This document explains the internal structure of libmceliece, and explains how to add new instruction sets and new implementations. The libmceliece infrastructure is adapted from the lib25519 infrastructure.

Primitives

The directories crypto_*/* inside libmceliece define the following primitives:

• crypto_xof/bitwrite16: crypto_xof_bitwrite16(out,outlen,in,inlen) computes bytes out[0], out[1], ..., out[outlen-1] from bytes in[0], in[1], ..., in[inlen-1] as follows. The first 2 input bytes are viewed (in little-endian form) as a 16-bit integer; the next 2 input bytes are viewed as a 16-bit integer; and so on, with any remaining byte ignored. Bit position i in the output is 1 if i modulo 65536 is one of these 16-bit integers, otherwise 0.

• crypto_xof/shake256: crypto_xof_shake256(out,outlen,in,inlen) computes bytes out[0], out[1], ..., out[outlen-1] as the first outlen bytes of the (infinitely long) SHAKE256 hash of bytes in[0], in[1], ..., in[inlen-1].

• crypto_sort/int16: crypto_sort_int16(x,n) sorts the int16 values x[0], x[1], ..., x[n-1].

• crypto_sort/int32: crypto_sort_int32(x,n) sorts the int32 values x[0], x[1], ..., x[n-1].

• crypto_sort/int64: crypto_sort_int64(x,n) sorts the int64 values x[0], x[1], ..., x[n-1].

• crypto_kem/*: crypto_kem_6960119_keypair(pk,sk) is key generation for the 6960119 parameter set, and is provided by the stable API as mceliece6960119_keypair. Similar comments apply to enc and dec, to the f variants, and to sizes other than 6960119.

libmceliece includes a command-line utility mceliece-test that runs some tests for each of these primitives, and another utility mceliece-speed that measures cycle counts for each of these primitives.

As in SUPERCOP and NaCl, message lengths intentionally use long long, not size_t. In libmceliece, as in lib25519, message lengths are signed.

Implementations

A single primitive can, and usually does, have multiple implementations. Each implementation is in its own subdirectory. The implementations are required to have exactly the same input-output behavior, and to some extent this is tested, although it is not yet formally verified (except for some components such as crypto_sort).

Different implementations typically offer different tradeoffs between portability, simplicity, and efficiency. For example, crypto_kem/6960119/vec is portable; crypto_kem/6960119/avx is faster and less portable.

Each unportable implementation has an architectures file. Each line in this file identifies a CPU instruction set (and ABI) where the implementation works. For example, crypto_kem/6960119/avx/architectures has one line

amd64 sse3 ssse3 sse41 popcnt avx bmi1 bmi2 avx2

meaning that the implementation works on CPUs that have the Intel/AMD 64-bit instruction set with the SSE3, SSSE3, SSE4.1, POPCNT, AVX, BMI1, BMI2, and AVX2 instruction-set extensions. The top-level compilers directory shows (among other things) the allowed instruction-set names such as bmi2.

At run time, libmceliece checks the CPU where it is running, and selects an implementation where architectures is compatible with that CPU. Each primitive makes its own selection once per program startup, using the compiler's ifunc mechanism (or constructor on platforms that do not support ifunc). This type of run-time selection means, for example, that an amd64 CPU without AVX2 can share binaries with an amd64 CPU with AVX2. However, correctness requires instruction sets to be preserved by migration across cores via the OS kernel, VM migration, etc.

The compiler has a target mechanism that makes an ifunc selection based on CPU architectures. Instead of using the target mechanism, libmceliece uses a more sophisticated mechanism that also accounts for benchmarks collected in advance of compilation.

Compilers

libmceliece tries different C compilers for each implementation. For example, compilers/default lists the following compilers:

gcc -Wall -fPIC -fwrapv -O2
clang -Wall -fPIC -fwrapv -Qunused-arguments -O2

Sometimes gcc produces better code, and sometimes clang produces better code.

As another example, compilers/amd64+sse3+ssse3+sse41+popcnt+avx+bmi1+bmi2+avx2 lists the following compilers:

gcc -Wall -fPIC -fwrapv -O2 -mmmx -msse -msse2 -msse3 -mssse3 -msse4.1 -msse4.2 -mavx -mbmi -mbmi2 -mpopcnt -mavx2 -mtune=haswell
clang -Wall -fPIC -fwrapv -Qunused-arguments -O2 -mmmx -msse -msse2 -msse3 -mssse3 -msse4.1 -msse4.2 -mavx -mbmi -mbmi2 -mpopcnt -mavx2 -mtune=haswell

The -mavx2 option tells these compilers that they are free to use the AVX2 instruction-set extension.

