!boards/nsh: Replace reversible TEA password storage in /etc/passwd with one-way PBKDF2-HMAC-SHA256 hashing

[Abhishekmishra2808]

Summary

Replace reversible TEA password storage in /etc/passwd with one-way PBKDF2-HMAC-SHA256 hashing in modular crypt format (MCF): $pbkdf2-sha256$<iterations>$<base64url-salt>$<base64url-hash>

BREAKING CHANGE: TEA-encoded /etc/passwd entries no longer verify.
Regenerate with tools/mkpasswd or NSH passwd/useradd after upgrading nuttx-apps.

Changes:

• Add apps/crypto/pbkdf2 (PBKDF2-HMAC-SHA256; Mbed TLS backend when available, self-contained SHA-256 fallback otherwise)
• Rewrite passwd_encrypt.c / passwd_verify.c to use PBKDF2 and timingsafe_bcmp() for verification
• Add shared passwd_base64.c for base64url encode/decode
• Enforce password complexity when setting passwords (min 8 chars, upper, lower, digit, special character)
• Add testing/pbkdf2 (RFC 6070 vectors + passwd round-trip test)

Companion nuttx PR (host mkpasswd tool and ROMFS build integration): apache/nuttx-apps#3557

Impact

Security

• Passwords are stored as one-way hashes with per-password random salt and configurable iteration count (default 10000).
• Plaintext passwords are never written to /etc/passwd.

Compatibility (breaking)

• Existing TEA-encoded /etc/passwd entries will not verify. Users must regenerate passwords with NSH passwd / useradd or build-time mkpasswd after upgrading.
• Depends on companion nuttx PR for ROMFS autogen and host tooling.

Build

• Selects CRYPTO_PBKDF2 automatically when FSUTILS_PASSWD is enabled.

Runtime

• Login latency increases slightly on low MHz MCUs due to PBKDF2 iterations (tunable via CONFIG_FSUTILS_PASSWD_PBKDF2_ITERATIONS).

Testing

Host: WSL2 Linux, x86_64

Sim (sim:nsh or login-enabled config)

• Build succeeds with FSUTILS_PASSWD and TESTING_PBKDF2

abhishek@Lethallaptop:~/nuttx$ time make -j$(nproc)
Create version.h
LN: platform/board to /home/abhishek/nuttx-apps/platform/dummy
Register: gpio
Register: dd
Register: dumpstack
Register: nsh
Register: sh
Register: hello
CP:  /home/abhishek/nuttx/include/nuttx/config.h
CP:  /home/abhishek/nuttx/include/nuttx/fs/hostfs.h
ROMFS root password (min 8 characters): 
LD:  nuttx
Pac SIM with dynamic libs..
'/lib/x86_64-linux-gnu/libz.so.1' -> 'sim-pac/libs/libz.so.1'
'/lib/x86_64-linux-gnu/libc.so.6' -> 'sim-pac/libs/libc.so.6'
'/lib64/ld-linux-x86-64.so.2' -> 'sim-pac/ld-linux-x86-64.so.2'
SIM elf with dynamic libs archive in nuttx.tgz

real    0m51.470s
user    3m38.379s
sys     2m34.115s

ESP32-C3 (esp32c3-devkit:login, local test config, not in this PR)

• ROMFS /etc/passwd with PBKDF2 hash
• NSH console login over USB serial (/dev/ttyACM0) succeeds with configured password; wrong password rejected

abhishek@Lethallaptop:~/nuttx$ make -j$(nproc) CROSSDEV=riscv32-esp-elf-
Create version.h
Cloning Espressif HAL for 3rd Party Platforms
Clone: chip/esp-hal-3rdparty LN: platform/board to /home/abhishek/nuttx-apps/platform/dummy
Register: dumpstack
Register: nsh
Register: sh
Register: dd
Register: ostest
Register: pbkdf2_test
Register: getprime
Espressif HAL for 3rd Party Platforms: cleaning current repository...
Espressif HAL for 3rd Party Platforms: 0eb59f7e02a4735f2b9a78797e691b66740fcadb
ROMFS root password (min 8 characters): 
LD: nuttx
Memory region         Used Size  Region Size  %age Used
             ROM:      310900 B         4 MB      7.41%
     iram0_0_seg:       14768 B     321296 B      4.60%
     irom0_0_seg:      139684 B         4 MB      3.33%
     dram0_0_seg:       24512 B     321296 B      7.63%
     drom0_0_seg:      245328 B         4 MB      5.85%
    rtc_iram_seg:          52 B       8168 B      0.64%
rtc_reserved_seg:          0 GB         24 B      0.00%
CP: nuttx.hex
MKIMAGE: NuttX binary
esptool.py -c esp32c3 elf2image --ram-only-header -fs 4MB -fm dio -ff "80m" -o nuttx.bin nuttx
Warning: DEPRECATED: 'esptool.py' is deprecated. Please use 'esptool' instead. The '.py' suffix will be removed in a future major release.
esptool v5.3.0
Creating ESP32-C3 image...
Image has only RAM segments visible. ROM segments are hidden and SHA256 digest is not appended.
Merged 1 ELF section.
Successfully created ESP32-C3 image.
Generated: nuttx.bin

Manual verification

• useradd / passwd reject weak passwords (no special char, too short)
• Valid password produces $pbkdf2-sha256$... hash in /etc/passwd

[github-actions]

MemBrowse Memory Report

No memory changes detected for:

• arduino-mega2560
• esp32-devkitc
• hifive1-revb
• mirtoo
• qemu-armv8a
• qemu-intel64
• rx65n-rsk2mb
• s698pm-dkit
• stm32-nucleo-f103rb

[Abhishekmishra2808]

@acassis, Checks are failing because the password is currently empty, and CI expects some passwd value.
What would be the right way to fix CI?

[acassis]

@acassis, Checks are failing because the password is currently empty, and CI expects some passwd value. What would be the right way to fix CI?

Let's wait for @simbit18 suggestion. I think one option will be removing the board profile from CI testing

[simbit18]

Hi @Abhishekmishra2808 Is this a cross-platform solution (board_romfs_mkpasswd.sh)?

[Abhishekmishra2808]

Yes, on supported NuttX host environments

[simbit18]

This shell script (board_romfs_mkpasswd.sh) used in CMake (cmake/nuttx_add_romfs.cmake ) is not a cross-platform solution

      COMMAND
        ${NUTTX_DIR}/tools/board_romfs_mkpasswd.sh ${NUTTX_DIR}
        ${ROMFS_PASSWD_FILE} ${MKPASSWD_BIN} ${GENPASSWD_OUTPUT} --user
        "${CONFIG_BOARD_ETC_ROMFS_PASSWD_USER}" --uid
        ${CONFIG_BOARD_ETC_ROMFS_PASSWD_UID} --gid
        ${CONFIG_BOARD_ETC_ROMFS_PASSWD_GID} --home
        "${CONFIG_BOARD_ETC_ROMFS_PASSWD_HOME}"
      DEPENDS ${MKPASSWD_BIN} ${NUTTX_DIR}/.config
      COMMENT "Generating /etc/passwd from Kconfig values")

https://github.com/Abhishekmishra2808/nuttx/blob/084e0432d7943ad96c163d34dd5cbaea909fbdd6/cmake/nuttx_add_romfs.cmake#L318-L326

[Abhishekmishra2808]

I mixed up "works on supported NuttX Unix hosts" with "cross-platform in CMake," which isn't the same thing :(
Now I have updated cmake/nuttx_add_romfs.cmake to call the host mkpasswd binary directly again, reading the passwd and iterations from Kconfig