see #13 add optional paramter that specifies algorithm to verify function

src/clj_jwt/core.clj

@@ -49,7 +49,7 @@
   "Protocol for JonWebSignature"
   (set-alg [this alg] "Set algorithm name to JWS Header Parameter")
   (sign    [this key] [this alg key] "Set signature to this token")
-  (verify  [this] [this key] "Verify this token"))
+  (verify  [this] [this key] [this algorithm key] "Verify this token"))
 
 (extend-protocol JsonWebSignature
   JWT
@@ -76,7 +76,11 @@
                data    (str (encoded-header this) "." (encoded-claims this))]
            (verify-fn key data (:signature this)))
 
-         :else (throw (Exception. "Unkown signature")))))))
+         :else (throw (Exception. "Unkown signature")))))
+    ([this algorithm key]
+     (if (= algorithm (-> this :header :alg keyword))
+       (verify this key)
+       false))))
 
 ; =jwt
 (defn jwt [claim] (init (->JWT "" "" "") claim))

test/clj_jwt/attack_test.clj

@@ -0,0 +1,40 @@
+(ns clj-jwt.attack-test
+  (:require
+    [clj-jwt.core   :refer :all]
+    [clj-jwt.sign   :refer :all]
+    [clj-jwt.key    :refer [private-key public-key]]
+    [clojure.string :as str]
+    [clojure.test   :refer :all]))
+
+(def pub-key-path "test/files/rsa/no_pass.pub.key")
+(def rsa-prv-key  (private-key "test/files/rsa/no_pass.key"))
+(def rsa-pub-key  (public-key  pub-key-path))
+
+(deftest test-algorithm->none-attack
+  (let [key "secret"
+        original (-> {:foo "bar"} jwt (sign :HS256 key))
+        attacked (update-in original [:header :alg] (constantly "none"))]
+    (testing "attack"
+      (is (verify original key))
+      (is (not (verify attacked key))))
+
+    (testing "defense"
+      (is (verify original :HS256 key))
+      (is (not (verify original :RS256 key)))
+      (is (not (verify attacked :HS256 key))))))
+
+(deftest test-rsa->hmac-attack
+  (let [base      (jwt {:foo "bar"})
+        original  (sign base :RS256 rsa-prv-key)
+        hmac-sign (-> base (sign :HS256 (str/trim (slurp pub-key-path))) :signature)
+        attacked  (-> original
+                      (update-in [:header :alg] (constantly "HS256"))
+                      (update-in [:signature] (constantly hmac-sign)))]
+    (testing "attack"
+      (is (verify original rsa-pub-key))
+      (is (thrown? Exception (verify attacked rsa-pub-key))))
+
+    (testing "defense"
+      (is (verify original :RS256 rsa-pub-key))
+      (is (not (verify original :HS256 rsa-pub-key)))
+      (is (not (verify attacked :RS256 rsa-pub-key))))))