diff --git a/briar-core/src/main/java/org/briarproject/briar/socialbackup/DarkCrystal.java b/briar-core/src/main/java/org/briarproject/briar/socialbackup/DarkCrystal.java
index 9bac55e28..da991acbe 100644
--- a/briar-core/src/main/java/org/briarproject/briar/socialbackup/DarkCrystal.java
+++ b/briar-core/src/main/java/org/briarproject/briar/socialbackup/DarkCrystal.java
@@ -4,10 +4,12 @@ import org.briarproject.bramble.api.crypto.SecretKey;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 import org.briarproject.briar.api.socialbackup.Shard;
 
+import java.security.GeneralSecurityException;
 import java.util.List;
 
 @NotNullByDefault
 interface DarkCrystal {
 
 	List<Shard> createShards(SecretKey secret, int shards, int threshold);
+	SecretKey combineShards(List<Shard> shards) throws GeneralSecurityException;
 }
diff --git a/briar-core/src/main/java/org/briarproject/briar/socialbackup/DarkCrystalStub.java b/briar-core/src/main/java/org/briarproject/briar/socialbackup/DarkCrystalStub.java
index fbb90ce3e..6671b14ff 100644
--- a/briar-core/src/main/java/org/briarproject/briar/socialbackup/DarkCrystalStub.java
+++ b/briar-core/src/main/java/org/briarproject/briar/socialbackup/DarkCrystalStub.java
@@ -4,7 +4,9 @@ import org.briarproject.bramble.api.crypto.SecretKey;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 import org.briarproject.briar.api.socialbackup.Shard;
 
+import java.security.GeneralSecurityException;
 import java.util.ArrayList;
+import java.util.Arrays;
 import java.util.List;
 import java.util.Random;
 
@@ -33,4 +35,19 @@ class DarkCrystalStub implements DarkCrystal {
 		}
 		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();
+		}
+
+		Random random = new Random();
+		byte[] secretBytes = new byte[SecretKey.LENGTH];
+		random.nextBytes(secretId);
+		return new SecretKey(secretBytes);
+	}
 }