(This is a copy of the readme file from the repository.)

Mask is a small library to prevent secrets from being logged, printed or leaked in any similar way. Ships tags for Clojure, EDN and Aero.

Why? Because I’ve been in such a situation three times, namely:

  • We don’t mask the secrets.
  • Someone logs the entire config.
  • Secrets have leaked!
  • Rotate all the keys, tokens, etc.
  • Change the team and face the same.

This library is an attempt to break this vicious circle.



[com.github.igrishaev/mask "0.1.0"]

Clojure CLI/deps.edn:

com.github.igrishaev/mask {:mvn/version "0.1.0"}


The mask.core namespace provides mask and unmask functions. Pass a value to mask to make it safe for logging or printing in REPL:

(in-ns 'mask.core)

(def -m (mask "Secret123"))

<< masked >>

(str "The password is " -m)
"The password is << masked >>"

Masking is idempotent meaning that you can mask the same value multiple times but the result will be one-level masked value:

(-> -m mask mask mask)
<< masked >>

To release a value from a mask, unmask it:

(unmask -m)

Unmasking is idempotent a well:

(-> -m unmask unmask unmask)

Note: the library treats nil as an error value that cannot be masked. You’ll get an exception:

(mask nil)
Execution error (IllegalArgumentException) at ... (core.clj:34).
Cannot mask a nil value

Masking an empty value signals you’re doing something wrong. Most likely you’ve missed a corresponding key or an environment variable. Thus, the further work makes no sense.


The mask.spec module provides the ::mask spec that checks if a value is really masked. An example from the tests:

(let [config
      {:username "Ivan"
       :password #mask "secret"}]

  (is (s/valid? ::config config)))

;; true

Clojure tag

The built-in #mask tag wraps any value with a mask:

=> {:token #mask "abc123" :password "SecretABC"}

{:token << masked >>, :password "SecretABC"}

EDN tag

There is a reader-edn function that acts like an EDN reader for the same tag:

(let [source (-> "{:foo #mask 42}")]
  (edn/read-string {:readers {'mask reader-edn}}

;; {:foo << masked >>}

Aero tag

To extend Aero with the #mask tag, import the mask.aero namespace:

(require 'mask.aero)

Then read a config with the tag:

;; config.edn
{:foo #mask #env "SOME_PASSWORD"}

;; code
(aero/read-config (io/resource "config.edn"))

;; {:foo << masked >>}

The Aero dependency is not included. You’ve got to provide it by your own.

Ivan Grishaev, 2023