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Microsoft SEAL is an easy-to-use and powerful homomorphic encryption library.
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Microsoft SEAL

Microsoft SEAL is an easy-to-use open-source (MIT licensed) homomorphic encryption library developed by the Cryptography Research group at Microsoft. Microsoft SEAL is written in modern standard C++ and has no external dependencies, making it easy to compile and run in many different environments. For more information about the Microsoft SEAL project, see sealcrypto.org.

This document pertains to Microsoft SEAL version 3.3. Users of previous versions of the library should look at the list of changes.

Contents

Introduction

Core Concepts

Most encryption schemes consist of three functionalities: key generation, encryption, and decryption. Symmetric-key encryption schemes use the same secret key for both encryption and decryption; public-key encryption schemes use separately a public key for encryption and a secret key for decryption. Therefore, public-key encryption schemes allow anyone who knows the public key to encrypt data, but only those who know the secret key can decrypt and read the data. Symmetric-key encryption can be used for efficiently encrypting very large amounts of data, and enables secure outsourced cloud storage. Public-key encryption is a fundamental concept that enables secure online communication today, but is typically much less efficient than symmetric-key encryption.

While traditional symmetric- and public-key encryption can be used for secure storage and communication, any outsourced computation will necessarily require such encryption layers to be removed before computation can take place. Therefore, cloud services providing outsourced computation capabilities must have access to the secret keys, and implement access policies to prevent unauthorized employees from getting access to these keys.

Homomorphic Encryption

Homomorphic encryption refers to encryption schemes that allow the cloud to compute directly on the encrypted data, without requiring the data to be decrypted first. The results of such encrypted computations remain encrypted, and can be only decrypted with the secret key (by the data owner). Multiple homomorphic encryption schemes with different capabilities and trade-offs have been invented over the past decade; most of these are public-key encryption schemes, although the public-key functionality may not always be needed.

Homomorphic encryption is not a generic technology: only some computations on encrypted data are possible. It also comes with a substantial performance overhead, so computations that are already very costly to perform on unencrypted data are likely to be infeasible on encrypted data. Moreover, data encrypted with homomorphic encryption is many times larger than unencrypted data, so it may not make sense to encrypt, e.g., entire large databases, with this technology. Instead, meaningful use-cases are in scenarios where strict privacy requirements prohibit unencrypted cloud computation altogether, but the computations themselves are fairly lightweight.

Typically, homomorphic encryption schemes have a single secret key which is held by the data owner. For scenarios where multiple different private data owners wish to engage in collaborative computation, homomorphic encryption is probably not a reasonable solution.

Homomorphic encryption cannot be used to enable data scientist to circumvent GDPR. For example, there is no way for a cloud service to use homomorphic encryption to draw insights from encrypted customer data. Instead, results of encrypted computations remain encrypted and can only be decrypted by the owner of the data, e.g., a cloud service customer.

Microsoft SEAL

Microsoft SEAL is a homomorphic encryption library that allows additions and multiplications to be performed on encrypted integers or real numbers. Other operations, such as encrypted comparison, sorting, or regular expressions, are in most cases not feasible to evaluate on encrypted data using this technology. Therefore, only specific privacy-critical cloud computation parts of programs should be implemented with Microsoft SEAL.

It is not always easy or straightfoward to translate an unencrypted computation into a computation on encrypted data, for example, it is not possible to branch on encrypted data. Microsoft SEAL itself has a steep learning curve and requires the user to understand many homomorphic encryption specific concepts, even though in the end the API is not too complicated. Even if a user is able to program and run a specific computation using Microsoft SEAL, the difference between efficient and inefficient implementations can be several orders of magnitude, and it can be hard for new users to know how to improve the performance of their computation.

Microsoft SEAL comes with two different homomorphic encryption schemes with very different properties. The BFV scheme allows modular arithmetic to be performed on encrypted integers. The CKKS scheme allows additions and multiplications on encrypted real or complex numbers, but yields only approximate results. In applications such a summing up encrypted real numbers, evaluating machine learning models on encrypted data, or computing distances of encrypted locations CKKS is going to be by far the best choice. For applications where exact values are necessary, the BFV scheme is the only choice.

