diff --git a/meerkat-common/src/main/java/meerkat/crypto/mixnet/Mix2ZeroKnowledgeProver.java b/meerkat-common/src/main/java/meerkat/crypto/mixnet/Mix2ZeroKnowledgeProver.java
new file mode 100644
index 0000000..1113bfd
--- /dev/null
+++ b/meerkat-common/src/main/java/meerkat/crypto/mixnet/Mix2ZeroKnowledgeProver.java
@@ -0,0 +1,18 @@
+package meerkat.crypto.mixnet;
+
+import meerkat.protobuf.Crypto;
+import meerkat.protobuf.Mixing;
+
+/**
+ * Prove in zero knowledge that two ciphertexts are a mix of two original ciphertexts.
+ */
+public interface Mix2ZeroKnowledgeProver {
+    public Mixing.ZeroKnowledgeProof prove(Crypto.RerandomizableEncryptedMessage in1,
+                                           Crypto.RerandomizableEncryptedMessage in2,
+                                           Crypto.RerandomizableEncryptedMessage out1,
+                                           Crypto.RerandomizableEncryptedMessage out2,
+                                           boolean switched,
+                                           Crypto.EncryptionRandomness r1,
+                                           Crypto.EncryptionRandomness r2);
+
+}
diff --git a/meerkat-common/src/main/java/meerkat/crypto/mixnet/Mix2ZeroKnowledgeVerifier.java b/meerkat-common/src/main/java/meerkat/crypto/mixnet/Mix2ZeroKnowledgeVerifier.java
new file mode 100644
index 0000000..dd3c251
--- /dev/null
+++ b/meerkat-common/src/main/java/meerkat/crypto/mixnet/Mix2ZeroKnowledgeVerifier.java
@@ -0,0 +1,23 @@
+package meerkat.crypto.mixnet;
+
+import meerkat.protobuf.Crypto;
+import meerkat.protobuf.Mixing;
+
+/**
+ * Verify the two-ciphertext mix proof
+ */
+public interface Mix2ZeroKnowledgeVerifier {
+    /**
+     * Return true iff the proof is valid.
+     * @param in1
+     * @param in2
+     * @param out1
+     * @param out2
+     * @return
+     */
+    boolean verify(Crypto.RerandomizableEncryptedMessage in1,
+                   Crypto.RerandomizableEncryptedMessage in2,
+                   Crypto.RerandomizableEncryptedMessage out1,
+                   Crypto.RerandomizableEncryptedMessage out2,
+                   Mixing.ZeroKnowledgeProof proof);
+}
diff --git a/meerkat-common/src/main/proto/meerkat/mixing.proto b/meerkat-common/src/main/proto/meerkat/mixing.proto
new file mode 100644
index 0000000..50cffc7
--- /dev/null
+++ b/meerkat-common/src/main/proto/meerkat/mixing.proto
@@ -0,0 +1,12 @@
+syntax = "proto3";
+
+package meerkat;
+
+option java_package = "meerkat.protobuf";
+
+import 'meerkat/crypto.proto';
+
+// TODO:
+message ZeroKnowledgeProof {
+    bytes data = 1;
+}
\ No newline at end of file