seed RNG from entropy source (#11)

seed RNG from entropy source + code/doc improvements

CMakeLists.txt

@@ -39,6 +39,7 @@ if(${PICO_BOARD} STREQUAL pico_w)
   target_link_libraries(${PICO_IMAGE}
     pico_stdlib
     pico_unique_id
+    pico_rand
     hardware_flash
     hardware_sync
     tinyusb_device
@@ -50,6 +51,7 @@ elseif(${PICO_BOARD} STREQUAL pico2)
   target_link_libraries(${PICO_IMAGE}
     pico_stdlib
     pico_unique_id
+    pico_rand
     pico_sha256
     hardware_flash
     hardware_sync
@@ -61,6 +63,7 @@ else()
   target_link_libraries(${PICO_IMAGE}
     pico_stdlib
     pico_unique_id
+    pico_rand
     hardware_flash
     hardware_sync
     tinyusb_device

README.md

@@ -16,6 +16,11 @@ I'm not a security expert and the device/software is almost certainly not harden
 
 Pico, Pico W, Tiny2040 and Pico2 boards are known to work. Other RP2040/RP2350 boards have not been tested but are likely to (mostly) work. E.g. the Pico W requires the wifi driver purely for the LED (which is connected to the wifi chip) to function (though neither wifi nor bluetooth are enabled.)
 
+## Update v1.4.1
+
+- Use standard library implementation of `minstd_rand` and seed it with the TRNG from `pico_rand` (used by ECDSA).
+- Minor code improvements
+
 ## Update v1.4.0
 
 - Updates pico SDK to v2.0
@@ -102,7 +107,7 @@ You will then need to:
   sudo apt install gcc-arm-none-eabi cmake
   ```
 
-  NB 13.2.0 is recommended. A prebuilt RISC-V toolchain can be found [here](https://www.embecosm.com/resources/tool-chain-downloads/#riscv-stable).
+  NB 13.2.0 is recommended, but 10.3.1 and 14 seem to work too. A prebuilt RISC-V toolchain can be found [here](https://github.com/raspberrypi/pico-sdk-tools/releases/tag/v2.0.0-1).
 
 - download [pico-sdk](https://github.com/raspberrypi/pico-sdk) >= 2, see [here](https://www.raspberrypi.org/documentation/pico/getting-started/). NB This project uses a tagged release of pico-sdk, so download and extract e.g. [2.0.0](hhttps://github.com/raspberrypi/pico-sdk/archive/refs/tags/2.0.0.tar.gz)
 
@@ -118,16 +123,14 @@ You will then need to:
 
 - [**pico2 only**] tinyUSB 0.16.0 doesn't work. I used a cloned SDK with submodules (rather than a release) for pico2 builds
 
-- download [mbedtls](https://tls.mbed.org/api/): see also their [repo](https://github.com/ARMmbed/mbedtls). Currently using the 3.6.0 release/tag.
+- download [mbedtls](https://tls.mbed.org/api/): see also their [repo](https://github.com/ARMmbed/mbedtls). Currently using the 3.6.0 release/tag. This must be kept separate from the implementation in the SDK (which still on v2) and requires a custom configuration.
 
-  create a symlinks in the project root to the pico SDK and mbedtls, e.g.:
+  create a symlink in the project root to the pico SDK and mbedtls, e.g.:
 
   ```sh
   ln -s ../mbedtls-3.6.0 mbedtls
   ```
 
-  Not sure why, but I couldn't get it to work with the symlink inside pico-sdk like tinyusb.
-
 You should now have a structure something like this:
 
 ```txt

pico_crypto_key/build.py

@@ -27,12 +27,6 @@ def _get_config(board: str) -> dict[str, str]:
     return cfg["default"] | cfg[board]
 
 
-@app.command()
-def configure():
-    """Configure the project."""
-    raise NotImplementedError("TODO...")
-
-
 def _check_symlink(path: str) -> bool:
     path = Path(path)
     print(f"Checking {path} ", end="")
@@ -42,13 +36,14 @@ def _check_symlink(path: str) -> bool:
         print(f"-> {link}")
         return True
     elif path.is_dir():
+        print("")
         return True
     else:
         print("NOT FOUND")
         return False
 
 
-@app.command()
+@app.command(no_args_is_help=True)
 def check(board: str = typer.Option(help="the target board")):
     """Check the project configuration."""
 