Installing Microsoft SEAL

Windows

Microsoft SEAL comes with a Microsoft Visual Studio 2017 solution file SEAL.sln that can be used to conveniently build the library, examples, and unit tests.

Debug and Release builds

You can easily switch from Visual Studio build configuration menu whether Microsoft SEAL should be built in Debug mode (no optimizations) or in Release mode. Please note that Debug mode should not be used except for debugging SEAL itself, as the performance will be orders of magnitude worse than in Release mode.

Library

Build the SEAL project native\src\SEAL.vcxproj from SEAL.sln. This results in the static library seal.lib to be created in native\lib\$(Platform)\$(Configuration). When linking with applications, you need to add native\src\ (full path) as an include directory for SEAL header files.

Examples

Build the SEALExamples project native\examples\SEALExamples.vcxproj from SEAL.sln. This results in an executable sealexamples.exe to be created in native\bin\$(Platform)\$(Configuration).

Unit tests

The unit tests require the Google Test framework to be installed. The appropriate NuGet package is already listed in native\tests\packages.config, so once you attempt to build the SEALTest project native\tests\SEALTest.vcxproj from SEAL.sln Visual Studio will automatically download and install it for you.

Linux and macOS

Microsoft SEAL is very easy to configure and build in Linux and macOS using CMake (>= 3.10). A modern version of GNU G++ (>= 6.0) or Clang++ (>= 5.0) is needed. In macOS the Xcode toolchain (>= 9.3) will work.

In macOS you will need CMake with command line tools. For this, you can either

  1. install the cmake package with Homebrew, or
  2. download CMake directly from https://cmake.org/download and enable command line tools.

Below we give instructions for how to configure, build, and install SEAL either system-wide (global install), or for a single user (local install). A system-wide install requires elevated (root) privileges.

Debug and Release builds

You can easily switch from CMake configuration options whether Microsoft SEAL should be built in Debug mode (no optimizations) or in Release mode. Please note that Debug mode should not be used except for debugging Microsoft SEAL itself, as the performance will be orders of magnitude worse than in Release mode.

Global install

Library

If you have root access to the system you can install Microsoft SEAL system-wide as follows:

cd native/src
cmake .
make
sudo make install
cd ../..

Examples

To build the examples do:

cd native/examples
cmake .
make
cd ../..

After completing the above steps the sealexamples executable can be found in native/bin/. See native/examples/CMakeLists.txt for how to link Microsoft SEAL with your own project using CMake.

Unit tests

To build the unit tests you will need the GoogleTest framework, which is included in Microsoft SEAL as a git submodule. To download the GoogleTest source files, do:

git submodule update --init

This needs to be executed only once, and can be skipped if Microsoft SEAL was cloned with git --recurse-submodules. To build the tests, do:

cd native/tests
cmake .
make
cd ../..

After completing these steps the sealtest executable can be found in native/bin/. All unit tests should pass successfully.

Local install

Library

To install Microsoft SEAL locally, e.g., to ~/mylibs/, do the following:

cd native/src
cmake -DCMAKE_INSTALL_PREFIX=~/mylibs .
make
make install
cd ../..

Examples

To build the examples do:

cd native/examples
cmake -DCMAKE_PREFIX_PATH=~/mylibs .
make
cd ../..

After completing the above steps the sealexamples executable can be found in native/bin/. See native/examples/CMakeLists.txt for how to link Microsoft SEAL with your own project using CMake.

Unit tests

To build the unit tests you will need the GoogleTest framework, which is included in Microsoft SEAL as a git submodule. To download the GoogleTest source files, do:

git submodule update --init

This needs to be executed only one, and can be skipped if Microsoft SEAL was cloned with git --recurse-submodules. Then do:

cd native/tests
cmake -DCMAKE_PREFIX_PATH=~/mylibs .
make
cd ../..

After completing these steps the sealtest executable can be found in native/bin/. All unit tests should pass successfully.

Installing Microsoft SEAL for .NET

Microsoft SEAL provides a .NET Standard library that wraps the functionality in Microsoft SEAL for use in .NET development.

