Lean Crypto Library

The leancrypto library is a cryptographic library that exclusively contains only PQC-resistant cryptographic algorithms. It is lean in every of its properties listed in the following:

• minimal dependencies: only minimal POSIX environment needed - function calls are abstracted into helper code that may need to be replaced for other environments (see the Linux kernel support in linux_kernel for replacing the POSIX calls)

• extractable: the algorithms can be extracted and compiled as part of a separate project,

• flexible: you can disable algorithms on an as-needed basis using meson configure,

• fully thread-safe when using different cipher contexts for an invocation: there is no global state maintained for the algorithms,

• stack-only support: all algorithms can be allocated on stack if needed. In addition, allocation functions for a usage on heap is also supported,

• size: minimizing footprint when statically linking by supporting dead-code stripping,

• performance: provide optimized code invoked with minimal overhead,

• testable: all algorithm implementations are directly accessible via their data structures at runtime, and

• side-channel-resistant: A valgrind-based dynamic side channel analysis is applied to find time-variant code paths based on secret data.

The one go-to library usable in any environment - validate it once and use it for all workloads. Currently supported envionments:

• User space (POSIX environments like Linux, macOS, BSDs; Windows MSYS and native support)

• Kernel space (Linux including registration into kernel crypto API)

• EFI (including secure boot)

• Rust

Status

Library Build

If you want to build the leancrypto shared library, use the provided Meson build system:

1. Setup: meson setup build

2. Compile: meson compile -C build

3. Test: meson test -C build

4. Install: meson install -C build

Linux Kernel Builds

Without DKMS

The leancrypto library can also be built as an independent Linux kernel module. This kernel module offers the same APIs and functions as the user space version of the library. This implies that a developer wanting to develop kernel and user space users of cryptographic mechanisms do not need to adjust to a new API.

Note: The user space and kernel space versions of leancrypto are fully independent of each other. Neither requires the presence of the other for full operation.

To build the leancrypto Linux kernel module, use the Makefile in the directory linux_kernel:

1. cd linux_kernel

2. make

3. the leancrypto library is provided with leancrypto.ko

Note, the compiled test kernel modules are only provided for regression testing and are not required for production use. Insert the kernel modules and check dmesg for the results. Unload the kernel modules afterwards.

The API specified by the header files installed as part of the meson install -C build command for the user space library is applicable to the kernel module as well. When compiling kernel code, the flag -DLINUX_KERNEL needs to be set.

For more details, see linux_kernel/README.md.

With DKMS

Please read the steps outlined in dkms.conf found in the root directory.

In-Tree Statically Bound into Linux Kernel

When the use of leancrypto as a kernel module is insufficient, e.g. when its services is required during boot time such as for kernel module integrity verification, it can be compiled statically into the kernel binary. To do that, the following steps have to be taken:

Copy the entire leancrypto tree into the Linux kernel source tree into the directory leancrypto:

cp -av leancrypto- linux-/leancrypto

Tell the Linux kernel build system to also use leancrypto by editing the file Kbuild found in the root of the Linux kernel tree and add the following line at the end of that file:

obj-y += leancrypto/linux_kernel/

Tell leancrypto to be compiled statically into the kernel by editing the file leancrypto/linux_kernel/Kbuild.config and modify the option CONFIG_LEANCRYPTO to y.

At this point, leancrypto will now be built statically into the Linux kernel when compiling it. Naturally, all Linux kernel options can be set as leancrypto does not depend on specific kernel options.

You may want to consider the option CONFIG_LEANCRYPTO_TEST_MODULES found in leancrypto/linux_kernel/Kbuild.config. When setting this option, all test kernel modules are built which is usually only desired during development or validation of the library. None of these kernel modules are relevant or used in production.

Library Build for EFI Environment

The leancrypto library is designed to run without any dependencies and thus can be used in environments like (U)EFI. To compile it for the EFI environment, configure the compilation with the following command:

meson setup build -Defi=enabled
meson compile -C build
meson compile -C build pkcs7_trust_tester.efi

The compilation uses the GNU-EFI environment and generates:

1. The static library leancrypto.a that could be bound into an EFI application compiled externally to the build environment.

2. A test application in build/efi/tests/pkcs7_trust_tester.efi which is statically linked with leancrypto.a and implements the test "PKCS7 Trust Validation - PKCS#7 with trust chain" from asn1/tests/meson.build. This application is a UEFI application:

   $ file ./build/efi/tests/pkcs7_trust_tester.efi
   ./build/efi/tests/pkcs7_trust_tester.efi: PE32+ executable for EFI (application), x86-64 (stripped to external PDB), 7 sections
   

Naturally, all other options offered by the meson build enviornment can be toggled for EFI support as well allowing leancrypto to be configured to implement the exact algorithms required.

When programming with leancrypto in the EFI environment, the following considerations must be applied:

• The API specified by the header files installed as part of the meson install -C build command for the user space library is applicable to the EFI environment as well.