Code compiled using the compilers in compilers/amd64+sse3+ssse3+sse41+popcnt+avx+bmi1+bmi2+avx2 will be considered at run time by the libmceliece selection mechanism if the supports() function in compilers/amd64+sse3+ssse3+sse41+popcnt+avx+bmi1+bmi2+avx2.c returns nonzero. This function checks whether the run-time CPU supports AVX2 (and SSE3 and so on, and OSXSAVE with XMM/YMM being saved; https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85100 says that all versions of gcc until 2018 handled this incorrectly in target). Similar comments apply to other compilers/* files.

If some compilers fail (for example, clang is not installed, or the compiler version is too old to support the compiler options used in libmceliece), the libmceliece compilation process will try its best to produce a working library using the remaining compilers, even if this means lower performance.

Trimming

By default, to reduce size of the compiled library, the libmceliece compilation process trims the library down to the implementations that are selected by libmceliece's selection mechanism.

For example, if the selection mechanism decides that CPUs with AVX2 should use 6960119/avx with clang and that other CPUs should use 6960119/vec with gcc, then trimming will remove 6960119/avx compiled with gcc and 6960119/vec compiled with clang.

This trimming is handled at link time rather than compile time to increase the chance that, even if some implementations are broken by compiler "upgrades", the library will continue to build successfully.

To avoid this trimming, pass the --no-trim option to ./configure. All implementations that compile are then included in the library, tested by mceliece-test, and measured by mceliece-speed. You'll want to avoid trimming if you're adding new instruction sets or new implementations (see below), so that you can run tests and benchmarks of code that isn't selected yet.

How to recompile after changes

If you make changes in the libmceliece source directory, the fully supported recompilation mechanism is to run ./configure again to clean and repopulate the build directory, and then run make again to recompile everything.

This can be on the scale of seconds if you have enough cores, but maybe you're developing on a slower machine. Three options are currently available to accelerate the edit-compile cycle:

• There is an experimental --no-clean option to ./configure that, for some simple types of changes, can produce a successful build without cleaning.

• Running make without ./configure can work for some particularly simple types of changes. However, not all dependencies are currently expressed in Makefile, and some types of dependencies that ./configure understands would be difficult to express in the Makefile language.

• You can disable the implementations you're not using by setting sticky bits on the source directories for those implementations: e.g., chmod +t crypto_kem/*/avx.

Make sure to reenable all implementations and do a full clean build if you're collecting data to add to the source benchmarks directory.

How to add new instruction sets

Adding another file compilers/amd64+foo, along with a supports() implementation in compilers/amd64+foo.c, will support a new instruction set. Do not assume that the new foo instruction set implies support for older instruction sets (the idea of "levels" of instruction sets); instead make sure to include the older instruction sets in + tags, as illustrated by compilers/amd64+sse3+ssse3+sse41+popcnt+avx+bmi1+bmi2+avx2.

In the compiler options, always make sure to include -fPIC to support shared libraries, and -fwrapv to switch to a slightly less dangerous version of C.

The foo tags don't have to be instruction sets. For example, if a CPU has the same instruction set but wants different optimizations because of differences in instruction timings, you can make a tag for those optimizations, using, e.g., CPU IDs or benchmarks in the corresponding supports() function to decide whether to enable those optimizations. Benchmarks tend to be more future-proof than a list of CPU IDs, but the time taken for benchmarks at program startup has to be weighed against the subsequent speedup from the resulting optimizations.

To see how well libmceliece performs with the new compilers, run mceliece-speed on the target machine and look for the foo lines in the output. If the new performance is better than the performance shown on the selected lines:

• Copy the mceliece-speed output into a file on the benchmarks directory, typically named after the hostname of the target machine.

• Run ./prioritize in the top-level directory to create priority files. These files tell libmceliece which implementations to select for any given architecture.

• Reconfigure (again with --no-trim), recompile, rerun mceliece-test, and rerun mceliece-speed to check that the selected lines now use the foo compiler.