Windows

The Microsoft Visual Studio 2017 solution file SEAL.sln contains the projects necessary to build the .NET assembly, a backing native shared library, .NET examples, and unit tests.

Native library

Microsoft SEAL for .NET requires a native library that is invoked by the managed .NET library. Build the SEALNetNative project dotnet\native\SEALNetNative.vcxproj from SEAL.sln. Building SEALNetNative results in the dynamic library sealnetnative.dll to be created in dotnet\lib\$(Platform)\$(Configuration). This library is meant to be used only by the .NET library, not by end users, and needs to be present in the same directory as your executable when developing a .NET application.

.NET library

Once you have built the shared native library (see above), build the SEALNet project dotnet\src\SEALNet.csproj from SEAL.sln. Building SEALNet results in the assembly SEALNet.dll to be created in dotnet\lib\$(Configuration)\netstandard2.0. This is the assembly you can reference in your application.

.NET examples

Build the SEALNetExamples project dotnet\examples\SEALNetExamples.csproj from SEAL.sln. This results in the assembly SEALNetExamples.dll to be created in dotnet\bin\$(Configuration)\netcoreapp2.1. The project takes care of copying the native SEALNetNative library to the output directory.

.NET unit tests

Build the SEALNet Test project dotnet\tests\SEALNetTest.csproj from SEAL.sln. This results in the SEALNetTest.dll assembly to be created in dotnet\lib\$(Configuration)\netcoreapp2.1. The project takes care of copying the native SEALNetNative library to the output directory.

Using Microsoft SEAL for .NET in your own application

To use Microsoft SEAL for .NET in your own application you need to:

  1. add a reference in your project to SEALNet.dll;
  2. ensure sealnetnative.dll is available for your application when run. The easiest way to ensure this is to copy sealnetnative.dll to the same directory where your application's executable is located.

Alternatively, you can build and use a NuGet package; see instructions in NUGET.md.

Linux and macOS

Microsoft SEAL for .NET relies on a native shared library that can be easily configured and built using CMake (>= 3.10) and a modern version of GNU G++ (>= 6.0) or Clang++ (>= 5.0). In macOS the Xcode toolchain (>= 9.3) will work.

For compiling .NET code you will need to install a .NET Core SDK (>= 2.1). You can follow these instructions for installing in Linux, or for installing in macOS.

Local use of shared native library

If you only intend to run the examples and unit tests provided with Microsoft SEAL, you do not need to install the native shared library, you only need to compile it. The SEALNetExamples and SEALNetTest projects take care of copying the native shared library to the appropriate assembly output directory.

To compile the native shared library you will need to:

  1. Compile Microsoft SEAL as a static or shared library with Position-Independent Code (PIC);
  2. Compile native shared library.

The instructions for compiling Microsoft SEAL are similar to the instructions described above for a global or local install. Make sure the CMake configuration option SEAL_LIB_BUILD_TYPE is set to either Static_PIC (default) or Shared. Assuming Microsoft SEAL was built and installed globally using the default CMake configuration options, we can immediately use it to compile the shared native library required for .NET:

cd dotnet/native
cmake .
make
cd ../..

If Microsoft SEAL was installed locally instead, use:

cd dotnet/native
cmake -DCMAKE_PREFIX_PATH=~/mylibs .
make
cd ../..

.NET library

To build the .NET Standard library, do the following:

cd dotnet/src
dotnet build
cd ../..

You can use the dotnet parameter --configuration <Debug|Release> to build either a Debug or Release version of the assembly. This will result in a SEALNet.dll assembly to be created in dotnet/lib/$(Configuration)/netstandard2.0. This assembly is the one you will want to reference in your own projects.

Examples

To build and run the .NET examples, do:

cd dotnet/examples
dotnet run
cd ../..

As mentioned before, the .NET project will copy the shared native library to the assembly output directory. You can use the dotnet parameter --configuration <Debug|Release> to run either Debug or Release versions of the examples.

Unit tests

To build and run the .NET unit tests, do:

cd dotnet/tests
dotnet test
cd ../..

All unit tests should pass. You can use the dotnet parameter --configuration <Debug|Release> to run Debug or Relase unit tests, and you can use --verbosity detailed to print the list of unit tests that are being run.