• As the EFI environment does not offer an automatic constructor functionality the leancrypto initialization function of lc_init must be called as the very first API call before calling any other leancrypto service function.

Library Build for Windows

Native

The leancrypto library can be built on Windows (x86_64 and ARMv8) natively using clang-cl along with MSVC. For this, install MSVC and add the support for clang-cl during its installation.

Then install Python to allow meson to be executed followed by meson itself.

Now you are ready to set up leancrypto with meson:

1. cmd: set CC=clang-cl or PowerShell: $Env:CC="clang-cl"

2. meson setup build

3. meson compile -C build

Applications linking against the leancrypto DLL must be compiled with -DLC_LINK_SHARED. It makes the header files declare the exported data symbols such as lc_sha3_256 with __declspec(dllimport) which Windows requires to resolve them through the import table. The generated pkg-config file carries the definition, i.e. consumers using pkg-config get it automatically. It must not be set when linking against the static library.

MSYS2

The leancrypto library can be built on Windows using MSYS2. Once MSYS2 is installed along with meson and the mingw compiler, the standard compilation procedure outlined above for meson can be used.

The support for full assembler acceleration is enabled.

Library Build for Other Environments

If you need leancrypto to work in other environments like small embedded systems, you need:

1. Adjust the build system as needed to compile and link it

2. Adjust the file ext_headers.h to point to the right header files and locations.

3. set the flag LC_MEM_ON_HEAP if your environment only has a limited stack size. When set, functions with large memory requirements use the heap instead of the stack for this memory. The maximum stack size used by a function is 2048 bytes and is verified by a compiler check.

An example on the approach is given with the Linux kernel support found in the directory linux_kernel.

Library Build Using Profile Guided Optimization

Using profile guided optimization with GCC is a two phase operation. First, et up leancrypto with profile measurements enabled and compile it.

meson setup build -Db_pgo=generate
meson compile -C build

Then execute the test environment with the regression test suite to create representative input:

meson test -C build --suite regression

Once that is done the compiler flags are changed to use the generated nformation and rebuild.

meson configure build -Db_pgo=use
meson compile -C build

Note, it is possible that for some source code files, no profiling data is creeated due to them not being excercised in the test. This should be ignored as the affected code is either auxiliary code or test code.

After these steps the resulting leancrypto library binary is fully optimized.

Cryptographic Algorithms

Leancrypto offers various cryptographic algorithms:

• Authenticated Encryption with Associated Data

  • AES-GCM

  • Ascon-AEAD128 (SP800-232)

  • Ascon Keccak 256, and 512 AEAD, full specification provided with aead/doc/Ascon-Keccak.pdf

  • ChaCha20-Poly1305

  • cSHAKE-based AEAD algorithm - algorithm devised with leancrypto, full specification provided with aead/doc/KMAC_cSHAKE_AEAD_algorithm.pdf

  • hash-based AEAD algorithm - algorithm devised with leancrypto, see hash_crypt.c for full specification

  • KMAC-based AEAD algorithm - algorithm devised with leancrypto, full specification provided with aead/doc/KMAC_cSHAKE_AEAD_algorithm.pdf

  • AES-based AEAD algorithm using SHA2 - see symhmac_crypt.c for full specification

  • AES-based AEAD algorithm using Keccak-based KMAC - see symkmac_crypt.c for full specification

• Pseudo Random Number Generators

  • XDRBG using either SHAKE-256 or Ascon-128 providing either 256 bits or 128 bits cryptographic strength respectively - see drng/doc/ToSC2024_1_01.pdf for full specification and proof

  • cSHAKE-based PRNG - see cshake_drng.c for full specification - it complies with the XDRBG specification given in drng/doc/ToSC2024_1_01.pdf

  • KMAC-based PRNG - see kmac_drng.c for full specification - it complies with the XDRBG specification given in drng/doc/ToSC2024_1_01.pdf

  • SHAKE-based PRNG

  • SP800-90A CTR, Hash and HMAC DRBG

  • ChaCha20-based PRNG - see https://www.chronox.de/lrng for specification

• Message Digest algorithms

  • SHA2-256, SHA2-512

  • SHA3-224, SHA3-256, SHA3-384, SHA3-512

  • SHAKE-128, SHAKE-256

  • cSHAKE-128, cSHAKE-256

  • Ascon-Hash256 (SP800-232)

  • Ascon-XOF128 (SP800-232)

• Keyed Message Digest algorithms

  • HMAC

  • KMAC

• Key Derivation Functions

  • HKDF

  • SP800-108 KDF (counter, feedback, double pipelining mode)

  • PBKDF2

• Key Encapsulation Mechanism

  • ML-KEM (Kyber) Key Encapsulation Mechanism (KEM)

  • ML-KEM (Kyber) Key Exchange Mechanism (KEX)

