caesium

caesium is a modern cryptography library for Clojure. It is a direct jnr-ffi binding to libsodium, which in turn is a more convenient fork of the original NaCl library by djb.

NOTE: Install libsodium 1.0.18+ before trying to use caesium.

Minimum viable snippet

Here's a sample of how you can use secretbox:

(ns minimum-viable-secretbox
  (:require [caesium.crypto.secretbox :as sb]))

(def key (sb/new-key!))
(def plaintext "Hello caesium!")
(def nonce (sb/int->nonce 0))
(def ciphertext (sb/encrypt key nonce (.getBytes plaintext)))
(def roundtrip (String. (sb/decrypt key nonce ciphertext)))
(assert (= plaintext roundtrip))

Documentation

The most important documentation for caesium is actually the documentation for libsodium. Since it's all just relatively small wrappers around that, everything in it applies.

Password hashing

Here's an example of how you can use pwhash:

(ns pwhash-usage
  (:require [caesium.crypto.pwhash :as pwhash]
            [caesium.randombytes :as rb]
            [caesium.byte-bufs :as bb]
            [caesium.util :as u]
            [caesium.crypto.secretbox :as sb]))

;; helper function for creating salts from integers. may be useful for deterministic
;; key derivation, incrementing subkeys from 0.
(def int->salt (partial u/n->bytes pwhash/saltbytes))

;; hashing passwords
(def password "example")
(def hashed-password (pwhash/pwhash-str password 
                                        pwhash/opslimit-sensitive
                                        pwhash/memlimit-sensitive))
(assert (= 0 (pwhash/pwhash-str-verify hashed-password password)))

;; key derivation
(def salt (rb/randombytes pwhash/saltbytes)) ; changing salt means changed derived key
(def derived-key (pwhash/pwhash sb/keybytes
                                password
                                salt
                                pwhash/opslimit-sensitive
                                pwhash/memlimit-sensitive
                                pwhash/alg-default))
(def message (.getBytes "hello, world!"))
(def encrypted-message (sb/encrypt derived-key (sb/int->nonce 0) message))
(def decrypted-message (sb/decrypt derived-key (sb/int->nonce 0) encrypted-message))
(assert (bb/bytes= message decrypted-message))

Usage with Github Actions secrets

Here is how you can create or update a repository secret for GitHub actions:

(require '[caesium.crypto.box])
(require '[clj-http.client :as http])
(require '[jsonista.core :as json])
(import '(java.util Base64))

(def public-key
  "The public key of the repository of which you want to create or update a secret"
  (let [payload (-> {:request-method :get
                     :url "https://api.github.com/repos/{owner}/{repo}/actions/secrets/public-key"
                     :basic-auth ["{user}" "{GITHUB_TOKEN}"]
                     :headers {"Content-Type" "application/json"
                               "Accept" "application/vnd.github.v3+json"}}
                    http/request
                    :body
                    json/read-value)
        ^String encoded-key (get payload "key")]
    {:decoded-key (.decode (Base64/getDecoder) (.getBytes encoded-key))
     :key-id (get payload "key_id")}))

(let [{:keys [^String decoded-key ^String key-id]} public-key
      plaintext "MY_SECRET_VALUE"
      cyphertext (caesium.crypto.box/box-seal
                   (byte-streams/to-byte-array plaintext)
                   decoded-key)]
  (http/request
    {:request-method :put
     :url "https://api.github.com/repos/{owner}/{repo}/actions/secrets/{MY_SECRET}"
     :body (json/write-value-as-string
             {:encrypted_value (.encodeToString (Base64/getEncoder) cyphertext)
              :key_id key-id})
     :basic-auth ["{user}" "{GITHUB_TOKEN}"]
     :headers {"Content-Type" "application/json"
               "Accept" "application/vnd.github.v3+json"}}))

Differences with other bindings

Instead of making specific claims about specific libraries which may become outdated, here are a few properties you may care about:

• caesium is written by a cryptographer who has experience binding cryptographic libraries.
• caesium has continuous integration and a fairly extensive test suite with very high form/line coverage.
• caesium does not provide magic layers on top of libsodium that prevent you from writing secure software because of JVM memory semantics, while not getting in your way if you want the default good-enough behavior.
• caesium uses jnr-ffi pinning correctly; resulting in zero-copy behavior between JVM and C land at the call site.
• All APIs take byte[] and in some cases ByteBuffer, never String. This gives you the option of zeroing byte arrays out once you're done. caesium doesn't hide the no-magic C APIs from you; but you have to understand libsodium to use them. The upside of that is that this library provides the APIs necessary to use libsodium safely; e.g. with locked buffers with canaries, secure memset, et cetera.
• caesium's APIs match libsodium's behavior. If libsodium hashes a seed to produce a keypair, caesium will hash a seed to produce a keypair. If libsodium uses the default output size of a particular hash function, caesium will use the default output size of that hash function. (These were at time of writing not true for at least 1 other library).

caesium tries to just give you the libsodium experience from Clojure. C pseudo-namespaces are mapped to real Clojure namespaces. It usually maps fns to predictable names; sodium_crypto_secretbox_open_easy will be called caesium.crypto.secretbox/open-easy. Formally: take the C pseudo-namespace, turn it into a real namespace, replace the leading sodium with caesium, replace underscores with dashes. Exceptions where this doesn't work out:

• sometimes, the last part of the C pseudo-namespace is repeated. This happens for functions that have the same name as a C pseudo-namespace, e.g. crypto_generichash (which is also the pseudo-namespace for e.g. crypto_generichash_init). These would be available in the caesium.crypto.generichash namespace, as generichash and init. This is also repeated for some functions where there is a small suffix, e.g. the function name for the "easy secretbox opener" is secretbox-easy-open, not easy-open.
• some functions map to the same underlying C functions, but have different Java APIs. For example, one of them might cast to ByteBuffer, while others assume byte arrays, while others rely on reflection to call the right thing. Other pairs of functions might expect you to produce the output buffer, or manage the output buffer for you. Since these are only JVM-level differences, these often need different names at the JVM/Clojure level. (This is always done as a fairly descriptive suffix.)
• functions designed to make a #define constant available are accessible as values, they don't need to be called. For example, you can access the crypto_generichash_KEYBYTES_MIN constant via the libsodium size_t crypto_generichash_keybytes_min(void); function, but in caesium, it's just caesium.crypto.generichash/keybytes-min (not a function you have to call).
• some families of functions in libsodium are a consequence of C not supporting multi-arity functions; e.g. scalarmult in libsodium has two functions: one with the fixed base point and one with an explicit base point; caesium just has one function with two arities.
• caesium sometimes takes a little artistic license with some of the exposed names when that makes more sense than the original; generally fns will be available under both the "official" name and an alias.

Compatibility

caesium uses semver.

Since this is a security-sensitive library, I will actively remove functions or APIs that have serious security problems, instead of simply documenting the problem.

License

Copyright © the caesium authors (see AUTHORS)

Distributed under the Eclipse Public License either version 1.0 or (at your option) any later version.