If the foo implementation is outperformed by other implementations, then these steps don't help except for documenting this fact. The same implementation might turn out to be useful for subsequent foo CPUs.

How to add new implementations

Taking full advantage of the foo instruction set usually requires writing new implementations. Sometimes there are also ideas for taking better advantage of existing instruction sets.

Structurally, adding a new implementation of a primitive is a simple matter of adding a new subdirectory with the code for that implementation. Most of the work is optimizing the use of foo intrinsics in .c files or foo instructions in .S files. Make sure to include an architectures file saying, e.g., amd64 avx2 foo.

Names of implementation directories can use letters, digits, dashes, and underscores. Do not use two implementation names that are the same when dashes and underscores are removed.

All .c and .S files in the implementation directory are compiled and linked. There is no need to edit a separate list of these files. You can also use .h files via the C preprocessor.

If an implementation is actually more restrictive than indicated in architectures then the resulting compiled library will fail on some machines (although perhaps that implementation will not be used by default). Putting unnecessary restrictions into architectures will not create such failures, but can unnecessarily limit performance.

Some, but not all, mistakes in architectures will produce warnings from the checkinsns script that runs automatically when libmceliece is compiled. Running the mceliece-test program tries all implementations, but only on the CPU where mceliece-test is being run; also, mceliece-test does not guarantee code coverage.

amd64 implies little-endian, and implies architectural support for unaligned loads and stores. Beware, however, that the Intel/AMD vectorized load/store intrinsics (and the underlying movdqa instruction) require alignment; if in doubt, use loadu/storeu (and movdqu). The mceliece-test program checks unaligned inputs and outputs, but can miss issues with unaligned stack variables.

To test your implementation, compile everything, check for compiler warnings and errors, run mceliece-test (or just mceliece-test xof to test a crypto_xof implementation), and check for a line saying all tests succeeded. To use AddressSanitizer (for catching, at run time, buffer overflows in C code), add -fsanitize=address to the gcc and clang lines in compilers/*; you may also have to add return; at the beginning of the limits() function in command/limits.inc.

To see the performance of your implementation, run mceliece-speed. If the new performance is better than the performance shown on the selected lines, follow the same steps as for a new instruction set: copy the mceliece-speed output into a file on the benchmarks directory; run ./prioritize in the top-level directory to create priority files; reconfigure (again with --no-trim); recompile; rerun mceliece-test; rerun mceliece-speed; check that the selected lines now use the new implementation.

How to handle namespacing

As in SUPERCOP and NaCl, to call crypto_sort_int32(), you have to include crypto_sort_int32.h; but to write an implementation of crypto_sort_int32(), you have to instead include crypto_sort.h and define crypto_sort. Similar comments apply to other primitives.

The function name that's actually linked might end up as, e.g., libmceliece_sort_int32_avx2_C2 where avx2 indicates the implementation and C2 indicates the compiler. Don't try to build this name into your implementation.

If you have another global symbol x (for example, a non-static function in a .c file, or a non-static variable outside functions in a .c file), you have to replace it with CRYPTO_NAMESPACE(x), for example with #define x CRYPTO_NAMESPACE(x).

For global symbols in .S files and shared-*.c files, use CRYPTO_SHARED_NAMESPACE instead of CRYPTO_NAMESPACE. For .S files that define both x and _x to handle platforms where x in C is _x in assembly, use CRYPTO_SHARED_NAMESPACE(x) and _CRYPTO_SHARED_NAMESPACE(x); CRYPTO_SHARED_NAMESPACE(_x) is not sufficient.

libmceliece includes a mechanism to recognize files that are copied across implementations (possibly of different primitives) and to unify those into a file compiled only once, reducing the overall size of the compiled library and possibly improving cache utilization. To request this mechanism, include a line

// linker define x

for any global symbol x defined in the file, and a line

// linker use x

for any global symbol x used in the file from the same implementation (not crypto_* subroutines that you're calling, randombytes, etc.). This mechanism tries very hard, perhaps too hard, to avoid improperly unifying files: for example, even a slight difference in a .h file included by a file defining a used symbol will disable the mechanism.

Typical namespacing mistakes will produce either linker failures or warnings from the checknamespace script that runs automatically when libmceliece is compiled.

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Version: This is version 2024.07.21 of the "Internals" web page.