  • ML-KEM (Kyber) Integrated Encryption Schema (IES) - algorithm devised with leancrypto, see kyber_ies.c for full specification

  • ML-KEM (Kyber) composite KEM / KEX with Curve25519 and Curve448

  • BIKE Key Encapsulation Mechanism (KEM)

  • HQC Key Encapsulation Mechanism (KEM) (NIST round 4 winner)

• One-Time Password (OTP) algorithms

  • HMAC-based One-Time Password Algorithm (HOTP)

  • Time-based One-Time Password Algorithm (TOTP)

• Signature algorithms

  • ML-DSA (Dilithium)

  • ML-DSA (Dilithium) composite signature operation with Curve25519 and Curve448

  • SLH-DSA (Sphincs Plus) with SHAKE and Ascon

• Symmetric algorithms

  • AES: ECB, CBC, CTR, KW, XTS

  • ChaCha20

API Documentation

The complete API documentation is provided in the different header files lc_*.h.

The Doxygen documentation is automatically compiled if the Doxygen binary is found during the compilation run.

The various header files contain data structures which are provided solely for the purpose that appropriate memory on stack can be allocated. These data structures do not constitute an API in the sense that calling applications should access member variables directly. If access to member variables is desired, proper accessor functions are available. This implies that changes to the data structures in newer versions of the library are not considered as API changes!

X.509 and PKCS#7 Support

The library offers an X.509 and PKCS#7 support with lc_x509_parser.h, lc_x509_generator.h, lc_pkcs7_parser.h, and lc_pkcs7_generator.h.

X.509 support includes:

• X.509 parsing

• X.509 generation

The operations are offered via APIs as well as via the lc_x509_generator application.

The following services are offered with the PKCS#7 support:

• Parsing PKCS#7 messages:

  • Signature verification

  • Enforcement of key usage / EKU, time stamps

  • Certificate chain validation

  • Trust store handling

• Generating PKCS#7 messages:

  • Signature generation

  • Certificate chain

The operations are offered via APIs as well as via the lc_pkcs7_generator application.

Compile-Time Options

The following compile time options provide support for various features.

Secure Execution

By enabling the compile-time option secure_execution, security features provided by the underlying OS for the process leancrypto executes in are enabled. As these options are partially very costly, the option is disabled by default. Currently supported options:

• Linux: enabling of the following

  • Speculative Store Bypass

  • Indirect Branch Speculation

  • Flush L1D Cache on context switch out of the task

Testing

Functional Testing

The command meson test -C build performs a full regression testing of all algorithms and all code paths.

When using the code coverage analysis support enabled by meson setup build -Db_coverage=true followed by meson test -C build and ninja coverage-html -C build, it is shown that almost all code paths in the library are covered (the test code contains error code paths which are not all tested, naturally).

To perform testing of the Linux kernel module of leancrypto, insmod the module leancrypto.ko followed by insmod'ing all remaining kernel modules found in the linux_kernel directory and review the kernel log via dmesg. After completion of testing, these modules can be removed.

Memory Leak Testing

Using valgrind, the memory leak testing can be applied. Valgrind shows no leaks possible for any code path.

For the Linux kernel, kmemleak can be applied showing no leaks during test execution.

ASAN Testing

Using ASAN address testing with the help of meson, no issues were identified.

Similarly, KASAN can be used for the same type of testing inside the Linux kernel where no issues were identified.

UBSAN Testing

Using UBSAN undefined operation testing with the help of meson, no issues were identified.

Similarly, UASAN can be used for the same type of testing inside the Linux kernel where no issues were identified.

Clang Static Code Analysis

Using the clang-scan tool with the command ninja -C build/ scan-build shows no issues.

Linux Hints

Linux offers the memfd_secret(2) system call which would be used by leancrypto for secure memory allocation, if it is available. This system call, however, is only available when the kernel is booted with securemem.enable=1. With this option enabled, according to the kernel documentation suspend is disabled as long as one or more memory with memfd_secret is in use.

If the memfd_secret is not available, leancrypto will use mlock to protect memory with sensitive data.

ACVP Testing

ACVP certificates covering all ciphers and their implementations testable by NIST:

The testing covered the following platforms:

• x86_64 systems: Intel Tiger Lake i7, Intel Alder Lake i7, Intel Meteor Lake Ultra 7

• RISC-V system: Spacemit X60 K1

• ARMv7: NXP Cortex-A7 i.MX6ULZ

• ARMv8: Apple M4 Max, Broadcom BCM2711, Broadcom BCM2712

The test harness is available at https://github.com/smuellerDD/acvpparser covering all algorithm implementations of algorithms that are testable.

The testing covers user space as well as Linux kernel space.

Only the current CAVP certificates are listed below. Older CAVP certificates are listed at the CAVP leancrypto website.

Version 1.6.0

leancrypto CAVP Certificates

Author

Stephan Müller smueller@chronox.de