Support for ephemeral provisioning communication channels.

// FREEBIE

src/main/java/org/whispersystems/textsecuregcm/websocket/ProvisioningAddress.java

@@ -0,0 +1,38 @@
+package org.whispersystems.textsecuregcm.websocket;
+
+import org.whispersystems.textsecuregcm.util.Base64;
+
+import java.security.NoSuchAlgorithmException;
+import java.security.SecureRandom;
+
+public class ProvisioningAddress extends WebsocketAddress {
+
+  private static final String PREFIX = ">>ephemeral-";
+
+  private final String address;
+
+  public ProvisioningAddress(String address) throws InvalidWebsocketAddressException {
+    super(address, 0);
+    this.address = address;
+
+    if (address == null || !address.startsWith(PREFIX)) {
+      throw new InvalidWebsocketAddressException(address);
+    }
+  }
+
+  public String getAddress() {
+    return address;
+  }
+
+  public static ProvisioningAddress generate() {
+    try {
+      byte[] random = new byte[16];
+      SecureRandom.getInstance("SHA1PRNG").nextBytes(random);
+
+      return new ProvisioningAddress(PREFIX + Base64.encodeBytesWithoutPadding(random)
+                                                    .replace('+', '-').replace('/', '_'));
+    } catch (NoSuchAlgorithmException | InvalidWebsocketAddressException e) {
+      throw new AssertionError(e);
+    }
+  }
+}