Credential detector
Private key (PEM block): format and detection
Short answer
A PEM private key is an armoured block from a line reading BEGIN … PRIVATE KEY to the line reading END … KEY, with at least 64 characters between. Maskflare matches the whole block, whatever the key type, and masks it as one value.
What it is
RSA, EC, PKCS8, OpenSSH and PGP private key blocks are all caught by one detector, because they share the same armour lines. A private key gives full control of whatever it signs or decrypts, so it is masked by default in the Credentials pack.
In Maskflare this is the detector private_key, labelled "Private key", category password or access key, in the Credentials pack.
Format
- Five hyphens, BEGIN, up to 100 characters of label (such as RSA or EC), PRIVATE KEY, optionally BLOCK, five hyphens.
- At least 64 characters of body, lines included.
- The closing armour line ending in KEY and five hyphens.
How Maskflare recognises it
The match is case-insensitive and spans lines. There is no entropy floor, because the armour lines are the signal.
While a stream is being masked, text after a BEGIN line is held back until it is clear whether a key body follows.
Why this page shows no sample key
A realistic sample key would look like a live credential to secret scanners and to anyone skimming the page, and an obviously fake one would not pass this detector's length and entropy rules. The format above is enough to recognise the key: the prefix, the length and the character set.
Regex and validation caveats
- A truncated block with no END line does not match, so a key pasted partially is not caught by this rule; a second detector, private_key_block, covers a shorter body between the lines.
- A key shown with its line breaks replaced by spaces or escaped characters still matches, since the body class accepts them.
- Public keys and certificates have different armour lines and are not matched.
How to detect it in LLM traffic
Private keys are pasted when someone asks an assistant to debug a TLS or SSH failure, or when a coding agent cats a key file while exploring a repository.
Maskflare reads the whole request body, including the system prompt, earlier messages and tool arguments, and replaces each match with a token such as mf_mask_d7e0 before the request goes out. Coding agents are covered through hooks that mask tool inputs and results, so a key printed by a shell command does not travel onward in the next prompt. On ChatGPT, browser protection blocks prompts that contain API keys, tokens or private keys outright.
If you build the detection yourself
- Scan the raw text rather than parsed fields: keys sit in code, JSON strings, headers, shell output and pasted logs, often with no label at all.
- Match the prefix and the exact length for the Private key (PEM block). Where the key has no fixed prefix, require the provider's name and an assignment next to the value instead of matching any long string.
- Compute the Shannon entropy of the candidate and reject low values, and keep an allow list of placeholders (${KEY}, repeated characters, the alphabet run), because documentation and templates are full of them.
- Mask the value before the text is sent anywhere. If the key was already sent, treat it as exposed and rotate it; masking later does not recall it.
How Maskflare handles it
- Protect now, Finance - Protect now, Healthcare, Developer and Monitor first: mask (a reversible token).
The available actions are mask (reversible token), redact, partial, hash, format-preserving encryption, generalize, alert and block. Masked values are kept encrypted for 30 days so that answers can be restored, then they expire; prompts and responses are not logged. The full list of detectors is in the detector reference.
Compliance profiles that include it
All 6 compliance profiles include the Credentials pack, and applying them turns this detector on: HIPAA Safe Harbor, GDPR - EU/EEA, UK GDPR, PCI DSS, US personal data and Switzerland - nDSG health data. A profile is a floor: it never turns anything off or weakens a rule that is already stricter.
Frequently asked questions
Which key types does Maskflare detect?
Any block whose armour reads PRIVATE KEY, including RSA, EC, OpenSSH and PKCS8 keys, and PGP private key blocks.
Are public keys or certificates masked?
No. Their armour lines do not say PRIVATE KEY.
Is the Private key (PEM block) detector on by default?
Yes. It belongs to the Credentials pack, which the default Protect now template turns on with the action mask.
Does Maskflare store the keys it masks?
Prompts and responses are not logged. A masked value is kept encrypted for 30 days so the answer can be restored, then it expires.
Rules on this page describe what Maskflare's detectors and validators check. They are read from the detection catalogue and are not the specification of the body that issues the identifier. No sample key is shown on purpose.