@@ -84,7 +79,7 @@ def check(board: str = typer.Option(help="the target board")):
     print("check ok" if ok else "configuration errors found")
 
 
-@app.command()
+@app.command(no_args_is_help=True)
 def clean(board: str = typer.Option(help="the target board")) -> None:
     """Clean intermediate build files."""
     _get_config(board)  # load config just to check board name is valid
@@ -93,7 +88,7 @@ def clean(board: str = typer.Option(help="the target board")) -> None:
         shutil.rmtree(_build_dir(board))
 
 
-@app.command()
+@app.command(no_args_is_help=True)
 def build(board: str = typer.Option(help="the target board")) -> None:
     """Build the pico-crypto-key image."""
 
@@ -124,7 +119,7 @@ def build(board: str = typer.Option(help="the target board")) -> None:
     assert result.returncode == 0
 
 
-@app.command()
+@app.command(no_args_is_help=True)
 def install(
     device_path: str = typer.Argument(..., help="the path to the device storage, e.g. /media/${USER}/RPI-RP2"),
     board: str = typer.Option(help="the target board"),
@@ -145,7 +140,7 @@ def install(
     assert result.returncode == 0
 
 
-@app.command()
+@app.command(no_args_is_help=True)
 def reset_pin(
     device_path: str = typer.Argument(..., help="the path to the device storage, e.g. /media/${USER}/RPI-RP2"),
     board: str = typer.Option(help="the target board"),

pyproject.toml

@@ -7,7 +7,7 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name = "pico-crypto-key"
-version = "1.4.0"
+version = "1.4.1"
 authors = [
   { name="Andrew Smith", email="[email protected]" },
 ]

src/ecdsa.cpp

@@ -9,6 +9,10 @@
 #include "mbedtls/asn1write.h" // for local ecdsa_signature_to_asn1
 #include "mbedtls/error.h"     // for local ecdsa_signature_to_asn1
 
+#include "pico/rand.h"
+
+#include <random>
+
 #include <cstdint>
 #include <cstring>
 
@@ -38,12 +42,15 @@ int ecdsa_signature_to_asn1(const mbedtls_mpi* r, const mbedtls_mpi* s, unsigned
   return 0;
 }
 
-// int (*f_rng_blind)(void *, unsigned char *, size_t)
+#if !defined(PICO_RAND_SEED_ENTROPY_SRC_ROSC) | !defined(PICO_RAND_ENTROPY_SRC_TIME)
+#error Entropy sources not enabled
+#endif
+
+// ECDSA requires this signature
 extern "C" int minstd_rand(void*, byte* p, size_t n) {
-  static uint32_t r = 1;
+  static std::minstd_rand rng(get_rand_32());
   for (size_t i = 0; i < n; ++i) {
-    r = r * 48271 % 2147483647;
-    p[i] = static_cast<byte>(r); // % 256;
+    p[i] = static_cast<byte>(rng());
   }
   return 0;
 }

src/sha256.cpp

@@ -2,9 +2,10 @@
 #include "error.h"
 #include "usb_cdc.h"
 
-#include "mbedtls/sha256.h"
 #ifdef PICO_RP2350
 #include "pico/sha256.h"
+#else
+#include "mbedtls/sha256.h"
 #endif
 
 namespace {
@@ -50,7 +51,6 @@ bytes sha256::hash_in() {
 
   pico_sha256_finish(&state, &result);
   bytes hash(result.bytes, result.bytes + SHA256_RESULT_BYTES);
-
 #else
   wrap<mbedtls_sha256_context> ctx(mbedtls_sha256_init, mbedtls_sha256_free);
   mbedtls_sha256_starts(&ctx, 0);

src/utils.h

@@ -9,7 +9,7 @@ typedef std::vector<byte> bytes;
 
 namespace std
 {
-// force byte to print as character 
+// force byte to print as character
 inline std::string to_string(uint8_t c)
 {
   return std::string(1, c);
@@ -34,8 +34,8 @@ using namespace std::string_literals;
 template <typename T> class wrap final {
 public:
   typedef T value_type;
-  typedef void (*f_init_t)(T*);
-  typedef void (*f_free_t)(T*);
+  typedef void (*f_init_t)(value_type*);
+  typedef void (*f_free_t)(value_type*);
 
   wrap(f_init_t init, f_free_t free) : m_struct(), m_deleter(free) { init(&m_struct); }
 
@@ -58,6 +58,6 @@ template <typename T> class wrap final {
   const value_type& operator*() const { return m_struct; }
 
 private:
-  T m_struct;
+  value_type m_struct;
   f_free_t m_deleter;
 };