Add ability to search for keys using WKD protocol

If a search pattern that looks like an email address is found
an additional query using Web Key Directory will be performed.

Implements basic flow described in "Key Discovery" [0] I-D.
Querying SRV records is not supported.

Fixes partially #2270.

[0]: https://tools.ietf.org/html/draft-koch-openpgp-webkey-service-05#section-3.1

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

@@ -348,6 +348,7 @@ public class Preferences {
         return new CloudSearchPrefs(mSharedPreferences.getBoolean(Pref.SEARCH_KEYSERVER, true),
                 mSharedPreferences.getBoolean(Pref.SEARCH_KEYBASE, true),
                 false,
+                true,
                 getPreferredKeyserver());
     }
 
@@ -365,6 +366,7 @@ public class Preferences {
         public final boolean searchKeyserver;
         public final boolean searchKeybase;
         public final boolean searchFacebook;
+        public final boolean searchWebKeyDirectory;
         public final HkpKeyserverAddress keyserver;
 
         /**
@@ -373,10 +375,12 @@ public class Preferences {
          * @param keyserver       the keyserver url authority to search on
          */
         public CloudSearchPrefs(boolean searchKeyserver, boolean searchKeybase,
-                                boolean searchFacebook, HkpKeyserverAddress keyserver) {
+                                boolean searchFacebook, boolean searchWebKeyDirectory,
+                                HkpKeyserverAddress keyserver) {
             this.searchKeyserver = searchKeyserver;
             this.searchKeybase = searchKeybase;
             this.searchFacebook = searchFacebook;
+            this.searchWebKeyDirectory = searchWebKeyDirectory;
             this.keyserver = keyserver;
         }
 
@@ -384,6 +388,7 @@ public class Preferences {
             searchKeyserver = in.readByte() != 0x00;
             searchKeybase = in.readByte() != 0x00;
             searchFacebook = in.readByte() != 0x00;
+            searchWebKeyDirectory = in.readByte() != 0x00;
             keyserver = in.readParcelable(HkpKeyserverAddress.class.getClassLoader());
         }
 
@@ -397,6 +402,7 @@ public class Preferences {
             dest.writeByte((byte) (searchKeyserver ? 0x01 : 0x00));
             dest.writeByte((byte) (searchKeybase ? 0x01 : 0x00));
             dest.writeByte((byte) (searchFacebook ? 0x01 : 0x00));
+            dest.writeByte((byte) (searchWebKeyDirectory ? 0x01 : 0x00));
             dest.writeParcelable(keyserver, flags);
         }
 

OpenKeychain/src/main/java/org/sufficientlysecure/keychain/util/ZBase32.java

@@ -0,0 +1,45 @@
+package org.sufficientlysecure.keychain.util;
+
+/**
+ * Utilities for handling ZBase-32 encoding.
+ *
+ * @see <a href="https://tools.ietf.org/html/rfc6189#section-5.1.6">Z-Base32 encoding as used in RFC 6189</a>
+ */
+public final class ZBase32 {
+
+    private static final char[] ALPHABET = "ybndrfg8ejkmcpqxot1uwisza345h769".toCharArray();
+    private static final int SHIFT = Integer.numberOfTrailingZeros(ALPHABET.length);
+    private static final int MASK = ALPHABET.length - 1;
+
+    public static String encode(byte[] data) {
+        if (data.length == 0) {
+            return "";
+        }
+
+        StringBuilder result = new StringBuilder();
+
+        int buffer = data[0];
+        int index = 1;
+        int bitsLeft = 8;
+        while (bitsLeft > 0 || index < data.length) {
+            if (bitsLeft < SHIFT) {
+                if (index < data.length) {
+                    buffer <<= 8;
+                    buffer |= (data[index++] & 0xff);
+                    bitsLeft += 8;
+                } else {
+                    int pad = SHIFT - bitsLeft;
+                    buffer <<= pad;
+                    bitsLeft += pad;
+                }
+            }
+            bitsLeft -= SHIFT;
+            result.append(ALPHABET[MASK & (buffer >> bitsLeft)]);
+        }
+        return result.toString();
+    }
+
+    private ZBase32() {
+    }
+
+}