

Components

SignChain consists of four cooperating components:

• Desktop App -- The signing interface. All document operations happen locally on the signer's machine.
• API Server -- Relays transactions to the blockchain and stores encrypted payloads.
• Smart Contract -- The on-chain anchor registry. Immutable once deployed.
• Verification Web App -- A lightweight page opened by scanning the QR code on a signed document.