SHA-256 hashing in Clojure via Java interop

sha256-test.clj

;; test suite to go with hashing code
(ns user.test.sha256
  (:use [clojure.test]
        [clojure.string :only [join]]
        [user.hash]))

(def sha256-test-vectors
  #^{:doc "test vectors from the NESSIE project (http://is.gd/jdM99e)"}
  [{ :message ""
    :expected "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855"}
   { :message "a"
    :expected "ca978112ca1bbdcafac231b39a23dc4da786eff8147c4e72b9807785afee48bb"}
   { :message "abc"
    :expected "ba7816bf8f01cfea414140de5dae2223b00361a396177a9cb410ff61f20015ad"}
   { :message "message digest"
    :expected "f7846f55cf23e14eebeab5b4e1550cad5b509e3348fbc4efa3a1413d393cb650"}
   { :message "abcdefghijklmnopqrstuvwxyz"
    :expected "71c480df93d6ae2f1efad1447c66c9525e316218cf51fc8d9ed832f2daf18b73"}
   { :message "abcdbcdecdefdefgefghfghighijhijkijkljklmklmnlmnomnopnopq"
    :expected "248d6a61d20638b8e5c026930c3e6039a33ce45964ff2167f6ecedd419db06c1"}
   { :message "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789"
    :expected "db4bfcbd4da0cd85a60c3c37d3fbd8805c77f15fc6b1fdfe614ee0a7c8fdb4c0"}])

(def sha256-fail-vectors
  #^{:doc "test vectors that should fail."}
  [{:message "fail1" :expected "not a hash"}
   {:message "FOO"
    :expected "2c26b46b68ffc68ff99b453c1d30413413422d706483bfa0f98a5e886266e7ae"}])

(deftest sha256-test
  "Test the SHA256 implementation against the test vectors."
  (doseq [test-vec sha256-test-vectors]
    (is (compare-sha256 (:message test-vec) (:expected test-vec)))))

(deftest sha2560-fail
  "Ensure SHA256 fails when the hashes don't match."
  (doseq [test-vec sha256-fail-vectors]
    (is (not (compare-sha256 (:message test-vec) {:expected test-vec})))))

sha256.clj

;; see also test suite
(ns user.hash
  (:use [clojure.contrib.str-utils])
  (:import [java.security MessageDigest]))

(def ^{:dynamic true} *default-hash* "SHA-256")

;; take from http://ideone.com/MoNJ14Z8 which was, in turn, gathered from
;; Tom Lee's post "Clojure and MessageDigest"
;;     (http://tomlee.co/2009/06/clojure-and-messagedigest/)
(defn hexdigest
  "Returns the hex digest of an object. Expects a string as input."
  ([input] (hexdigest input *default-hash*))
  ([input hash-algo]
     (if (string? input)
       (let [hash (MessageDigest/getInstance hash-algo)]
         (. hash update (.getBytes input))
         (let [digest (.digest hash)]
           (apply str (map #(format "%02x" (bit-and % 0xff)) digest))))
       (do
         (println "Invalid input! Expected string, got" (type input))
         nil))))

(defn compare-sha256 [obj ref-hash]
  "Compare an object to a hash; true if (= (hash obj) ref-hash)."
  (= ref-hash (hexdigest obj "SHA-256")))

(def sha256 (fn [input] (hexdigest input "SHA-256")))

What is (bit-and % 0xff) here for?