Blog — Technical Guide
How Document Timestamps Work
A plain-language explanation of the cryptography, blockchain anchoring, and verification process behind tamper-proof document timestamps.
What Is a Document Timestamp?
A document timestamp is a cryptographic proof that a specific file existed at a specific point in time. It is not the clock on your computer, not a date printed on a PDF, and not a file's "created" or "modified" metadata. It is a mathematical fingerprint anchored to an independent, public record that anyone can verify.
File metadata — the creation date, modification date, and other properties stored by your operating system — is trivially easy to change. Anyone with basic technical knowledge can alter a file's apparent creation date in seconds. This makes metadata unreliable as proof of when a document actually existed.
A cryptographic timestamp solves this problem by producing a fingerprint of the file's exact contents and recording that fingerprint in a place no one can alter after the fact. The result is a proof that can be independently verified by anyone, at any time, without trusting the person who created the timestamp or the service that facilitated it.
How Cryptographic Hashing Works
At the core of every document timestamp is a cryptographic hash function called SHA-256. When you feed any file into SHA-256 — a one-page contract, a 500-megabyte video, a single-pixel image — it produces a fixed-length, 256-bit fingerprint that is unique to that file's exact contents.
This fingerprint has a critical property: changing even a single byte of the original file produces a completely different hash. Add a space to a document, crop one pixel from an image, or alter one character in a spreadsheet, and the resulting hash will be unrecognizably different from the original. There is no way to make a "small" change that produces a "similar" hash.
SHA-256 is also a one-way function. Given a hash, there is no way to reconstruct the original file. This is what makes the process privacy-preserving: when your file is timestamped, only the hash is ever transmitted or recorded. The file itself stays on your device. No one — not the timestamping service, not the blockchain, not anyone else — ever sees the contents of your document.
SHA-256 is the same algorithm that secures every Bitcoin transaction. It has been subjected to decades of cryptographic scrutiny and remains unbroken.
Anchoring to the Bitcoin Blockchain
Once the hash is generated, it is submitted to OpenTimestamps, an open protocol for creating Bitcoin-anchored timestamps. OpenTimestamps is free to use and does not require an account — it is public infrastructure, like email or the web itself.
OpenTimestamps does not create a separate Bitcoin transaction for every document. Instead, it aggregates many document hashes into a data structure called a Merkle tree — a binary tree of hashes where each parent is the hash of its two children. The root of this tree represents all the documents submitted during that period. Only this single root hash is written to the Bitcoin blockchain, making the process efficient and inexpensive.
Once the Bitcoin transaction is confirmed in a block, the timestamp becomes permanent. Bitcoin's proof-of-work consensus mechanism means that reversing a confirmed transaction would require re-doing all the computational work performed since that block was mined — an effort that would cost billions of dollars in energy and hardware. No individual, company, or government can alter what has been recorded.
The proof is stored in a small .ots file that contains the mathematical path from your document's hash through the Merkle tree to the Bitcoin transaction. This file is all that is needed to verify the timestamp.
Verification: How to Prove a Document Is Unaltered
To verify a timestamped document, you need two things: the original file and the .ots proof file. The verification process re-computes the SHA-256 hash of the file and then follows the path in the proof file to check whether that hash is consistent with what was recorded in the Bitcoin blockchain.
If the file has been altered in any way since it was timestamped — even by a single byte — the re-computed hash will not match the hash in the proof, and the verification will fail. There is no ambiguity: either the file is identical to what was timestamped, or it is not.
This verification process does not require trusting any company, server, or third party. Bitcoin is the neutral witness. The blockchain is public, the OpenTimestamps protocol is open-source, and the cryptographic operations can be performed by anyone with freely available tools. Even if EverCert were to cease operations entirely, every timestamp it ever created would remain independently verifiable.
You can verify timestamps using the OpenTimestamps website, command-line tools, or any compatible third-party verifier. The proof is yours to keep and use however you choose.
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