From 85c1cdf50b13ead9b52a0af73c77ad88d2b04740 Mon Sep 17 00:00:00 2001
From: Hanno Becker <beckphan@amazon.co.uk>
Date: Mon, 16 Dec 2024 20:14:52 +0000
Subject: [PATCH] Add example for custom config and custom FIPS-202 backend

This commit adds another minimal example to `examples/`, demonstrating
how to use a custom configuration file and a custom FIPS-202 backend.

Signed-off-by: Hanno Becker <beckphan@amazon.co.uk>
---
 .github/workflows/ci.yml                      |   3 +
 examples/README.md                            |   5 +
 examples/custom_backend/Makefile              |  64 ++++++
 examples/custom_backend/README.md             |  34 +++
 examples/custom_backend/main.c                |  56 +++++
 .../mlkem_native/custom_config.h              | 113 ++++++++++
 .../mlkem_native/fips202/LICENSE              |   1 +
 .../mlkem_native/fips202/fips202.c            |   1 +
 .../mlkem_native/fips202/fips202.h            |   1 +
 .../mlkem_native/fips202/fips202_backend.h    |   1 +
 .../mlkem_native/fips202/fips202x4.c          |   1 +
 .../mlkem_native/fips202/fips202x4.h          |   1 +
 .../mlkem_native/fips202/keccakf1600.c        |   1 +
 .../mlkem_native/fips202/keccakf1600.h        |   1 +
 .../mlkem_native/fips202/native/api.h         |   1 +
 .../fips202/native/custom/custom.h            |  25 +++
 .../fips202/native/custom/src/LICENSE         |  23 ++
 .../fips202/native/custom/src/Makefile        |  28 +++
 .../fips202/native/custom/src/README.md       |  63 ++++++
 .../fips202/native/custom/src/custom_impl.h   |  22 ++
 .../fips202/native/custom/src/sha3.c          | 197 ++++++++++++++++++
 .../fips202/native/custom/src/sha3.h          |  50 +++++
 .../custom_backend/mlkem_native/mlkem/LICENSE |   1 +
 .../mlkem_native/mlkem/arith_backend.h        |   1 +
 .../custom_backend/mlkem_native/mlkem/cbd.c   |   1 +
 .../custom_backend/mlkem_native/mlkem/cbd.h   |   1 +
 .../custom_backend/mlkem_native/mlkem/cbmc.h  |   1 +
 .../mlkem_native/mlkem/common.h               |   1 +
 .../mlkem_native/mlkem/config.h               |   1 +
 .../custom_backend/mlkem_native/mlkem/debug   |   1 +
 .../mlkem_native/mlkem/indcpa.c               |   1 +
 .../mlkem_native/mlkem/indcpa.h               |   1 +
 .../custom_backend/mlkem_native/mlkem/kem.c   |   1 +
 .../custom_backend/mlkem_native/mlkem/kem.h   |   1 +
 .../mlkem_native/mlkem/namespace.h            |   1 +
 .../custom_backend/mlkem_native/mlkem/ntt.c   |   1 +
 .../custom_backend/mlkem_native/mlkem/ntt.h   |   1 +
 .../mlkem_native/mlkem/params.h               |   1 +
 .../custom_backend/mlkem_native/mlkem/poly.c  |   1 +
 .../custom_backend/mlkem_native/mlkem/poly.h  |   1 +
 .../mlkem_native/mlkem/polyvec.c              |   1 +
 .../mlkem_native/mlkem/polyvec.h              |   1 +
 .../mlkem_native/mlkem/randombytes.h          |   1 +
 .../mlkem_native/mlkem/reduce.h               |   1 +
 .../mlkem_native/mlkem/rej_uniform.c          |   1 +
 .../mlkem_native/mlkem/rej_uniform.h          |   1 +
 .../mlkem_native/mlkem/symmetric.h            |   1 +
 .../custom_backend/mlkem_native/mlkem/sys.h   |   1 +
 .../mlkem_native/mlkem/verify.c               |   1 +
 .../mlkem_native/mlkem/verify.h               |   1 +
 .../custom_backend/mlkem_native/mlkem/zetas.c |   1 +
 .../test_only_rng/notrandombytes.c            |  96 +++++++++
 scripts/ci/lint                               |   2 +-
 53 files changed, 818 insertions(+), 1 deletion(-)
 create mode 100644 examples/custom_backend/Makefile
 create mode 100644 examples/custom_backend/README.md
 create mode 100644 examples/custom_backend/main.c
 create mode 100644 examples/custom_backend/mlkem_native/custom_config.h
 create mode 120000 examples/custom_backend/mlkem_native/fips202/LICENSE
 create mode 120000 examples/custom_backend/mlkem_native/fips202/fips202.c
 create mode 120000 examples/custom_backend/mlkem_native/fips202/fips202.h
 create mode 120000 examples/custom_backend/mlkem_native/fips202/fips202_backend.h
 create mode 120000 examples/custom_backend/mlkem_native/fips202/fips202x4.c
 create mode 120000 examples/custom_backend/mlkem_native/fips202/fips202x4.h
 create mode 120000 examples/custom_backend/mlkem_native/fips202/keccakf1600.c
 create mode 120000 examples/custom_backend/mlkem_native/fips202/keccakf1600.h
 create mode 120000 examples/custom_backend/mlkem_native/fips202/native/api.h
 create mode 100644 examples/custom_backend/mlkem_native/fips202/native/custom/custom.h
 create mode 100644 examples/custom_backend/mlkem_native/fips202/native/custom/src/LICENSE
 create mode 100644 examples/custom_backend/mlkem_native/fips202/native/custom/src/Makefile
 create mode 100644 examples/custom_backend/mlkem_native/fips202/native/custom/src/README.md
 create mode 100644 examples/custom_backend/mlkem_native/fips202/native/custom/src/custom_impl.h
 create mode 100644 examples/custom_backend/mlkem_native/fips202/native/custom/src/sha3.c
 create mode 100644 examples/custom_backend/mlkem_native/fips202/native/custom/src/sha3.h
 create mode 120000 examples/custom_backend/mlkem_native/mlkem/LICENSE
 create mode 120000 examples/custom_backend/mlkem_native/mlkem/arith_backend.h
 create mode 120000 examples/custom_backend/mlkem_native/mlkem/cbd.c
 create mode 120000 examples/custom_backend/mlkem_native/mlkem/cbd.h
 create mode 120000 examples/custom_backend/mlkem_native/mlkem/cbmc.h
 create mode 120000 examples/custom_backend/mlkem_native/mlkem/common.h
 create mode 120000 examples/custom_backend/mlkem_native/mlkem/config.h
 create mode 120000 examples/custom_backend/mlkem_native/mlkem/debug
 create mode 120000 examples/custom_backend/mlkem_native/mlkem/indcpa.c
 create mode 120000 examples/custom_backend/mlkem_native/mlkem/indcpa.h
 create mode 120000 examples/custom_backend/mlkem_native/mlkem/kem.c
 create mode 120000 examples/custom_backend/mlkem_native/mlkem/kem.h
 create mode 120000 examples/custom_backend/mlkem_native/mlkem/namespace.h
