Initial implementation of social backup client.

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

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+package org.briarproject.briar.api.socialbackup;
+
+import org.briarproject.bramble.api.db.DbException;
+
+/**
+ * Thrown when an attempt is made to create a social account backup but a
+ * backup already exists. This exception may occur due to concurrent updates
+ * and does not indicate a database error.
+ */
+public class BackupExistsException extends DbException {
+}

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

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+package org.briarproject.briar.api.socialbackup;
+
+import org.briarproject.bramble.api.crypto.SecretKey;
+import org.briarproject.bramble.api.identity.Author;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import java.util.List;
+
+import javax.annotation.concurrent.Immutable;
+
+@Immutable
+@NotNullByDefault
+public class BackupMetadata {
+
+	private final SecretKey secret;
+	private final List<Author> custodians;
+	private final int threshold, version;
+
+	public BackupMetadata(SecretKey secret, List<Author> custodians,
+			int threshold, int version) {
+		this.secret = secret;
+		this.custodians = custodians;
+		this.threshold = threshold;
+		this.version = version;
+	}
+
+	public SecretKey getSecret() {
+		return secret;
+	}
+
+	public List<Author> getCustodians() {
+		return custodians;
+	}
+
+	public int getThreshold() {
+		return threshold;
+	}
+
+	public int getVersion() {
+		return version;
+	}
+}

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

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+package org.briarproject.briar.api.socialbackup;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import javax.annotation.concurrent.Immutable;
+
+@Immutable
+@NotNullByDefault
+public class Shard {
+
+	private final byte[] secretId, shard;
+	private final int numShards, threshold;
+
+	public Shard(byte[] secretId, int numShards, int threshold,
+			byte[] shard) {
+		this.secretId = secretId;
+		this.numShards = numShards;
+		this.threshold = threshold;
+		this.shard = shard;
+	}
+
+	public byte[] getSecretId() {
+		return secretId;
+	}
+
+	public int getNumShards() {
+		return numShards;
+	}
+
+	public int getThreshold() {
+		return threshold;
+	}
+
+	public byte[] getShard() {
+		return shard;
+	}
+}

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

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+package org.briarproject.briar.api.socialbackup;
+
+import org.briarproject.bramble.api.contact.ContactId;
+import org.briarproject.bramble.api.db.DbException;
+import org.briarproject.bramble.api.db.Transaction;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.sync.ClientId;
+
+import java.util.List;
+
+import javax.annotation.Nullable;
+
+@NotNullByDefault
+public interface SocialBackupManager {
+
+	/**
+	 * The unique ID of the social backup client.
+	 */
+	ClientId CLIENT_ID = new ClientId("pw.darkcrystal.backup");
+
+	/**
+	 * The current major version of the social backup client.
+	 */
+	int MAJOR_VERSION = 0;
+
+	/**
+	 * The current minor version of the social backup client.
+	 */
+	int MINOR_VERSION = 0;
+
+	/**
+	 * Returns the metadata for this device's backup, or null if no backup has
+	 * been created.
+	 */
+	@Nullable
+	BackupMetadata getBackupMetadata(Transaction txn) throws DbException;
+
+	/**
+	 * Creates a backup for this device using the given custodians and
+	 * threshold. The encrypted backup and a shard of the backup key will be
+	 * sent to each custodian.
+	 *
+	 * @throws BackupExistsException If a backup already exists
+	 */
+	void createBackup(Transaction txn, List<ContactId> custodianIds,
+			int threshold) throws DbException;
+}