backup payload and returned shard payload

briar-api/src/main/java/org/briarproject/briar/api/socialbackup/BackupPayload.java

@@ -1,4 +1,4 @@
-package org.briarproject.briar.socialbackup;
+package org.briarproject.briar.api.socialbackup;
 
 import org.briarproject.bramble.api.Bytes;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
@@ -7,9 +7,9 @@ import javax.annotation.concurrent.Immutable;
 
 @Immutable
 @NotNullByDefault
-class BackupPayload extends Bytes {
+public class BackupPayload extends Bytes {
 
-	BackupPayload(byte[] payload) {
+	public BackupPayload(byte[] payload) {
 		super(payload);
 	}
 }

briar-api/src/main/java/org/briarproject/briar/api/socialbackup/ReturnShardPayload.java

@@ -1,4 +1,25 @@
 package org.briarproject.briar.api.socialbackup;
 
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import javax.annotation.concurrent.Immutable;
+
+@Immutable
+@NotNullByDefault
 public class ReturnShardPayload {
+	private final Shard shard;
+	private final BackupPayload backupPayload;
+
+	public ReturnShardPayload(Shard shard, BackupPayload backupPayload) {
+		this.shard = shard;
+		this.backupPayload = backupPayload;
+	}
+
+	public Shard getShard() {
+		return shard;
+	}
+
+	public BackupPayload getBackupPayload() {
+		return backupPayload;
+	}
 }

briar-api/src/main/java/org/briarproject/briar/api/socialbackup/SocialBackupExchangeManager.java

@@ -1,4 +1,37 @@
 package org.briarproject.briar.api.socialbackup;
 
+import org.briarproject.bramble.api.crypto.SecretKey;
+import org.briarproject.bramble.api.db.DbException;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.plugin.duplex.DuplexTransportConnection;
+
+import java.io.IOException;
+
+@NotNullByDefault
 public interface SocialBackupExchangeManager {
+
+	/**
+	 * Return a shard and encrypted backup to a secret-owner's new device
+	 * @param conn
+	 * @param masterKey
+	 * @param verified
+	 * @throws IOException
+	 * @throws DbException
+	 */
+	public void sendReturnShard(DuplexTransportConnection conn,
+			SecretKey masterKey,
+			boolean verified) throws IOException, DbException;
+
+	/**
+	 * Receive a returned shard and encrypted backup from a custodian
+	 * @param conn
+	 * @param masterKey
+	 * @param verified
+	 * @return the shard and encrypted backup
+	 * @throws IOException
+	 * @throws DbException
+	 */
+	public ReturnShardPayload receiveReturnShard(DuplexTransportConnection conn,
+			SecretKey masterKey, boolean verified)
+			throws IOException, DbException;
 }