A user on Hacker News posed an intriguing question: how can one foreshadow specific information and only reveal it at a future date, while also providing irrefutable proof that the information was recorded before its eventual public disclosure? This problem touches upon concepts of trust, verification, and the integrity of time-stamped data.

The discussion yielded a practical and widely accepted cryptographic solution, detailed in a key comment. Here's a breakdown of the proposed method:

The Cryptographic Proof-of-Prior-Knowledge Method

The core idea revolves around using cryptographic hashes to create a "digital fingerprint" of your secret information, publishing this fingerprint publicly, and then revealing the original information later.

1. Prepare Your Document:

   • Compile the information you wish to foreshadow into a digital document. This could be a text file, a PDF, or any other file format.
   • Crucial Tip: The content of this document should be substantial. As the commenter noted, if the text is too short (e.g., a single sentence or a few words), it might be possible for someone to brute-force guess the content that produces the same hash, undermining your proof. Longer text makes this computationally infeasible.

2. Generate a Cryptographic Hash:

   • Use a strong cryptographic hash function, like SHA256 (as suggested in the example Sha256 a9774a1a6ebf564cc408cfd86b5f2c06c13d830e143989714d958d34f325db13), to compute a unique hash sum of your document. This hash is a fixed-length string of characters that acts as a digital fingerprint. Even a tiny change in the document will result in a completely different hash.

3. Publish the Hash Publicly and Securely:

   • This is the "foreshadowing" step. Publish the generated hash in a place that is:
     • Publicly accessible: So others can see it.
     • Timestamped: So there's a clear record of when it was published.
     • Immutable (or difficult to alter post-facto): To ensure the integrity of the timestamp.
   • Examples of suitable platforms mentioned or implied by the discussion include:
     • Social media (e.g., Mastodon timeline, Reddit, Hacker News comments)
     • A public blockchain transaction (e.g., in an OP_RETURN field – though not explicitly mentioned, it fits the criteria)
     • An advertisement in a national printed newspaper (a traditional, but effective, method for a widely accepted timestamp).
   • When publishing, include a clear statement, such as the example provided: “My prediction will be revealed on Dec 31 2025. Sha256 a9774a1a6ebf564cc408cfd86b5f2c06c13d830e143989714d958d34f325db13.”

4. Reveal the Original Document:

   • On or after the specified future date, publish the original document you created in Step 1.
   • Anyone can then independently download this document, calculate its SHA256 hash (or whichever algorithm was used), and compare it to the hash you published earlier. If the hashes match, it provides strong proof that the document (and its contents) existed in that exact form at the time the hash was originally published.

Why This Method Works

This technique leverages the fundamental properties of cryptographic hash functions:

• Deterministic: The same input will always produce the same hash.
• Pre-image resistance: It's computationally infeasible to find the original input (your document) given only its hash, especially for non-trivial documents.
• Second pre-image resistance/Collision resistance: It's computationally infeasible to find a different document that produces the same hash.

By publishing the hash first, you commit to the content without revealing it. The public, timestamped nature of the hash publication prevents you from changing the document later and claiming it was the original. This method offers a robust way to foreshadow events, make sealed predictions, or establish prior art for an idea, ensuring that when the time comes for the reveal, you have verifiable proof of when your information was first recorded.