Support for Signal calls.

Merge in RedPhone

// FREEBIE

src/org/thoughtcrime/redphone/crypto/Otp.java

@@ -0,0 +1,49 @@
+/*
+ * Copyright (C) 2011 Whisper Systems
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+package org.thoughtcrime.redphone.crypto;
+
+import org.thoughtcrime.redphone.util.Base64;
+
+import java.security.InvalidKeyException;
+import java.security.NoSuchAlgorithmException;
+
+import javax.crypto.Mac;
+import javax.crypto.spec.SecretKeySpec;
+
+/**
+ * A utility class for calcuating a HOTP token from a user's
+ * password and the current counter value.
+ *
+ * @author Moxie Marlinspike
+ *
+ */
+public class Otp {
+  public static String calculateOtp(String password, long counter) {
+    try {
+      SecretKeySpec key = new SecretKeySpec(password.getBytes(), "HmacSHA1");
+      Mac mac           = Mac.getInstance("HmacSHA1");
+      mac.init(key);
+
+      return Base64.encodeBytes(mac.doFinal((counter+"").getBytes()));
+    } catch (NoSuchAlgorithmException nsae) {
+      throw new AssertionError(nsae);
+    } catch (InvalidKeyException e) {
+      throw new AssertionError(e);
+    }
+  }
+}