 create mode 120000 examples/custom_backend/mlkem_native/mlkem/ntt.c
 create mode 120000 examples/custom_backend/mlkem_native/mlkem/ntt.h
 create mode 120000 examples/custom_backend/mlkem_native/mlkem/params.h
 create mode 120000 examples/custom_backend/mlkem_native/mlkem/poly.c
 create mode 120000 examples/custom_backend/mlkem_native/mlkem/poly.h
 create mode 120000 examples/custom_backend/mlkem_native/mlkem/polyvec.c
 create mode 120000 examples/custom_backend/mlkem_native/mlkem/polyvec.h
 create mode 120000 examples/custom_backend/mlkem_native/mlkem/randombytes.h
 create mode 120000 examples/custom_backend/mlkem_native/mlkem/reduce.h
 create mode 120000 examples/custom_backend/mlkem_native/mlkem/rej_uniform.c
 create mode 120000 examples/custom_backend/mlkem_native/mlkem/rej_uniform.h
 create mode 120000 examples/custom_backend/mlkem_native/mlkem/symmetric.h
 create mode 120000 examples/custom_backend/mlkem_native/mlkem/sys.h
 create mode 120000 examples/custom_backend/mlkem_native/mlkem/verify.c
 create mode 120000 examples/custom_backend/mlkem_native/mlkem/verify.h
 create mode 120000 examples/custom_backend/mlkem_native/mlkem/zetas.c
 create mode 100644 examples/custom_backend/test_only_rng/notrandombytes.c

diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index e0c7e65e7..e63ce068b 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -119,6 +119,9 @@ jobs:
       - name: bring_your_own_fips202
         run: |
           make run -C examples/bring_your_own_fips202
+      - name: custom_backend
+        run: |
+          make run -C examples/custom_backend
   build_kat:
     needs: [quickcheck, quickcheck-windows]
     strategy:
diff --git a/examples/README.md b/examples/README.md
index 3412042d1..ba73ec584 100644
--- a/examples/README.md
+++ b/examples/README.md
@@ -12,3 +12,8 @@ See [mlkem_native_as_code_package](mlkem_native_as_code_package).
 
 See [bring_your_own_fips202](bring_your_own_fips202) for an example of how to use mlkem-native with your own FIPS-202
 implementation.
+
+## Using mlkem-native as a code package, custom config + custom FIPS-202 backend
+
+See [custom_backend](custom_backend) for an example of how to use mlkem-native with a custom configuration file and a
+custom FIPS-202 backend.
diff --git a/examples/custom_backend/Makefile b/examples/custom_backend/Makefile
new file mode 100644
index 000000000..b1dad8549
--- /dev/null
+++ b/examples/custom_backend/Makefile
@@ -0,0 +1,64 @@
+# (SPDX-License-Identifier: CC-BY-4.0)
+
+.PHONY: build run clean
+
+# Part A:
+#
+# mlkem-native source and header files
+#
+# If you are not concerned about minimizing for a specific backend,
+# you can just include _all_ source files into your build.
+MLKEM_NATIVE_SOURCE=$(wildcard          \
+	mlkem_native/**/*.c	  	\
+	mlkem_native/**/*.c		\
+	mlkem_native/**/**/*.c		\
+	mlkem_native/**/**/**/*.c	\
+	mlkem_native/**/**/**/**/*.c)
+
+INC=
+INC+=-Imlkem_native/
+INC+=-Imlkem_native/mlkem
+INC+=-Imlkem_native/mlkem/native
+INC+=-Imlkem_native/fips202
+INC+=-Imlkem_native/fips202/native
+
+# Part B:
+#
+# Random number generator
+#
+# !!! WARNING !!!
+#
+# The randombytes() implementation used here is for TESTING ONLY.
+# You MUST NOT use this implementation outside of testing.
+#
+# !!! WARNING !!!
+RNG_SOURCE=$(wildcard test_only_rng/*.c)
+
+# Part C:
+#
+# Your application source code
+APP_SOURCE=$(wildcard *.c)
+
+ALL_SOURCE=$(MLKEM_NATIVE_SOURCE) $(RNG_SOURCE) $(APP_SOURCE)
+
+BUILD_DIR=build
+BIN=test_binary
+
+CFLAGS=-DMLKEM_NATIVE_CONFIG_FILE="\"custom_config.h\""
+
+BINARY_NAME_FULL=$(BUILD_DIR)/$(BIN)
+
+$(BINARY_NAME_FULL): $(ALL_SOURCE)
+	echo "$@"
+	mkdir -p $(BUILD_DIR)
+	$(CC) $(CFLAGS) $(INC) $^ -o $@
+
+all: run
+
+build: $(BINARY_NAME_FULL)
+
+run: $(BINARY_NAME_FULL)
+	./$(BINARY_NAME_FULL)
+
+clean:
+	rm -rf $(BUILD_DIR)
diff --git a/examples/custom_backend/README.md b/examples/custom_backend/README.md
new file mode 100644
index 000000000..b93ea95ad
--- /dev/null
+++ b/examples/custom_backend/README.md
@@ -0,0 +1,34 @@
+[//]: # (SPDX-License-Identifier: CC-BY-4.0)
+
+# Using a custom configuration and FIPS-202 backend
+
+This directory contains a minimal example for how to use mlkem-native as a code package, with a custom FIPS-202
+backend and a custom configuration. We use the [tiny_sha3](https://github.com/mjosaarinen/tiny_sha3/) by Markku-J. O.
+Saarinen as an example.
+
+## Components
+
+An application using mlkem-native with a custom FIPS-202 backend and custom configuration needs the following:
+
+1. Arithmetic part of the mlkem-native source tree: [`mlkem/`](../../mlkem). In this example, we disable arithmetic
+   backends, hence it is safe to remove the entire `native` subfolder.
+2. A secure pseudo random number generator, implementing [`randombytes.h`](../../mlkem/randombytes.h).
+3. FIPS-202 part of the mlkem-native source tree, [`fips/`](../../fips202). If you only want to use your backend,
+   you can remove all existing backends; that's what this example does.
+4. A custom FIPS-202 backend. In this example, the metadata file is
+   [custom.h](mlkem_native/fips202/native/custom/custom.h), the implementation shim is
+   [custom_impl.h](mlkem_native/fips202/native/custom/src/custom_impl.h), wrapping the
+   [sha3.c](mlkem_native/fips202/native/custom/src/sha3.c) and setting `MLKEM_USE_FIPS101_X1_NATIVE` to indicate that we
+   replace 1-fold Keccak-F1600.
+5. Either modify the existing [config.h](mlkem_native/mlkem/config.h), or register a new config. In this example, we add
+   a new config [custom_config.h](mlkem_native/custom_config.h) and register it from the command line for
+   `-DMLKEM_NATIVE_CONFIG_FILE="custom_config.h"` -- no further changes to the build are needed. For the sake of
+   demonstration, we set a custom namespace. We set `MLKEM_NATIVE_FIPS202_BACKEND` to point to our custom FIPS-202
+   backend, but leave `MLKEM_NATIVE_ARITH_BACKEND` undefined to indicate that we wish to use the C backend.
