dbmask

Discover which columns hold sensitive data, mask them with realistic deterministic fakes, then verify the masking actually happened — one auditable workflow for making safe copies of SQL databases.

pip install dbmask

Production data constantly leaks into places with weaker controls: dev and test systems, demo environments, analytics warehouses, vendor handoffs, AI pipelines. dbmask is for the moment you copy that data: it finds the sensitive columns, rewrites them with consistent fakes, and then checks its own work row by row.

---

60-second tour

Everything below runs locally against a throwaway SQLite file (bash syntax; use your own database URL for the real thing).

# 0. A demo database
python -c "
import sqlite3
db = sqlite3.connect('demo.db')
db.executescript('''
CREATE TABLE customers (id INTEGER PRIMARY KEY, full_name TEXT, email TEXT);
INSERT INTO customers (full_name, email) VALUES
  ('Mary Johnson', 'mary.johnson@corp.example'),
  ('Robert Smith', 'robert.smith@corp.example'),
  ('Linda Davis',  'linda.davis@corp.example');
'''); db.commit()"

# 1. A minimal config
cat > dbmask.yaml <<'EOF'
database:
  url: sqlite:///demo.db
source_database:
  url: sqlite:///demo_original.db   # untouched copy, used by `validate`
detection:
  skip_column_patterns: ["^id$"]    # surrogate keys aren't sensitive
masking:
  seed: pick-a-private-seed
EOF

# 2. Which columns are sensitive? (read-only)
dbmask scan --config dbmask.yaml
#   [ok       ] main.customers.id (skip, conf=1.00)
#   [SENSITIVE] main.customers.full_name -> full_name (pattern, conf=0.90)
#   [SENSITIVE] main.customers.email -> email (pattern, conf=1.00)

# 3. Preview the changes (dry run; originals are shown redacted)
dbmask mask --config dbmask.yaml

# 4. Keep an untouched copy, then actually mask
cp demo.db demo_original.db
dbmask mask --config dbmask.yaml --apply

# 5. Prove it worked: row counts, schema, and per-row value comparison
dbmask validate --config dbmask.yaml --strict
#   RESULT: PASSED ✓

Prefer code over a shell? python examples/quickstart.py runs the same story end-to-end, and the library API mirrors the CLI.

---

Safe by default

These are behaviors, not aspirations — each one has a regression test:

• mask never writes without --apply. The flag is the single source of truth; a config file cannot turn a preview into a write.
• An incomplete scan aborts masking. If any column could not be analyzed, mask refuses to run (exit 2) rather than silently leaving that column unmasked. --allow-partial is the explicit escape hatch.
• "Could not tell" is not "not sensitive." Inconclusive columns are reported as UNKNOWN, are never masked, never persisted, and both scan and mask tell you to review them.
• Unmaskable columns are announced. A sensitive column that is also the primary key cannot be rewritten in place — you get a loud NOT MASKED warning instead of a preview that pretends otherwise.
• Previews don't leak. Original values are redacted (***-**) in output by default (--show-values to reveal), dry runs have no side effects, and validation reports carry shape-redacted samples only.
• Verification is a real gate. validate exits non-zero on failure; --strict also fails on anything it could not verify.

---

How a decision is made

For each column, manual overrides take precedence. Historical decisions are reused only after review and while their type, expiry and strategy remain applicable. Machine suggestions stay pending until an analyst imports a reviewed file.

flowchart TD
    A[Column] --> B{Manual override?}
    B -->|yes| Z[Decision]
    B -->|no| C{Imported history?}
    C -->|approved and applicable| Z
    C -->|pending or invalid| R[Human review required]
    C -->|none| D[Patterns and optional LLM]
    D --> S[Pending suggestions]
    S --> H[Human review and file import]
    H --> C

• Overrides (config/dbmask.fields.yaml): a human decision always wins.
• History: exact database/schema/table/column records, separate analyst and reviewer IDs, explicit masking strategies, expiry/type checks and revisions. Import/export CSV, XLSX (pip install "dbmask[excel]"), or a strict Markdown table. Set history.source_file to keep that original file authoritative; history-writeback merges human-approved review rows back into it with a backup and conflict checks. See the history workflow.
• Patterns: explicit format checks plus column-name and database-type context, with no weighted winner selection. Defaults: at least 20 nonblank samples and a match ratio of at least 90%. Reports show counts and ratios for the SQL connector's distinct-value samples, not whole-column row percentages. Conflicts and ambiguous identifiers need review. See pattern detection for supported formats and migration.
• LLM (optional, off by default): for the long tail. Works with OpenAI or an OpenAI-compatible endpoint — or a fully local model (Ollama, LM Studio, vLLM), so nothing leaves your network. llm.send_values: false restricts even a remote provider to column names only. The CLI warns explicitly before any values would leave the machine.

