Get salt and rounds

src/main/java/org/mindrot/jbcrypt/BCrypt.java

@@ -770,6 +770,61 @@ public static boolean checkpw(String plaintext, String hashed) {
         return equalsNoEarlyReturn(hashed, hashpw(plaintext, hashed));
     }
 
+    /**
+     * Retrieve the salt from a hashed password
+     * @param hashed the previously-hashed password
+     * @return the salt the password has been hashed with
+     */
+    public static String getsalt(String hashed) {
+        int minor, off;
+        if (hashed.charAt(0) != '$' || hashed.charAt(1) != '2') {
+            throw new IllegalArgumentException("Invalid salt version");
+        }
+        if (hashed.charAt(2) == '$') {
+            off = 3;
+        } else {
+            minor = hashed.charAt(2);
+            if (minor != 'a' || hashed.charAt(3) != '$') {
+                throw new IllegalArgumentException("Invalid salt revision");
+            }
+            off = 4;
+        }
+        if (hashed.charAt(off + 2) != '$') {
+            throw new IllegalArgumentException("Missing salt rounds");
+        }
+        if (hashed.length() - off < 25) {
+            throw new IllegalArgumentException("Invalid salt");
+        }
+        return hashed.substring(0, off + 25);
+    }
+
+    /**
+     * Retrieve the log2 of the number of rounds of hashing applied
+     * @param saltOrHashed the salt generated by BCrypt.gensalt() or
+     *                     the previously-hashed password
+     * @return the log2 of the number of rounds of hashing applied.
+     */
+    public static int getlogrounds(String saltOrHashed) {
+        int log_rounds, minor, off;
+        if (saltOrHashed.charAt(0) != '$' || saltOrHashed.charAt(1) != '2') {
+            throw new IllegalArgumentException("Invalid saltOrHashed version");
+        }
+        if (saltOrHashed.charAt(2) == '$') {
+            off = 3;
+        } else {
+            minor = saltOrHashed.charAt(2);
+            if (minor != 'a' || saltOrHashed.charAt(3) != '$') {
+                throw new IllegalArgumentException("Invalid saltOrHashed revision");
+            }
+            off = 4;
+        }
+        if (saltOrHashed.charAt(off + 2) != '$') {
+            throw new IllegalArgumentException("Missing saltOrHashed rounds");
+        }
+        log_rounds = Integer.parseInt(saltOrHashed.substring(off, off + 2));
+        return log_rounds;
+    }
+
     static boolean equalsNoEarlyReturn(String a, String b) {
         char[] caa = a.toCharArray();
         char[] cab = b.toCharArray();

src/test/java/org/mindrot/jbcrypt/BCryptTest.java

@@ -341,4 +341,31 @@ public void equalsOnStringsIsCorrect()
 
         assertFalse(BCrypt.equalsNoEarlyReturn("test", "pass"));
     }
+
+    @Test
+    public void testGetSalt()
+    {
+        String password = "password";
+        String salt = BCrypt.gensalt();
+        String hashedPassword = BCrypt.hashpw(password, salt);
+        assertEquals(salt, BCrypt.getsalt(hashedPassword));
+    }
+
+    @Test
+    public void testGetLogRoundsFromSalt()
+    {
+        int logRounds = 11;
+        String salt = BCrypt.gensalt(logRounds);
+        assertEquals(logRounds, BCrypt.getlogrounds(salt));
+    }
+
+    @Test
+    public void testGetLogRoundsFromHashed()
+    {
+        String password = "password";
+        int logRounds = 11;
+        String salt = BCrypt.gensalt(logRounds);
+        String hashed = BCrypt.hashpw(password, salt);
+        assertEquals(logRounds, BCrypt.getlogrounds(hashed));
+    }
 }