+
+**WARNING:** The `randombytes()` implementation used here is for TESTING ONLY. You MUST NOT use this implementation
+outside of testing.
+
+## Usage
+
+Build this example with `make build`, run with `make run`.
diff --git a/examples/custom_backend/main.c b/examples/custom_backend/main.c
new file mode 100644
index 000000000..d6bb92888
--- /dev/null
+++ b/examples/custom_backend/main.c
@@ -0,0 +1,56 @@
+/*
+ * Copyright (c) 2024 The mlkem-native project authors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <stdio.h>
+#include <string.h>
+
+#include <kem.h>
+
+int main(void)
+{
+  uint8_t pk[CRYPTO_PUBLICKEYBYTES];
+  uint8_t sk[CRYPTO_SECRETKEYBYTES];
+  uint8_t ct[CRYPTO_CIPHERTEXTBYTES];
+  uint8_t key_a[CRYPTO_BYTES];
+  uint8_t key_b[CRYPTO_BYTES];
+
+  printf("Generating keypair ... ");
+
+  /* Alice generates a public key */
+  crypto_kem_keypair(pk, sk);
+
+  printf("DONE\n");
+  printf("Encaps... ");
+
+  /* Bob derives a secret key and creates a response */
+  crypto_kem_enc(ct, key_b, pk);
+
+  printf("DONE\n");
+  printf("Decaps... ");
+
+  /* Alice uses Bobs response to get her shared key */
+  crypto_kem_dec(key_a, ct, sk);
+
+  printf("DONE\n");
+  printf("Compare... ");
+
+  if (memcmp(key_a, key_b, CRYPTO_BYTES))
+  {
+    printf("ERROR\n");
+    return 1;
+  }
+
+  printf("OK\n");
+
+  printf("Shared secret: ");
+  {
+    int i;
+    for (i = 0; i < sizeof(key_a); i++)
+      printf("%02x", key_a[i]);
+  }
+  printf("\n");
+
+  return 0;
+}
diff --git a/examples/custom_backend/mlkem_native/custom_config.h b/examples/custom_backend/mlkem_native/custom_config.h
new file mode 100644
index 000000000..d9ecfa9fa
--- /dev/null
+++ b/examples/custom_backend/mlkem_native/custom_config.h
@@ -0,0 +1,113 @@
+/*
+ * Copyright (c) 2024 The mlkem-native project authors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#ifndef MLKEM_NATIVE_CONFIG_H
+#define MLKEM_NATIVE_CONFIG_H
+
+/******************************************************************************
+ * Name:        MLKEM_K
+ *
+ * Description: Determines the security level for ML-KEM
+ *              - MLKEM_K=2 corresponds to ML-KEM-512
+ *              - MLKEM_K=3 corresponds to ML-KEM-768
+ *              - MLKEM_K=4 corresponds to ML-KEM-1024
+ *
+ *              This can also be set using CFLAGS.
+ *
+ *****************************************************************************/
+#define MLKEM_K 4 /* We want MLKEM-1024 */
+
+/******************************************************************************
+ * Name:        MLKEM_NATIVE_CONFIG_FILE
+ *
+ * Description: If defined, this is a header that will be included instead
+ *              of mlkem/config.h.
+ *
+ *              When you need to build mlkem-native in multiple configurations,
+ *              this can be a convenient alternative to configuration via
+ *              CFLAGS.
+ *
+ *****************************************************************************/
+/* No need to set this -- we _are_ already in a custom config */
+/* #define MLKEM_NATIVE_CONFIG_FILE "config.h" */
+
+/******************************************************************************
+ * Name:        MLKEM_NAMESPACE
+ *              _MLKEM_NAMESPACE
+ *
+ * Description: The macros to use to namespace global symbols
+ *              from mlkem/.
+ *****************************************************************************/
+#define __CONC(a, b) a##b
+#define CONC(a, b) __CONC(a, b)
+
+#define MLKEM_NAMESPACE(sym) CONC(CUSTOM_TINY_SHA3_, sym)
+#define _MLKEM_NAMESPACE(sym) CONC(_CUSTOM_TINY_SHA3_, sym)
+
+/******************************************************************************
+ * Name:        FIPS202_NAMESPACE
+ *              _FIPS202_NAMESPACE
+ *
+ * Description: The macros to use to namespace global symbols
+ *              from fips202/.
+ *****************************************************************************/
+#define FIPS202_NAMESPACE(sym) CONC(CUSTOM_TINY_SHA3_, sym)
+#define _FIPS202_NAMESPACE(sym) CONC(_CUSTOM_TINY_SHA3_, sym)
+
+/******************************************************************************
+ * Name:        MLKEM_USE_NATIVE
+ *
+ * Description: Determines whether a native backend should
+ *              be used, if available.
+ *
+ *              This can also be set using CFLAGS.
+ *
+ *****************************************************************************/
+#define MLKEM_USE_NATIVE
+
+/******************************************************************************
+ * Name:        MLKEM_NATIVE_ARITH_BACKEND
+ *
+ * Description: The arithmetic backend to use.
+ *
+ *              This must be the filename of an arithmetic
+ *              backend. The backend is expected to define
+ *
+ *              - MLKEM_NATIVE_ARITH_BACKEND_NAME
+ *
+ *                The name of the backend as used in the default namespace.
+ *
+ *              - MLKEM_NATIVE_ARITH_BACKEND_IMPL
+ *
+ *                The filename of the implementation of the arithmetic backend.
+ *
+ *              See the existing backends for more information.
+ *
+ *****************************************************************************/
+/* Let's pretend we don't want an arithmetic backend */
+/* #define MLKEM_NATIVE_ARITH_BACKEND "native/default.h" */
+
+/******************************************************************************
+ * Name:        MLKEM_NATIVE_FIPS202_BACKEND
+ *
+ * Description: The FIPS-202 backend to use.
+ *
+ *              This must be the filename of an FIPS-202
+ *              backend. The backend is expected to define
+ *
+ *              - MLKEM_NATIVE_FIPS202_BACKEND_NAME
+ *
+ *                The name of the backend as used in the default namespace.
+ *
+ *              - MLKEM_NATIVE_FIPS202_BACKEND_IMPL
+ *
+ *                The filename of the implementation of the FIPS-202 backend.
+ *
+ *              See the existing backends for more information.
