diff --git a/implementations/encrypted/EncryptedBlock.h b/implementations/encrypted/EncryptedBlock.h
index b5414217..cb0e10a4 100644
--- a/implementations/encrypted/EncryptedBlock.h
+++ b/implementations/encrypted/EncryptedBlock.h
@@ -9,6 +9,7 @@
 #include "messmer/cpp-utils/macros.h"
 #include <memory>
 #include <boost/optional.hpp>
+#include "ciphers/Cipher.h"
 
 namespace blockstore {
 namespace encrypted {
@@ -19,6 +20,7 @@ template<class Cipher> class EncryptedBlockStore;
 template<class Cipher>
 class EncryptedBlock: public Block {
 public:
+  BOOST_CONCEPT_ASSERT((CipherConcept<Cipher>));
   static std::unique_ptr<EncryptedBlock> TryCreateNew(BlockStore *baseBlockStore, const Key &key, Data data, const typename Cipher::EncryptionKey &encKey);
   static std::unique_ptr<EncryptedBlock> TryDecrypt(std::unique_ptr<Block> baseBlock, const typename Cipher::EncryptionKey &key);
 
diff --git a/implementations/encrypted/ciphers/AES256_CFB.h b/implementations/encrypted/ciphers/AES256_CFB.h
index 5a817f35..154106ce 100644
--- a/implementations/encrypted/ciphers/AES256_CFB.h
+++ b/implementations/encrypted/ciphers/AES256_CFB.h
@@ -6,6 +6,7 @@
 #include "../../../utils/Data.h"
 #include <cryptopp/cryptopp/aes.h>
 #include <boost/optional.hpp>
+#include "Cipher.h"
 
 namespace blockstore {
 namespace encrypted {
@@ -13,11 +14,11 @@ namespace encrypted {
 //TODO Add contract/interface for ciphers
 class AES256_CFB {
 public:
+  BOOST_CONCEPT_ASSERT((CipherConcept<AES256_CFB>));
+
   using EncryptionKey = FixedSizeData<32>;
   static_assert(32 == CryptoPP::AES::MAX_KEYLENGTH, "If AES offered larger keys, we should offer a variant with it");
 
-  AES256_CFB(const EncryptionKey &key);
-
   static constexpr unsigned int ciphertextSize(unsigned int plaintextBlockSize) {
     return plaintextBlockSize + IV_SIZE;
   }
diff --git a/implementations/encrypted/ciphers/Cipher.h b/implementations/encrypted/ciphers/Cipher.h
new file mode 100644
index 00000000..08936427
--- /dev/null
+++ b/implementations/encrypted/ciphers/Cipher.h
@@ -0,0 +1,32 @@
+#pragma once
+#ifndef MESSMER_BLOCKSTORE_IMPLEMENTATIONS_ENCRYPTED_CIPHERS_CIPHER_H_
+#define MESSMER_BLOCKSTORE_IMPLEMENTATIONS_ENCRYPTED_CIPHERS_CIPHER_H_
+
+#include <boost/concept_check.hpp>
+#include <cstdint>
+
+namespace blockstore {
+namespace encrypted {
+
+template<class X>
+struct CipherConcept {
+public:
+  BOOST_CONCEPT_USAGE(CipherConcept) {
+    same_type(UINT32_C(0), X::ciphertextSize(UINT32_C(5)));
+    same_type(UINT32_C(0), X::plaintextSize(UINT32_C(5)));
+    typename X::EncryptionKey key = X::EncryptionKey::CreateRandom();
+    same_type(Data(0), X::encrypt((byte*)nullptr, UINT32_C(0), key));
+    same_type(boost::optional<Data>(Data(0)), X::decrypt((byte*)nullptr, UINT32_C(0), key));
+  }
+
+private:
+  // Type deduction will fail unless the arguments have the same type.
+  template <typename T> void same_type(T const&, T const&);
+};
+
+}
+}
+
+
+
+#endif
diff --git a/test/implementations/encrypted/CipherTest.cpp b/test/implementations/encrypted/CipherTest.cpp
index f1bccee3..a4385998 100644
--- a/test/implementations/encrypted/CipherTest.cpp
+++ b/test/implementations/encrypted/CipherTest.cpp
@@ -1,5 +1,6 @@
 #include <google/gtest/gtest.h>
 #include "../../../implementations/encrypted/ciphers/AES256_CFB.h"
+#include "../../../implementations/encrypted/ciphers/Cipher.h"
 
 #include "../../testutils/DataBlockFixture.h"
 #include "../../../utils/Data.h"
@@ -10,6 +11,7 @@ using blockstore::Data;
 template<class Cipher>
 class CipherTest: public ::testing::Test {
 public:
+  BOOST_CONCEPT_ASSERT((CipherConcept<Cipher>));
   typename Cipher::EncryptionKey encKey = createRandomKey();
 
   static typename Cipher::EncryptionKey createRandomKey(int seed = 0) {
@@ -82,7 +84,7 @@ TYPED_TEST_P(CipherTest, Size_1048576) {
   EXPECT_EQ(1048576, TypeParam::plaintextSize(TypeParam::ciphertextSize(1048576)));
 }
 
-constexpr std::initializer_list<unsigned int> SIZES = {0, 1, 100, 1024, 5000, 1048576, 52428800};
+constexpr std::initializer_list<unsigned int> SIZES = {0, 1, 100, 1024, 5000, 1048576, 20971520};
 
 TYPED_TEST_P(CipherTest, EncryptThenDecrypt_Zeroes) {
   for (auto size: SIZES) {