Using Microsoft SEAL for .NET in your own application

To use Microsoft SEAL for .NET in your own application you need to:

  1. add a reference in your project to SEALNet.dll;
  2. ensure the native shared library is available for your application when run. The easiest way to ensure this is to copy libsealnetnative.so to the same directory where your application's executable is located.

In Linux or macOS, if you have root access to the system, you have the option to install the native shared library globally. Then your application will always be able to find and load it.

Assuming Microsoft SEAL is build and installed globally, you can install the shared native library globally as follows:

cd dotnet/native
cmake  .
make
sudo make install
cd ../..

Getting Started

Using Microsoft SEAL will require the user to invest some time in learning fundamental concepts in homomorphic encryption. The code comes with heavily commented examples that are designed to gradually teach such concepts as well as to demonstrate much of the API. The code examples are available (and identical) in C++ and C#, and are divided into several source files in native/examples/ (C++) and dotnet/examples/ (C#), as follows:

C++ C# Description
examples.cpp Examples.cs The example runner application
1_bfv_basics.cpp 1_BFV_Basics.cs Encrypted modular arithmetic using the BFV scheme
2_encoders.cpp 2_Encoders.cs Encoding more complex data into Microsoft SEAL plaintext objects
3_levels.cpp 3_Levels.cs Introduces the concept of levels; prerequisite for using the CKKS scheme
4_ckks_basics.cpp 4_CKKS_Basics.cs Encrypted real number arithmetic using the CKKS scheme
5_rotation.cpp 5_Rotation.cs Performing cyclic rotations on encrypted vectors in the BFV and CKKS schemes
6_performance.cpp 6_Performance.cs Performance tests for Microsoft SEAL

It is recommeded to read the comments and the code snippets along with command line printout from running an example. For easier navigation, command line printout provides the line number in the associated source file where the associated code snippets start.

WARNING: It is impossible to use Microsoft SEAL correctly without reading all examples or by simply re-using the code from examples. Any developer attempting to do so will inevitably produce code that is vulnerable, malfunctioning, or extremely slow.

Contributing

This project welcomes contributions and suggestions. Most contributions require you to agree to a Contributor License Agreement (CLA) declaring that you have the right to, and actually do, grant us the rights to use your contribution. For details, visit https://cla.microsoft.com.

When you submit a pull request, a CLA-bot will automatically determine whether you need to provide a CLA and decorate the PR appropriately (e.g., label, comment). Simply follow the instructions provided by the bot. You will only need to do this once across all repos using our CLA.

Pull requests must be submitted to the branch called contrib.

This project has adopted the Microsoft Open Source Code of Conduct. For more information see the Code of Conduct FAQ or contact opencode@microsoft.com with any additional questions or comments.

Citing Microsoft SEAL

To cite Microsoft SEAL in academic papers, please use the following BibTeX entries.

Version 3.3

@misc{sealcrypto,
    title = {{M}icrosoft {SEAL} (release 3.3)},
    howpublished = {\url{https://github.com/Microsoft/SEAL}},
    month = june,
    year = 2019,
    note = {Microsoft Research, Redmond, WA.},
    key = {SEAL}
}

Version 3.2

@misc{sealcrypto,
    title = {{M}icrosoft {SEAL} (release 3.2)},
    howpublished = {\url{https://github.com/Microsoft/SEAL}},
    month = feb,
    year = 2019,
    note = {Microsoft Research, Redmond, WA.},
    key = {SEAL}
}

Version 3.1

@misc{sealcrypto,
    title = {{M}icrosoft {SEAL} (release 3.1)},
    howpublished = {\url{https://github.com/Microsoft/SEAL}},
    month = dec,
    year = 2018,
    note = {Microsoft Research, Redmond, WA.},
    key = {SEAL}
}

Version 3.0

@misc{sealcrypto,
    title = {{M}icrosoft {SEAL} (release 3.0)},
    howpublished = {\url{http://sealcrypto.org}},
    month = oct,
    year = 2018,
    note = {Microsoft Research, Redmond, WA.},
    key = {SEAL}
}
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