+ *
+ *****************************************************************************/
+#define MLKEM_NATIVE_FIPS202_BACKEND "fips202/native/custom/custom.h"
+
+#endif /* MLkEM_NATIVE_CONFIG_H */
diff --git a/examples/custom_backend/mlkem_native/fips202/LICENSE b/examples/custom_backend/mlkem_native/fips202/LICENSE
new file mode 120000
index 000000000..bcebb16c1
--- /dev/null
+++ b/examples/custom_backend/mlkem_native/fips202/LICENSE
@@ -0,0 +1 @@
+../../../../fips202/LICENSE
\ No newline at end of file
diff --git a/examples/custom_backend/mlkem_native/fips202/fips202.c b/examples/custom_backend/mlkem_native/fips202/fips202.c
new file mode 120000
index 000000000..34cf3813b
--- /dev/null
+++ b/examples/custom_backend/mlkem_native/fips202/fips202.c
@@ -0,0 +1 @@
+../../../../fips202/fips202.c
\ No newline at end of file
diff --git a/examples/custom_backend/mlkem_native/fips202/fips202.h b/examples/custom_backend/mlkem_native/fips202/fips202.h
new file mode 120000
index 000000000..164a3b7b8
--- /dev/null
+++ b/examples/custom_backend/mlkem_native/fips202/fips202.h
@@ -0,0 +1 @@
+../../../../fips202/fips202.h
\ No newline at end of file
diff --git a/examples/custom_backend/mlkem_native/fips202/fips202_backend.h b/examples/custom_backend/mlkem_native/fips202/fips202_backend.h
new file mode 120000
index 000000000..5397725dc
--- /dev/null
+++ b/examples/custom_backend/mlkem_native/fips202/fips202_backend.h
@@ -0,0 +1 @@
+../../../../fips202/fips202_backend.h
\ No newline at end of file
diff --git a/examples/custom_backend/mlkem_native/fips202/fips202x4.c b/examples/custom_backend/mlkem_native/fips202/fips202x4.c
new file mode 120000
index 000000000..fd9ec60d1
--- /dev/null
+++ b/examples/custom_backend/mlkem_native/fips202/fips202x4.c
@@ -0,0 +1 @@
+../../../../fips202/fips202x4.c
\ No newline at end of file
diff --git a/examples/custom_backend/mlkem_native/fips202/fips202x4.h b/examples/custom_backend/mlkem_native/fips202/fips202x4.h
new file mode 120000
index 000000000..4dc5c3b80
--- /dev/null
+++ b/examples/custom_backend/mlkem_native/fips202/fips202x4.h
@@ -0,0 +1 @@
+../../../../fips202/fips202x4.h
\ No newline at end of file
diff --git a/examples/custom_backend/mlkem_native/fips202/keccakf1600.c b/examples/custom_backend/mlkem_native/fips202/keccakf1600.c
new file mode 120000
index 000000000..53a5e8617
--- /dev/null
+++ b/examples/custom_backend/mlkem_native/fips202/keccakf1600.c
@@ -0,0 +1 @@
+../../../../fips202/keccakf1600.c
\ No newline at end of file
diff --git a/examples/custom_backend/mlkem_native/fips202/keccakf1600.h b/examples/custom_backend/mlkem_native/fips202/keccakf1600.h
new file mode 120000
index 000000000..fe6e51a4c
--- /dev/null
+++ b/examples/custom_backend/mlkem_native/fips202/keccakf1600.h
@@ -0,0 +1 @@
+../../../../fips202/keccakf1600.h
\ No newline at end of file
diff --git a/examples/custom_backend/mlkem_native/fips202/native/api.h b/examples/custom_backend/mlkem_native/fips202/native/api.h
new file mode 120000
index 000000000..b255ddf09
--- /dev/null
+++ b/examples/custom_backend/mlkem_native/fips202/native/api.h
@@ -0,0 +1 @@
+../../../../../fips202/native/api.h
\ No newline at end of file
diff --git a/examples/custom_backend/mlkem_native/fips202/native/custom/custom.h b/examples/custom_backend/mlkem_native/fips202/native/custom/custom.h
new file mode 100644
index 000000000..2ef194461
--- /dev/null
+++ b/examples/custom_backend/mlkem_native/fips202/native/custom/custom.h
@@ -0,0 +1,25 @@
+/*
+ * Copyright (c) 2024 The mlkem-native project authors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/* Default FIPS202 assembly profile for AArch64 systems */
+
+#ifdef FIPS202_NATIVE_PROFILE_H
+#error Only one FIPS202 assembly profile can be defined -- did you include multiple profiles?
+#else
+#define FIPS202_NATIVE_PROFILE_H
+
+/* Identifier for this backend so that source and assembly files
+ * in the build can be appropriately guarded. */
+#define MLKEM_NATIVE_FIPS202_BACKEND_CUSTOM_TINY_SHA3
+
+#define MLKEM_NATIVE_FIPS202_BACKEND_NAME TINY_SHA3
+
+/* Filename of the C backend implementation.
+ * This is not inlined here because this header is included in assembly
+ * files as well. */
+#define MLKEM_NATIVE_FIPS202_BACKEND_IMPL \
+  "fips202/native/custom/src/custom_impl.h"
+
+#endif /* FIPS202_NATIVE_PROFILE_H */
diff --git a/examples/custom_backend/mlkem_native/fips202/native/custom/src/LICENSE b/examples/custom_backend/mlkem_native/fips202/native/custom/src/LICENSE
new file mode 100644
index 000000000..35741e52a
--- /dev/null
+++ b/examples/custom_backend/mlkem_native/fips202/native/custom/src/LICENSE
@@ -0,0 +1,23 @@
+[//]: # (SPDX-License-Identifier: CC-BY-4.0)
+
+The MIT License (MIT)
+
+Copyright (c) 2015 Markku-Juhani O. Saarinen
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
diff --git a/examples/custom_backend/mlkem_native/fips202/native/custom/src/Makefile b/examples/custom_backend/mlkem_native/fips202/native/custom/src/Makefile
new file mode 100644
index 000000000..e855f986f
--- /dev/null
+++ b/examples/custom_backend/mlkem_native/fips202/native/custom/src/Makefile
@@ -0,0 +1,28 @@
+# SPDX-License-Identifier: MIT
+
+# Makefile
+# 19-Nov-11 Markku-Juhani O. Saarinen <mjos@iki.fi>
+
+BINARY          = sha3test
+OBJS     	= sha3.o main.o
+DIST            = tiny_sha3
+
+CC              = gcc
+CFLAGS		= -Wall -O3
+LIBS            =
+LDFLAGS         =
+INCLUDES	=
+
+$(BINARY):      $(OBJS)
+		$(CC) $(LDFLAGS) -o $(BINARY) $(OBJS) $(LIBS)
+
+.c.o:
+		$(CC) $(CFLAGS) $(INCLUDES) -c $< -o $@
+
+clean:
+		rm -rf $(DIST)-*.txz $(OBJS) $(BINARY) *~
+
+dist:		clean
+		cd ..; \
+		tar cfvJ $(DIST)/$(DIST)-`date -u "+%Y%m%d%H%M00"`.txz \
+			$(DIST)/*
diff --git a/examples/custom_backend/mlkem_native/fips202/native/custom/src/README.md b/examples/custom_backend/mlkem_native/fips202/native/custom/src/README.md
new file mode 100644
index 000000000..66684bec2
--- /dev/null
+++ b/examples/custom_backend/mlkem_native/fips202/native/custom/src/README.md
@@ -0,0 +1,63 @@
+[//]: # (SPDX-License-Identifier: MIT)
+
+# tiny_sha3
+Very small, readable implementation of the FIPS 202 and SHA3 hash function.
+Public domain.
