Timestamp

About browser-side timestamping

The Timestamp tool computes the SHA-256 fingerprint of any file you drop into the browser and packages it with a UTC ISO-8601 time stamp into a downloadable JSON receipt. Together, the file and the receipt let anyone later prove the file has not changed since the receipt was issued. This is content notarisation by hash, the same primitive Git uses to chain commits, Bitcoin uses to link blocks, and Linux distributions use to verify downloads. The computation happens entirely client-side through the Web Crypto API's crypto.subtle.digest, so no file leaves your device and the receipt is created without any network round-trip.

The receipt anchors what the file looked like, not when independently verifiable time occurred. For legally defensible time stamps (court evidence, intellectual-property dating, regulatory filings), use a Trusted Timestamping Authority (TSA) operating under IETF RFC 3161 (DigiCert, Sectigo, FreeTSA) or an EU eIDAS-qualified provider. Those services countersign your hash using their own certificate and an externally audited atomic clock, producing a token that survives the loss of the original signer's keys.

How the timestamp receipt is built

1. File dropped -> ArrayBuffer
2. Web Crypto: SHA-256 digest of bytes -> 256-bit value
3. Convert to 64-char hex string
4. Capture timestamp: new Date().toISOString() (UTC, ISO 8601)
5. Build receipt JSON, download as <filename>.timestamp.json

Receipt:
{
  "file": "thesis-v9.pdf",
  "size": 1842711,
  "sha256": "3f59c9... (64 hex chars)",
  "timestampUtc": "2026-05-28T15:08:42.317Z",
  "issuer": "3Tej.com (browser-side)"
}

• SHA-256 outputs 256 bits, expressed as 64 hex characters. The same input always produces the same hash.
• ISO 8601 ensures the timestamp parses identically across languages, OSs, and time zones.
• UTC avoids ambiguity from daylight-saving shifts and regional offsets.

Worked example: stamping a draft contract

You finish a 1.8 MB draft contract and want a record that the document existed in this exact form on 2026-05-28.

1. Drop the file on the drop zone. Browser reads bytes into an ArrayBuffer.
2. Compute SHA-256 via Web Crypto: 3f59c9c98c... e2a1d (64 hex chars).
3. Capture UTC time: 2026-05-28T15:08:42.317Z.
4. Receipt downloaded: thesis-v9.pdf.timestamp.json.
5. Archive both the original file and the receipt in your records or escrow.

Comparison: browser stamp vs RFC 3161 TSA

Common pitfalls when stamping files

• Losing the receipt. Without the JSON receipt, the original file cannot be matched. Store both files together.
• Editing the file post-stamp. Any change (even saving a PDF in a different reader) re-encodes bytes and breaks the hash.
• Trusting browser clock for legal use. A self-stamped receipt is weak evidence in court because the timestamp comes from your own device. Use RFC 3161 TSA when stakes are high.
• Stamping a draft. Each save of a Word document changes bytes. Wait until the file is final before stamping.
• Renaming the file. The filename is recorded in the receipt for human convenience but does not affect the hash. The file content is what matters.

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Frequently asked questions