We envision a scenario where contract signers put their portable document format (PDF) contract into a blockchain application that outputs a signed contract, while blockchain nodes don't know the contract content, contract signers' identities and contract signatures. Comparing to current centralized online contract signing services, blockchain applications could avoid single point of failure, internal attacks and data loss. More importantly, the application also provides fairness and privacy properties. By fairness, we mean that contract signers obtain a signed contract simultaneously, or obtain nothing, or some signer obtains a singed contract at the cost of paying contract coins. By privacy, we mean that contract contents, signatures, and signers' identities are hidden from blockchain nodes. At last, we support RSA signatures whose verification is embedded in most PDF readers, which makes the whole solution practical.
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