Security & Encryption

An independent product, not affiliated with Microsoft.

This page explains what ExcelRewind does and does not do with your spreadsheet data, in plain terms, with no compliance badges attached. Recording happens on your machine. The original workbook is never touched. An optional passphrase locks the recording it produces. Below is how each part actually works.

What does ExcelRewind do with my spreadsheet?

ExcelRewind is an Excel add-in (with a Google Sheets add-on) that records why the numbers in a spreadsheet change. It captures every cell edit with timing and a note explaining the reasoning, and saves them to a shareable, replayable .rewind companion file. It never modifies the original workbook and never uploads spreadsheet content¹.

Is the .rewind file encrypted by default?

Not automatically. A .rewind file is a compressed archive of JSON, similar in spirit to a .zip. Compression makes the file smaller; it is not encryption, and it does not stop someone with the file from opening it. If you need the file locked so only someone with a passphrase can read it, turn on end-to-end encryption before you share it. Without that step, treat a .rewind file the way you'd treat any other file you send: anyone who gets a copy can open it.

How does end-to-end encryption work?

When you enable it, ExcelRewind generates a random encryption key for that file and seals each part of the .rewind archive with it using AES-256-GCM. The key itself is then locked ("wrapped") under a key derived from the passphrase you choose, using PBKDF2 with SHA-256 and a per-file random salt. Your passphrase is never sent anywhere; the derivation happens on your device, and only the wrapped key travels inside the file.

This is a zero-knowledge design: ExcelRewind's servers see none of your passphrase, your derived key, or your file contents. That also means ExcelRewind cannot decrypt an encrypted .rewind file for you, on request or otherwise. There is no backdoor to remove, because none exists.

What happens if I lose the passphrase?

The file stays locked. Because the passphrase never leaves your device and ExcelRewind never stores it, there's no "forgot passphrase" reset for a .rewind file the way there is for a login. If the passphrase is gone, the recording is unrecoverable. Store it the way you'd store any password you can't afford to lose, in a password manager or a written note kept somewhere safe, not in the same email as the file.

How do I know a .rewind file hasn't been altered?

Every .rewind file carries a SHA-256 hash for each of its parts: the edit timeline, notes, audio if present, and so on. When the file is opened, the viewer or add-in recomputes those hashes and compares them against what's recorded. A mismatch means that part was changed or corrupted after it was written, and it is reported rather than played back silently.

What can the add-in see in my workbook?

The Excel add-in reads cell-change events in the workbook it is loaded into, using Office.js's built-in change-tracking APIs. It has no access to other open workbooks and cannot run scripts against them. What it captures is limited to cell edits with timing, plus any note you type; that's what goes into the .rewind file. Nothing about the workbook's contents is sent to a server as part of recording.

How does replaying an encrypted file work?

The web viewer replays a .rewind file entirely in your browser; it never uploads the file to a server to play it back. If the file is encrypted, the viewer asks for its passphrase before it can decrypt and replay anything, and that decryption also happens locally in the browser tab.

Want to send a recording without handing over the workbook itself? See sharing a walkthrough without sending the file, and what the .rewind format actually contains on the .rewind file page.

Frequently asked questions

Does ExcelRewind encrypt my Excel file?

No. ExcelRewind never touches or encrypts the original .xlsx or Google Sheet. Encryption, when you turn it on, applies only to the separate .rewind recording file, and the original workbook is never modified.

Is spreadsheet content ever uploaded to ExcelRewind's servers?

Not during normal recording, which happens locally on your machine; only account, licensing, and telemetry-level data touches ExcelRewind's servers. The one exception is Kiosk/Exam mode: when a student submits a proctored exam, the finished recording uploads to ExcelRewind's storage so the instructor can review it, sealed to the organization's encryption key when one is enabled. See the privacy page for what that covers.

How does end-to-end encryption work?

With end-to-end encryption enabled, each .rewind file gets a random encryption key, and that key is locked with a passphrase you choose (PBKDF2-SHA256 key derivation, AES-256-GCM to seal the data). ExcelRewind never receives your passphrase or the underlying key, so it cannot open an encrypted file.

What happens if I forget my passphrase?

The file becomes unreadable. Because ExcelRewind never stores or receives your passphrase, there is no reset link and no support-desk recovery. Losing the passphrase means losing access to that recording, so write it down somewhere safe before you share an encrypted file.

How does ExcelRewind detect if a .rewind file has been tampered with?

Every .rewind file carries a SHA-256 hash for each of its parts (timeline, notes, audio, and so on). The viewer recomputes and checks those hashes when it opens the file, and it will not silently play back a part whose hash doesn't match.

Install the add-in   or read what Excel's built-in tools miss and how ExcelRewind compares to Excel's own change tracking.

¹ The one exception: a Kiosk/Exam mode submission uploads the finished exam recording to ExcelRewind's storage for instructor review, sealed to the organization's encryption key when one is enabled.

Last updated: July 2026.

ExcelRewind is an independent product and is not affiliated with, endorsed by, or sponsored by Microsoft Corporation. Microsoft and Excel are trademarks of the Microsoft group of companies. Questions? [email protected].