Project 04

Messaging

Voting taught you to keep a choice private while disclosing an aggregate. Messaging asks a different question: how do you keep an entire piece of content private, forever, while still proving on-chain that something was sent?

What you'll learn

• The difference between hiding a value from the chain and hiding it from everyone except the recipient
• Why a message's content should usually live in private state, not be disclosed at all
• Using a public counter as proof-of-activity without proof-of-content

The contract

Save this as contracts/messaging.compact:

pragma language_version >= 0.23;

import CompactStandardLibrary;

witness getMessageContent(): Opaque<"string">;

export ledger messageCount: Uint<32>;

export circuit sendMessage(): [] {
    const content = getMessageContent();
    messageCount = messageCount + 1;
}

The important part is what's missing

Look closely: content is read from the witness, but never disclosed anywhere. It's used inside the circuit and then simply not written to ledger state. That's the whole trick: private data doesn't need a special "hide me" instruction, it just needs you to never call disclose() on it. The chain only ever learns that a message-sending event happened, via the incremented counter, never what was said.

Where a real version goes further

A production messaging app would also need actual encrypted delivery to a specific recipient (so only they can read the content off-chain), not just "nobody on-chain sees it." That's a real cryptography problem beyond this exercise, worth researching once this pattern feels natural.

Build it yourself

compact compile contracts/messaging.compact contracts/managed/messaging

Deploy, call sendMessage() a few times with different witness content each time, and confirm messageCount goes up correctly, while nothing about the actual message text appears anywhere in the ledger state you read back.