+
+### Updated 27-Dec-15:
+
+Added SHAKE128 and SHAKE256 code and test vectors. The code can actually do
+a XOF of arbitrary size (like "SHAKE512").
+
+
+### Updated 03-Sep-15:
+
+Made the implementation portable. The API is now pretty much the
+same that OpenSSL uses.
+
+
+### Updated 07-Aug-15:
+
+Now that SHA3 spec is out, I've updated the package to match with the
+new padding rules. There is literally one line difference between
+Keccak 3.0 and SHA-3 implementations:
+
+```
+    temp[inlen++] = 0x06;           // XXX Padding Changed from Keccak 3.0
+```
+
+The 0x06 constant there used to be 0x01. But this of course totally
+breaks compatibility and test vectors had to be revised.
+
+SHA-3 Spec: http://nvlpubs.nist.gov/nistpubs/FIPS/NIST.FIPS.202.pdf
+
+Cheers,
+- markku
+
+
+### Original README.TXT from 19-Nov-11:
+
+Hi.
+
+The SHA-3 competition is nearing it's end and I would personally like
+to support Keccak as the winner. I have a PhD in hash function cryptanalysis
+so don't take my word for it, go ahead and look into the code !
+
+Since I couldn't find a *compact* and/or *readable* implementation of Keccak
+anywhere, here's one I cooked up as a service to the curious.
+
+This implementation is intended for study of the algorithm, not for
+production use.
+
+The code works correctly on 64-bit little-endian platforms with gcc.
+Like your Linux box. The main.c module contains self-tests for all
+officially supported hash sizes.
+
+If you're looking for production code, the official multi-megabyte package
+covers everyting you could possibly need and too much much more:
+http://keccak.noekeon.org/
+
+Cheers,
+- Markku  19-Nov-11
+
+Dr. Markku-Juhani O. Saarinen <mjos@iki.fi>
diff --git a/examples/custom_backend/mlkem_native/fips202/native/custom/src/custom_impl.h b/examples/custom_backend/mlkem_native/fips202/native/custom/src/custom_impl.h
new file mode 100644
index 000000000..242d6b7f4
--- /dev/null
+++ b/examples/custom_backend/mlkem_native/fips202/native/custom/src/custom_impl.h
@@ -0,0 +1,22 @@
+/*
+ * Copyright (c) 2024 The mlkem-native project authors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/* Default FIPS202 assembly profile for AArch64 systems */
+
+#ifdef FIPS202_NATIVE_PROFILE_IMPL_H
+#error Only one FIPS202 assembly profile can be defined -- did you include multiple profiles?
+#else
+#define FIPS202_NATIVE_PROFILE_IMPL_H
+
+#include "sha3.h"
+
+/* Replace (single) Keccak-F1600 by tiny-SHA3's */
+#define MLKEM_USE_FIPS202_X1_NATIVE
+static INLINE void keccak_f1600_x1_native(uint64_t *state)
+{
+  sha3_keccakf(state);
+}
+
+#endif /* FIPS202_NATIVE_PROFILE_H */
diff --git a/examples/custom_backend/mlkem_native/fips202/native/custom/src/sha3.c b/examples/custom_backend/mlkem_native/fips202/native/custom/src/sha3.c
new file mode 100644
index 000000000..c21684ae5
--- /dev/null
+++ b/examples/custom_backend/mlkem_native/fips202/native/custom/src/sha3.c
@@ -0,0 +1,197 @@
+// SPDX-License-Identifier: MIT
+//
+// sha3.c
+// 19-Nov-11  Markku-Juhani O. Saarinen <mjos@iki.fi>
+
+// Revised 07-Aug-15 to match with official release of FIPS PUB 202 "SHA3"
+// Revised 03-Sep-15 for portability + OpenSSL - style API
+
+#include "sha3.h"
+
+// update the state with given number of rounds
+
+void sha3_keccakf(uint64_t st[25])
+{
+  // constants
+  const uint64_t keccakf_rndc[24] = {
+      0x0000000000000001, 0x0000000000008082, 0x800000000000808a,
+      0x8000000080008000, 0x000000000000808b, 0x0000000080000001,
+      0x8000000080008081, 0x8000000000008009, 0x000000000000008a,
+      0x0000000000000088, 0x0000000080008009, 0x000000008000000a,
+      0x000000008000808b, 0x800000000000008b, 0x8000000000008089,
+      0x8000000000008003, 0x8000000000008002, 0x8000000000000080,
+      0x000000000000800a, 0x800000008000000a, 0x8000000080008081,
+      0x8000000000008080, 0x0000000080000001, 0x8000000080008008};
+  const int keccakf_rotc[24] = {1,  3,  6,  10, 15, 21, 28, 36, 45, 55, 2,  14,
+                                27, 41, 56, 8,  25, 43, 62, 18, 39, 61, 20, 44};
+  const int keccakf_piln[24] = {10, 7,  11, 17, 18, 3, 5,  16, 8,  21, 24, 4,
+                                15, 23, 19, 13, 12, 2, 20, 14, 22, 9,  6,  1};
+
+  // variables
+  int i, j, r;
+  uint64_t t, bc[5];
+
+#if __BYTE_ORDER__ != __ORDER_LITTLE_ENDIAN__
+  uint8_t *v;
+
+  // endianess conversion. this is redundant on little-endian targets
+  for (i = 0; i < 25; i++)
+  {
+    v = (uint8_t *)&st[i];
+    st[i] = ((uint64_t)v[0]) | (((uint64_t)v[1]) << 8) |
+            (((uint64_t)v[2]) << 16) | (((uint64_t)v[3]) << 24) |
+            (((uint64_t)v[4]) << 32) | (((uint64_t)v[5]) << 40) |
+            (((uint64_t)v[6]) << 48) | (((uint64_t)v[7]) << 56);
+  }
+#endif
+
+  // actual iteration
+  for (r = 0; r < KECCAKF_ROUNDS; r++)
+  {
+    // Theta
+    for (i = 0; i < 5; i++)
+      bc[i] = st[i] ^ st[i + 5] ^ st[i + 10] ^ st[i + 15] ^ st[i + 20];
+
+    for (i = 0; i < 5; i++)
+    {
+      t = bc[(i + 4) % 5] ^ ROTL64(bc[(i + 1) % 5], 1);
+      for (j = 0; j < 25; j += 5)
+        st[j + i] ^= t;
+    }
+
+    // Rho Pi
+    t = st[1];
+    for (i = 0; i < 24; i++)
+    {
+      j = keccakf_piln[i];
+      bc[0] = st[j];
+      st[j] = ROTL64(t, keccakf_rotc[i]);
+      t = bc[0];
+    }
+
