Extract out contruction of Web Key Directory URLs

Moves `toWebKeyDirectoryURL` to a separate class adding unit tests
for URL correctness as well as support for spaces at the beginning
and end of the e-mail. Spaces are frequently automatically inserted
by soft keyboards.

OpenKeychain/src/main/java/org/sufficientlysecure/keychain/util/WebKeyDirectoryUtil.java

@@ -0,0 +1,52 @@
+package org.sufficientlysecure.keychain.util;
+
+import android.support.annotation.Nullable;
+
+import java.net.MalformedURLException;
+import java.net.URL;
+import java.security.MessageDigest;
+import java.security.NoSuchAlgorithmException;
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
+
+public class WebKeyDirectoryUtil {
+
+    private static final Pattern EMAIL_PATTERN = Pattern.compile("^\\s*([^\\s]+)@([^\\s]+)\\s*$");
+
+    private WebKeyDirectoryUtil() {
+    }
+
+    /**
+     * Tries to construct a Web Key Directory from a given name.
+     * Returns {@code null} if unsuccessful.
+     *
+     * @see <a href="https://tools.ietf.org/html/draft-koch-openpgp-webkey-service-05#section-3.1">Key Discovery</a>
+     */
+    @Nullable
+    public static URL toWebKeyDirectoryURL(String name) {
+        Matcher matcher = EMAIL_PATTERN.matcher(name);
+
+        if (!matcher.matches()) {
+            return null;
+        }
+
+        String localPart = matcher.group(1);
+        String encodedPart = ZBase32.encode(toSHA1(localPart.toLowerCase().getBytes()));
+        String domain = matcher.group(2);
+
+        try {
+            return new URL("https://" + domain + "/.well-known/openpgpkey/hu/" + encodedPart);
+        } catch (MalformedURLException e) {
+            return null;
+        }
+    }
+
+    private static byte[] toSHA1(byte[] input) {
+        try {
+            return MessageDigest.getInstance("SHA-1").digest(input);
+        } catch (NoSuchAlgorithmException e) {
+            throw new AssertionError("SHA-1 should always be available");
+        }
+    }
+
+}