Masking strategies

Deterministic by construction: the same input always maps to the same output (seeded from masking.seed), so Tesla masks identically in every table and joins survive.

Typed Python values (int, float, Decimal, date, datetime, UUID, bool) come back as their own type and valid for it — a masked DATE column never receives 8342-73-51. Register your own with register_strategy(...) and register_dictionary(...).

Which strategy applies? Per-column override → your rule mapping → built-in default for the detected rule → masking.default_strategy. Long free-text fields (notes, comments) are exactly where you should decide yourself — blank, redact, or format_random:

masking:
  column_strategies:
    notes: blank                 # used when no reviewed history strategy is set
  rule_strategies:
    email: fake_email            # per detected rule
  default_strategy: format_random

Consistency — the seed map

Determinism alone drifts: reorder a dictionary file, or change the seed, and every recomputed mapping silently changes. The seed map (on by default) writes each original → masked pair down the first time it is used — keyed by a salted hash, never the original value — and reuses it forever after. Last month's masked snapshot and today's agree; joins across databases stay intact.

masking:
  seed_map:
    enabled: true
    url:                    # blank = sqlite:///dbmask_seedmap.db
    salt: ${DBMASK_SEED_SALT}   # keep the salt out of the store (recommended)

Details, threat model, and the pair-tracking CLI (dbmask seeds): docs/seed-map.

Validation — check the work

dbmask validate compares the masked database against the untouched source and exits non-zero for CI gates:

Completeness is primary-key aligned: source and target rows are matched key-by-key and the sensitive column compared value-by-value, which catches a row where one field survived unmasked even though others changed. Tables without a usable key fall back to a documented heuristic whose clean result is a warning, not a pass — --strict turns any "could not verify" into a failure. Reports state their coverage explicitly.

Supported databases

The connector is a single SQLAlchemy code path, so PostgreSQL, MySQL/MariaDB, SQL Server, Oracle, SQLite and anything else with a SQLAlchemy dialect are wired up (pip install "dbmask[postgres]" etc.).

Honesty about testing: the automated suite currently exercises SQLite on CPython 3.9–3.14 (Linux + Windows). PostgreSQL and MySQL integration tests are the next roadmap item — until they land, treat those engines as "supported by construction, verified by early adopters", and please report anything that misbehaves.

Security model & limitations

Masking reduces exposure; it is not anonymization, and dbmask does not pretend otherwise:

• Deterministic masking is dictionary-attackable for guessable values. Anyone holding your masking.seed (or the default — the CLI warns) can recompute the mapping for values they can guess. Choose a private seed, and set the seed-map salt from the environment.
• Column-level scope. Mixed PII inside free text is flagged at the column level at best; the right treatment for notes is usually blank/redact, not clever faking.
• Primary-key columns are not masked (announced loudly). Restructure or drop such tables before sharing if the key itself is sensitive.
• Detection is heuristic. Patterns miss things; review UNKNOWN columns, keep overrides for what matters, and treat validate --strict as the gate.
• Run against a copy of production. Never point --apply at the primary.

Found a hole in any of these guarantees? That's a security report: SECURITY.md.

How dbmask compares

Different tools solve adjacent problems — this table is about workflow shape, not maturity (several of these are excellent and far more battle-tested):

If you need heavy-duty subsetting, synthesis, or enterprise scale today, those tools may serve you better — dbmask optimizes for one auditable pipeline you can read in an afternoon.

Configuration

Two YAML files (copy the *.example.yaml from config/, drop the .example): the main config (connection, detection, masking, validation — secrets via ${ENV_VAR}) and the field-override file (your manual sensitive/safe toggles). Every option is commented in the examples; full reference: docs/configuration.

Project status & roadmap

0.1.x — young and moving fast. The current release focused on making the safety envelope real (fail-closed scanning, PK-aligned verification, valid typed output, no PII in previews/logs — see the changelog). Near-term roadmap:

• PostgreSQL & MySQL integration tests in CI (testcontainers)
• A public detection benchmark (precision/recall per rule, fixed datasets)
• Run manifests: idempotency, resume, and "what exactly did this run touch"
• A small, stable public Python API (scan / apply / verify) with a deprecation policy
• Governance export (OpenMetadata / DataHub) so classifications feed the catalogs organizations already run

Using dbmask anywhere real? Add yourself to ADOPTERS.md or file adopter feedback — including "we chose something else because…". It steers the roadmap.

Contributing & development

git clone https://github.com/sealandseacat/dbmask.git
cd dbmask
python -m venv .venv && source .venv/bin/activate
pip install -e ".[dev]"
pytest

Every bug fix ships with a regression test that fails on the old code — the test suite doubles as documented history of every sharp edge found so far. See CONTRIBUTING.md.

Citing

If dbmask is useful in your work, cite it via the repository's CITATION.cff (GitHub's "Cite this repository" button).

License

MIT