+    //  Chi
+    for (j = 0; j < 25; j += 5)
+    {
+      for (i = 0; i < 5; i++)
+        bc[i] = st[j + i];
+      for (i = 0; i < 5; i++)
+        st[j + i] ^= (~bc[(i + 1) % 5]) & bc[(i + 2) % 5];
+    }
+
+    //  Iota
+    st[0] ^= keccakf_rndc[r];
+  }
+
+#if __BYTE_ORDER__ != __ORDER_LITTLE_ENDIAN__
+  // endianess conversion. this is redundant on little-endian targets
+  for (i = 0; i < 25; i++)
+  {
+    v = (uint8_t *)&st[i];
+    t = st[i];
+    v[0] = t & 0xFF;
+    v[1] = (t >> 8) & 0xFF;
+    v[2] = (t >> 16) & 0xFF;
+    v[3] = (t >> 24) & 0xFF;
+    v[4] = (t >> 32) & 0xFF;
+    v[5] = (t >> 40) & 0xFF;
+    v[6] = (t >> 48) & 0xFF;
+    v[7] = (t >> 56) & 0xFF;
+  }
+#endif
+}
+
+// Initialize the context for SHA3
+
+int sha3_init(sha3_ctx_t *c, int mdlen)
+{
+  int i;
+
+  for (i = 0; i < 25; i++)
+    c->st.q[i] = 0;
+  c->mdlen = mdlen;
+  c->rsiz = 200 - 2 * mdlen;
+  c->pt = 0;
+
+  return 1;
+}
+
+// update state with more data
+
+int sha3_update(sha3_ctx_t *c, const void *data, size_t len)
+{
+  size_t i;
+  int j;
+
+  j = c->pt;
+  for (i = 0; i < len; i++)
+  {
+    c->st.b[j++] ^= ((const uint8_t *)data)[i];
+    if (j >= c->rsiz)
+    {
+      sha3_keccakf(c->st.q);
+      j = 0;
+    }
+  }
+  c->pt = j;
+
+  return 1;
+}
+
+// finalize and output a hash
+
+int sha3_final(void *md, sha3_ctx_t *c)
+{
+  int i;
+
+  c->st.b[c->pt] ^= 0x06;
+  c->st.b[c->rsiz - 1] ^= 0x80;
+  sha3_keccakf(c->st.q);
+
+  for (i = 0; i < c->mdlen; i++)
+  {
+    ((uint8_t *)md)[i] = c->st.b[i];
+  }
+
+  return 1;
+}
+
+// compute a SHA-3 hash (md) of given byte length from "in"
+
+void *sha3(const void *in, size_t inlen, void *md, int mdlen)
+{
+  sha3_ctx_t sha3;
+
+  sha3_init(&sha3, mdlen);
+  sha3_update(&sha3, in, inlen);
+  sha3_final(md, &sha3);
+
+  return md;
+}
+
+// SHAKE128 and SHAKE256 extensible-output functionality
+
+void shake_xof(sha3_ctx_t *c)
+{
+  c->st.b[c->pt] ^= 0x1F;
+  c->st.b[c->rsiz - 1] ^= 0x80;
+  sha3_keccakf(c->st.q);
+  c->pt = 0;
+}
+
+void shake_out(sha3_ctx_t *c, void *out, size_t len)
+{
+  size_t i;
+  int j;
+
+  j = c->pt;
+  for (i = 0; i < len; i++)
+  {
+    if (j >= c->rsiz)
+    {
+      sha3_keccakf(c->st.q);
+      j = 0;
+    }
+    ((uint8_t *)out)[i] = c->st.b[j++];
+  }
+  c->pt = j;
+}
diff --git a/examples/custom_backend/mlkem_native/fips202/native/custom/src/sha3.h b/examples/custom_backend/mlkem_native/fips202/native/custom/src/sha3.h
new file mode 100644
index 000000000..fb5276059
--- /dev/null
+++ b/examples/custom_backend/mlkem_native/fips202/native/custom/src/sha3.h
@@ -0,0 +1,50 @@
+// SPDX-License-Identifier: MIT
+//
+// sha3.h
+// 19-Nov-11  Markku-Juhani O. Saarinen <mjos@iki.fi>
+
+#ifndef SHA3_H
+#define SHA3_H
+
+#include <stddef.h>
+#include <stdint.h>
+
+#ifndef KECCAKF_ROUNDS
+#define KECCAKF_ROUNDS 24
+#endif
+
+#ifndef ROTL64
+#define ROTL64(x, y) (((x) << (y)) | ((x) >> (64 - (y))))
+#endif
+
+// state context
+typedef struct
+{
+  union
+  {                  // state:
+    uint8_t b[200];  // 8-bit bytes
+    uint64_t q[25];  // 64-bit words
+  } st;
+  int pt, rsiz, mdlen;  // these don't overflow
+} sha3_ctx_t;
+
+// Compression function.
+void sha3_keccakf(uint64_t st[25]);
+
+// OpenSSL - like interfece
+int sha3_init(sha3_ctx_t *c, int mdlen);  // mdlen = hash output in bytes
+int sha3_update(sha3_ctx_t *c, const void *data, size_t len);
+int sha3_final(void *md, sha3_ctx_t *c);  // digest goes to md
+
+// compute a sha3 hash (md) of given byte length from "in"
+void *sha3(const void *in, size_t inlen, void *md, int mdlen);
+
+// SHAKE128 and SHAKE256 extensible-output functions
+#define shake128_init(c) sha3_init(c, 16)
+#define shake256_init(c) sha3_init(c, 32)
+#define shake_update sha3_update
+
+void shake_xof(sha3_ctx_t *c);
+void shake_out(sha3_ctx_t *c, void *out, size_t len);
+
+#endif
diff --git a/examples/custom_backend/mlkem_native/mlkem/LICENSE b/examples/custom_backend/mlkem_native/mlkem/LICENSE
new file mode 120000
index 000000000..8fae44d93
--- /dev/null
+++ b/examples/custom_backend/mlkem_native/mlkem/LICENSE
@@ -0,0 +1 @@
+../../../../mlkem/LICENSE
\ No newline at end of file
diff --git a/examples/custom_backend/mlkem_native/mlkem/arith_backend.h b/examples/custom_backend/mlkem_native/mlkem/arith_backend.h
new file mode 120000
index 000000000..c7abf8ecd
--- /dev/null
+++ b/examples/custom_backend/mlkem_native/mlkem/arith_backend.h
@@ -0,0 +1 @@
+../../../../mlkem/arith_backend.h
\ No newline at end of file
diff --git a/examples/custom_backend/mlkem_native/mlkem/cbd.c b/examples/custom_backend/mlkem_native/mlkem/cbd.c
new file mode 120000
index 000000000..66131480c
--- /dev/null
+++ b/examples/custom_backend/mlkem_native/mlkem/cbd.c
@@ -0,0 +1 @@
+../../../../mlkem/cbd.c
\ No newline at end of file
diff --git a/examples/custom_backend/mlkem_native/mlkem/cbd.h b/examples/custom_backend/mlkem_native/mlkem/cbd.h
new file mode 120000
index 000000000..941b8bd44
--- /dev/null
+++ b/examples/custom_backend/mlkem_native/mlkem/cbd.h
@@ -0,0 +1 @@
+../../../../mlkem/cbd.h
\ No newline at end of file
diff --git a/examples/custom_backend/mlkem_native/mlkem/cbmc.h b/examples/custom_backend/mlkem_native/mlkem/cbmc.h
new file mode 120000
index 000000000..1d7ad6bf2
--- /dev/null
