Support signatures on data packets

[greglook]

Add support for signing data packets. When unpacking a signed data packet, use a source of public keys (keyring?) to verify the signature values.

[greglook]

http://stackoverflow.com/questions/18772283/how-to-use-bouncycastle-pgpcontentsigner-to-clear-sign-a-byte-array

[tdavis]

Here's what I came up with. Hopefully it's useful to you!

(require '[clojure.java.io :as io]
         '[clj-pgp.keyring :as keyring]
         '[byte-streams :refer [to-byte-array]])
(import '[org.bouncycastle.openpgp PGPSignatureGenerator PGPSignatureSubpacketGenerator PGPUtil PGPSignature]
        '[org.bouncycastle.openpgp.operator.jcajce JcaPGPContentSignerBuilder]
        '[org.bouncycastle.bcpg ArmoredOutputStream BCPGOutputStream])

(defn clear-sign [content secret-keyring-path passphrase]
  "Produce a clear-signed version of `content'.
   The text will be signed with the secret keyright found at `secret-keyring-path', unlocked with `passphrase'"
  (let [bytea-out (java.io.ByteArrayOutputStream.)
        armor-out (ArmoredOutputStream. bytea-out)
        bcpg-out (BCPGOutputStream. armor-out)
        keyring (keyring/load-secret-keyring (io/file secret-keyring-path))
        pubkey (last (keyring/list-public-keys keyring))
        seckey (keyring/get-secret-key keyring (pgp/hex-id pubkey))
        privkey (pgp/unlock-key seckey passphrase)
        builder (JcaPGPContentSignerBuilder. (.getAlgorithm pubkey) PGPUtil/SHA256)
        sig-gen (PGPSignatureGenerator. (.setProvider builder "BC"))]
    (.init sig-gen PGPSignature/CANONICAL_TEXT_DOCUMENT privkey)
    (.beginClearText armor-out PGPUtil/SHA256)
    (.update sig-gen (to-byte-array content))
    ;; Without an additional newline (\n) the final pgp message will be invalid.
    ;; It's only to satisfy the asc format, however, so it should *not* be
    ;; considered as part of signature computation
    (.write armor-out (to-byte-array (str content "\n")))
    (.endClearText armor-out)
    (.encode (.generate sig-gen) bcpg-out)
    (.flush armor-out)
    (.close armor-out)
    (.toString bytea-out)))

[greglook]

Cool, I'll see if I can integrate something along these lines!