How to calculate the default password hash used by Authlogic

gistfile1.clj

(defn sha-512
  "Return the SHA-512 of the given string"
  [data]
  (let [md (. java.security.MessageDigest getInstance "SHA-512")]
    (. md update (.getBytes data))
    (let [bytes (. md digest)]
      (reduce #(str %1 (format "%02x" %2)) "" bytes))))

(defn hash-repeatedly
  "Hash the given string n number of times"
  ([str times] (hash-repeatedly str times 0))
  ([str times cur]
     (if (< (- times 1) cur)
       str
       (recur (sha-512 str) times (inc cur)))))

user> (hash-repeatedly (str password salt) 20)
"371df0048ddbc0da3f098437a65d100abcc5d709166840e017a92934f0cdd2c6bc5da156b782b4b45ddbab60838328cb6ab766f72eb6e38a844909392679c3d9"

Authlogic (a Rails plugin for authentication) defaults to applying the SHA-512 hashing algorithm 20 times to the concatenated users' password + salt. The above will give you the same result using Clojure and the java.security.MessageDigest library.