+++ b/examples/custom_backend/mlkem_native/mlkem/cbmc.h
@@ -0,0 +1 @@
+../../../../mlkem/cbmc.h
\ No newline at end of file
diff --git a/examples/custom_backend/mlkem_native/mlkem/common.h b/examples/custom_backend/mlkem_native/mlkem/common.h
new file mode 120000
index 000000000..f6ec75f66
--- /dev/null
+++ b/examples/custom_backend/mlkem_native/mlkem/common.h
@@ -0,0 +1 @@
+../../../../mlkem/common.h
\ No newline at end of file
diff --git a/examples/custom_backend/mlkem_native/mlkem/config.h b/examples/custom_backend/mlkem_native/mlkem/config.h
new file mode 120000
index 000000000..a71bed830
--- /dev/null
+++ b/examples/custom_backend/mlkem_native/mlkem/config.h
@@ -0,0 +1 @@
+../../../../mlkem/config.h
\ No newline at end of file
diff --git a/examples/custom_backend/mlkem_native/mlkem/debug b/examples/custom_backend/mlkem_native/mlkem/debug
new file mode 120000
index 000000000..cd511d740
--- /dev/null
+++ b/examples/custom_backend/mlkem_native/mlkem/debug
@@ -0,0 +1 @@
+../../../../mlkem/debug
\ No newline at end of file
diff --git a/examples/custom_backend/mlkem_native/mlkem/indcpa.c b/examples/custom_backend/mlkem_native/mlkem/indcpa.c
new file mode 120000
index 000000000..56b7fc666
--- /dev/null
+++ b/examples/custom_backend/mlkem_native/mlkem/indcpa.c
@@ -0,0 +1 @@
+../../../../mlkem/indcpa.c
\ No newline at end of file
diff --git a/examples/custom_backend/mlkem_native/mlkem/indcpa.h b/examples/custom_backend/mlkem_native/mlkem/indcpa.h
new file mode 120000
index 000000000..6bec2894b
--- /dev/null
+++ b/examples/custom_backend/mlkem_native/mlkem/indcpa.h
@@ -0,0 +1 @@
+../../../../mlkem/indcpa.h
\ No newline at end of file
diff --git a/examples/custom_backend/mlkem_native/mlkem/kem.c b/examples/custom_backend/mlkem_native/mlkem/kem.c
new file mode 120000
index 000000000..2677344d2
--- /dev/null
+++ b/examples/custom_backend/mlkem_native/mlkem/kem.c
@@ -0,0 +1 @@
+../../../../mlkem/kem.c
\ No newline at end of file
diff --git a/examples/custom_backend/mlkem_native/mlkem/kem.h b/examples/custom_backend/mlkem_native/mlkem/kem.h
new file mode 120000
index 000000000..6f74e1af6
--- /dev/null
+++ b/examples/custom_backend/mlkem_native/mlkem/kem.h
@@ -0,0 +1 @@
+../../../../mlkem/kem.h
\ No newline at end of file
diff --git a/examples/custom_backend/mlkem_native/mlkem/namespace.h b/examples/custom_backend/mlkem_native/mlkem/namespace.h
new file mode 120000
index 000000000..c41101c01
--- /dev/null
+++ b/examples/custom_backend/mlkem_native/mlkem/namespace.h
@@ -0,0 +1 @@
+../../../../mlkem/namespace.h
\ No newline at end of file
diff --git a/examples/custom_backend/mlkem_native/mlkem/ntt.c b/examples/custom_backend/mlkem_native/mlkem/ntt.c
new file mode 120000
index 000000000..693bc94f2
--- /dev/null
+++ b/examples/custom_backend/mlkem_native/mlkem/ntt.c
@@ -0,0 +1 @@
+../../../../mlkem/ntt.c
\ No newline at end of file
diff --git a/examples/custom_backend/mlkem_native/mlkem/ntt.h b/examples/custom_backend/mlkem_native/mlkem/ntt.h
new file mode 120000
index 000000000..5ea1982c5
--- /dev/null
+++ b/examples/custom_backend/mlkem_native/mlkem/ntt.h
@@ -0,0 +1 @@
+../../../../mlkem/ntt.h
\ No newline at end of file
diff --git a/examples/custom_backend/mlkem_native/mlkem/params.h b/examples/custom_backend/mlkem_native/mlkem/params.h
new file mode 120000
index 000000000..f9fc45bfb
--- /dev/null
+++ b/examples/custom_backend/mlkem_native/mlkem/params.h
@@ -0,0 +1 @@
+../../../../mlkem/params.h
\ No newline at end of file
diff --git a/examples/custom_backend/mlkem_native/mlkem/poly.c b/examples/custom_backend/mlkem_native/mlkem/poly.c
new file mode 120000
index 000000000..8f37481c9
--- /dev/null
+++ b/examples/custom_backend/mlkem_native/mlkem/poly.c
@@ -0,0 +1 @@
+../../../../mlkem/poly.c
\ No newline at end of file
diff --git a/examples/custom_backend/mlkem_native/mlkem/poly.h b/examples/custom_backend/mlkem_native/mlkem/poly.h
new file mode 120000
index 000000000..6aa17ed35
--- /dev/null
+++ b/examples/custom_backend/mlkem_native/mlkem/poly.h
@@ -0,0 +1 @@
+../../../../mlkem/poly.h
\ No newline at end of file
diff --git a/examples/custom_backend/mlkem_native/mlkem/polyvec.c b/examples/custom_backend/mlkem_native/mlkem/polyvec.c
new file mode 120000
index 000000000..358c967ff
--- /dev/null
+++ b/examples/custom_backend/mlkem_native/mlkem/polyvec.c
@@ -0,0 +1 @@
+../../../../mlkem/polyvec.c
\ No newline at end of file
diff --git a/examples/custom_backend/mlkem_native/mlkem/polyvec.h b/examples/custom_backend/mlkem_native/mlkem/polyvec.h
new file mode 120000
index 000000000..b3a001d95
--- /dev/null
+++ b/examples/custom_backend/mlkem_native/mlkem/polyvec.h
@@ -0,0 +1 @@
+../../../../mlkem/polyvec.h
\ No newline at end of file
diff --git a/examples/custom_backend/mlkem_native/mlkem/randombytes.h b/examples/custom_backend/mlkem_native/mlkem/randombytes.h
new file mode 120000
index 000000000..d1b4aeb9e
--- /dev/null
+++ b/examples/custom_backend/mlkem_native/mlkem/randombytes.h
@@ -0,0 +1 @@
+../../../../mlkem/randombytes.h
\ No newline at end of file
diff --git a/examples/custom_backend/mlkem_native/mlkem/reduce.h b/examples/custom_backend/mlkem_native/mlkem/reduce.h
new file mode 120000
index 000000000..6b2408820
--- /dev/null
+++ b/examples/custom_backend/mlkem_native/mlkem/reduce.h
@@ -0,0 +1 @@
