Hash & Anonymise Columns in Arrow Files Online
Anonymise or pseudonymise columns in Arrow files by replacing values with MD5, SHA-256, or DuckDB hashes — directly in your browser. Useful for GDPR compliance and sharing data without exposing PII — no upload required.
How to hash & Anonymise Columns in Arrow files
- Drop your file onto the upload area. The first 200 rows are shown with the row and column count.
- Choose a Hash algorithm: MD5 (32-character hex), SHA-256 (64-character hex) or DuckDB hash (a 64-bit integer written as text).
- Choose an Output mode. Replace swaps each selected column for its hash. Append keeps the original and adds a new column such as email_md5 at the end.
- Tick the columns to hash. Nothing is ticked by default.
- Click Hash Columns, check the preview, then download the file in the same format.
Your file is processed locally in your browser and is never uploaded. The free limit is 50 MB per file; larger files work if your device has the memory for them.
Worked example
A research team wants to share survey scores with an outside analyst without revealing respondent emails, while still showing which answers came from the same person.
Input (Arrow)
Arrow IPC file (binary, columnar) — shown as a table with its schema
| respondent_id | age_band | nps_score | |
|---|---|---|---|
| R-1001 | maria.lopez@example.org | 35-44 | 8 |
| R-1002 | j.okafor@example.net | 25-34 | 6 |
| R-1003 | maria.lopez@example.org | 35-44 | 9 |
| R-1004 | NULL | 45-54 | 7 |
Schema: respondent_id VARCHAR, email VARCHAR, age_band VARCHAR, nps_score BIGINT
Settings
- Hash algorithm: MD5
- Output mode: Replace original column with hash value
- Columns to hash: email
Result
| respondent_id | age_band | nps_score | |
|---|---|---|---|
| R-1001 | 0774b6f01f630684b8643132792f1c48 | 35-44 | 8 |
| R-1002 | 04010e734bdab7aa07d0f05cd693ef81 | 25-34 | 6 |
| R-1003 | 0774b6f01f630684b8643132792f1c48 | 35-44 | 9 |
| R-1004 | NULL | 45-54 | 7 |
These are the real MD5 values of each address. R-1001 and R-1003 get the same hash because they share an email, so the analyst can still group by person without seeing the address. The missing email stays empty, because MD5 and SHA-256 of NULL are NULL. The column keeps its name and position in Replace mode.
Frequently Asked Questions
Is hashing the same as anonymisation?
No. It is pseudonymisation. The same input always gives the same hash, and no salt is added. Anyone with a list of likely values, such as known email addresses, can hash them and look for matches. Treat hashed personal data as still personal.
Which algorithm should I choose?
SHA-256 for anything shared outside your team. MD5 is shorter and fine for internal join keys, but it is no longer considered secure. The DuckDB hash option is fast but not a standard algorithm, so other systems cannot reproduce it.
Can I add a salt before hashing?
Not in this tool. Use the SQL Query tool instead, with an expression such as sha256('my-secret-salt' || email). Keep the salt private and use the same one for every file you want to join.
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