DarkCrystal implementation which calls SecretSharingWrapper

briar-android/src/main/java/org/briarproject/briar/android/socialbackup/DarkCrystalImpl.java

@@ -1,4 +1,46 @@
 package org.briarproject.briar.android.socialbackup;
+import org.briarproject.bramble.api.crypto.SecretKey;
+import org.briarproject.briar.api.socialbackup.Shard;
+import org.briarproject.briar.socialbackup.DarkCrystal;
+import org.magmacollective.darkcrystal.secretsharingwrapper.SecretSharingWrapper;
 
-public class DarkCrystalImpl {
+import java.security.GeneralSecurityException;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+import java.util.Random;
+
+import static org.briarproject.briar.socialbackup.SocialBackupConstants.SECRET_ID_BYTES;
+
+public class DarkCrystalImpl implements DarkCrystal {
+
+	@Override
+	public List<Shard> createShards(SecretKey secret, int numShards,
+			int threshold) {
+		Random random = new Random();
+		byte[] secretId = new byte[SECRET_ID_BYTES];
+		random.nextBytes(secretId);
+		List<byte[]> shardsBytes = SecretSharingWrapper.share(secret.getBytes(), numShards, threshold);
+		List<Shard> shards = new ArrayList<>(numShards);
+		for (byte[] shardBytes : shardsBytes) {
+			shards.add(new Shard(secretId, shardBytes));
+		}
+		return shards;
+	}
+
+	@Override
+	public SecretKey combineShards(List<Shard> shards) throws
+			GeneralSecurityException {
+		// Check each shard has the same secret Id
+		byte[] secretId = shards.get(0).getSecretId();
+		for (Shard shard : shards) {
+			if (!Arrays.equals(shard.getSecretId(), secretId)) throw new GeneralSecurityException();
+		}
+        List<byte[]> shardsBytes = new ArrayList<>(shards.size());
+		for (Shard shard : shards) {
+			shardsBytes.add(shard.getShard());
+		}
+		byte[] secretBytes = SecretSharingWrapper.combine(shardsBytes);
+		return new SecretKey(secretBytes);
+	}
 }