+../../../../mlkem/reduce.h
\ No newline at end of file
diff --git a/examples/custom_backend/mlkem_native/mlkem/rej_uniform.c b/examples/custom_backend/mlkem_native/mlkem/rej_uniform.c
new file mode 120000
index 000000000..bcc45b521
--- /dev/null
+++ b/examples/custom_backend/mlkem_native/mlkem/rej_uniform.c
@@ -0,0 +1 @@
+../../../../mlkem/rej_uniform.c
\ No newline at end of file
diff --git a/examples/custom_backend/mlkem_native/mlkem/rej_uniform.h b/examples/custom_backend/mlkem_native/mlkem/rej_uniform.h
new file mode 120000
index 000000000..1d2f26cdb
--- /dev/null
+++ b/examples/custom_backend/mlkem_native/mlkem/rej_uniform.h
@@ -0,0 +1 @@
+../../../../mlkem/rej_uniform.h
\ No newline at end of file
diff --git a/examples/custom_backend/mlkem_native/mlkem/symmetric.h b/examples/custom_backend/mlkem_native/mlkem/symmetric.h
new file mode 120000
index 000000000..a44e26f71
--- /dev/null
+++ b/examples/custom_backend/mlkem_native/mlkem/symmetric.h
@@ -0,0 +1 @@
+../../../../mlkem/symmetric.h
\ No newline at end of file
diff --git a/examples/custom_backend/mlkem_native/mlkem/sys.h b/examples/custom_backend/mlkem_native/mlkem/sys.h
new file mode 120000
index 000000000..c6b25c7f2
--- /dev/null
+++ b/examples/custom_backend/mlkem_native/mlkem/sys.h
@@ -0,0 +1 @@
+../../../../mlkem/sys.h
\ No newline at end of file
diff --git a/examples/custom_backend/mlkem_native/mlkem/verify.c b/examples/custom_backend/mlkem_native/mlkem/verify.c
new file mode 120000
index 000000000..d49e9df42
--- /dev/null
+++ b/examples/custom_backend/mlkem_native/mlkem/verify.c
@@ -0,0 +1 @@
+../../../../mlkem/verify.c
\ No newline at end of file
diff --git a/examples/custom_backend/mlkem_native/mlkem/verify.h b/examples/custom_backend/mlkem_native/mlkem/verify.h
new file mode 120000
index 000000000..82ae31329
--- /dev/null
+++ b/examples/custom_backend/mlkem_native/mlkem/verify.h
@@ -0,0 +1 @@
+../../../../mlkem/verify.h
\ No newline at end of file
diff --git a/examples/custom_backend/mlkem_native/mlkem/zetas.c b/examples/custom_backend/mlkem_native/mlkem/zetas.c
new file mode 120000
index 000000000..4e1a91dda
--- /dev/null
+++ b/examples/custom_backend/mlkem_native/mlkem/zetas.c
@@ -0,0 +1 @@
+../../../../mlkem/zetas.c
\ No newline at end of file
diff --git a/examples/custom_backend/test_only_rng/notrandombytes.c b/examples/custom_backend/test_only_rng/notrandombytes.c
new file mode 100644
index 000000000..93807622d
--- /dev/null
+++ b/examples/custom_backend/test_only_rng/notrandombytes.c
@@ -0,0 +1,96 @@
+/*
+ * Copyright (c) 2024 The mlkem-native project authors
+ * SPDX-License-Identifier: LicenseRef-PD-hp OR CC0-1.0 OR 0BSD OR MIT-0 OR MI
+ * Based on https://cr.yp.to/papers.html#surf by Daniel. J. Bernstein
+ */
+
+/**
+ * WARNING
+ *
+ * The randombytes() implementation in this file is for TESTING ONLY.
+ * You MUST NOT use this implementation outside of testing.
+ *
+ */
+
+#warning !!! WARNING !!!
+#warning THIS BUILD IS USING A TEST-ONLY RANDOM NUMBER GENERATOR WHICH MUST NOT BE USED IN PRODUCTION
+
+#include <stdint.h>
+#include "randombytes.h"
+
+static uint32_t seed[32] = {3, 1, 4, 1, 5, 9, 2, 6, 5, 3, 5, 8, 9, 7, 9, 3,
+                            2, 3, 8, 4, 6, 2, 6, 4, 3, 3, 8, 3, 2, 7, 9, 5};
+static uint32_t in[12];
+static uint32_t out[8];
+static int32_t outleft = 0;
+
+#define ROTATE(x, b) (((x) << (b)) | ((x) >> (32 - (b))))
+#define MUSH(i, b) x = t[i] += (((x ^ seed[i]) + sum) ^ ROTATE(x, b));
+
+static void surf(void)
+{
+  uint32_t t[12];
+  uint32_t x;
+  uint32_t sum = 0;
+  int32_t r;
+  int32_t i;
+  int32_t loop;
+
+  for (i = 0; i < 12; ++i)
+  {
+    t[i] = in[i] ^ seed[12 + i];
+  }
+  for (i = 0; i < 8; ++i)
+  {
+    out[i] = seed[24 + i];
+  }
+  x = t[11];
+  for (loop = 0; loop < 2; ++loop)
+  {
+    for (r = 0; r < 16; ++r)
+    {
+      sum += 0x9e3779b9;
+      MUSH(0, 5)
+      MUSH(1, 7)
+      MUSH(2, 9)
+      MUSH(3, 13)
+      MUSH(4, 5)
+      MUSH(5, 7)
+      MUSH(6, 9)
+      MUSH(7, 13)
+      MUSH(8, 5)
+      MUSH(9, 7)
+      MUSH(10, 9)
+      MUSH(11, 13)
+    }
+    for (i = 0; i < 8; ++i)
+    {
+      out[i] ^= t[i + 4];
+    }
+  }
+}
+
+void randombytes(uint8_t *buf, size_t n)
+{
+  while (n > 0)
+  {
+    if (!outleft)
+    {
+      if (!++in[0])
+      {
+        if (!++in[1])
+        {
+          if (!++in[2])
+          {
+            ++in[3];
+          }
+        }
+      }
+      surf();
+      outleft = 8;
+    }
+    *buf = (uint8_t)out[--outleft];
+    ++buf;
+    --n;
+  }
+}
diff --git a/scripts/ci/lint b/scripts/ci/lint
index 406f00526..879ad13b2 100755
--- a/scripts/ci/lint
+++ b/scripts/ci/lint
@@ -85,7 +85,7 @@ check-spdx()
       success=false
     fi
   done
-  for file in $(git ls-files -- "*.[chsS]" "*.py" ":/!cbmc/*.py" ":/!examples/bring_your_own_fips202/custom_fips202/tiny_sha3/*"); do
+  for file in $(git ls-files -- "*.[chsS]" "*.py" ":/!cbmc/*.py" ":/!examples/bring_your_own_fips202/custom_fips202/tiny_sha3/*" ":/!examples/custom_backend/mlkem_native/fips202/native/custom/src/*"); do
     # Ignore symlinks
     if [[ ! -L $file && $(grep "Copyright (c) 2024 The mlkem-native project authors" $file | wc -l) == 0 ]]; then
       echo "::error file=$file,line=${line:-1},title=Missing copyright header error::$file is missing copyright header"