HTTPS client on STM32 Nucleo-F411RE

Posted on 2016/02/04

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Nucleo board wired with Ethernet shield.

Nucleo board wired with Ethernet shield.

One of my goal of the past months was to use an STM32 Nucleo board to get a webpage through HTTPS, using an Ethernet shield for TCP/IP stack. I was close last time, but I discovered that my Nucleo-F103RB did not have enough RAM. Now I have my hands on a Nucleo-F411RE that has 128KiB of RAM and 512KiB of Flash, and I was finally able to do it.

The hardware is composed of:

  • Nucleo-F411RE
    • STM32F411RE microcontroller
    • SPI master to communicate with other chips
    • USART serial port with STLink USB connection for standard I/O
    • JTAG with STLink USB connection to flash program
  • Ethernet shield
    • Wiznet W5100 that implements the TCP/IP and UDP/IP stacks.
    • SPI connection to the board below.

The software that I uploaded on board is composed of:

  • Low-level drivers provided by libopencm3
  • W5100 SPI driver to expose the chip registers and memories.
  • Socket API: POSIX socket library developed by me on top of W5100 functionality.
  • USART minimal driver to write/read from serial port through the STLink USB connection on the Nucleo.
  • DNS: I developed a minimal implementation of getaddrinfo() to retrieve the IP address of a website from its name.
  • TLS: I used mbedTLS 2.2.0 for the Transport Layer Security 1.2
  • The certificate of the CA, that is embedded statically as a binary blob into the program (I used Chrome to download it beforehand).
  • The application is simply an HTTP GET request.

The tools I used to build the software are:

  • GCC ARM Embedded toolchain (version 4.9 2015q1) to compile and link the code,
  • OpenOCD (version 0.9.0) to write the software image into the STM32 embedded flash,
  • Chrome to download the CA certificate,
  • minicom to connect with the serial port
  • All on my Debian (Stretch) Linux box

To build and run:

  1. git checkout my nucleo_tests repo (now at commit eb2cc1942618f745286028c3cc591012b92d56bc).
  2. Go in libopencm3 directory and run “make” to compile the low-level drivers.
  3. Download mbedTLS 2.2.0 and unpack it in nucleo_tests directory.
  4. Open “nucleo_tests/tests/test.mk“, comment the stm32f103rb line and uncomment the stm32f411re line.
  5. Go in “nucleo_tests/tests/mbedtls” directory and run “make” to build both the mbedTLS library and the application.
  6. Connect the Nucleo board + Ethernet shield to the PC with USB, and connect the Ethernet cable.
  7. In another terminal run “minicom -b 57600 -D /dev/ttyACM0” to connect to the serial port.
  8. Run “make flash” to write the program into the STM32 embedded flash (uses openocd program)
  9. Wait for the Link LED on the Ethernet shield to turn on.
  10. Go in the minicom terminal, and you should see a prompt; press a key to continue.
  11. The terminal should print the page that has been retrieved through HTTPS.

This is the output I get from the serial port:

Press any key to continue...
Certificate:
cert. version     : 3
serial number     : 02:0B:70:F4:68:67:1E:47:13:96:63:3D:99:2C:E5:CD
issuer name       : C=US, O=DigiCert Inc, CN=DigiCert SHA2 Secure Server CA
subject name      : C=US, ST=Cali
HTTP/1.1 200 OK
Server: nginx
Date: Thu, 04 Feb 2016 20:41:12 GMT
Content-Type: text/html; charset=UTF-8
Transfer-Encoding: chunked
Connection: keep-alive
Last-Modified: Mon, 09 Nov 2015 23:59:43 GMT
ETag: "687f8a-bd-52424631079c0"
Cache-Control: max-age=1209600
Expires: Wed, 10 Feb 2016 15:34:38 GMT
Vary: Accept-Encoding
X-Varnish: 324309826 304737767
Age: 709594
Via: 1.1 varnish
Strict-Transport-Security: max-age=31536000; includeSubdomains
X-Content-Type-Options: nosniff
X-Frame-Options: SAMEORIGIN

bd
<html>
    <head>
        <title>Site Under Maintenance</title>
    </head>
    <body>


<h1>This site is currently under maintenance. Please try back later.</h1>


    </body>
</html>

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To conclude, this proof of concept shows a very small platform that can be used to build IoT applications with a layer of security. Instead of having an application processor running Linux, we have a microcontroller running at a frequency as low as 16MHz, the base software is around 90KiB of code running in flash, and needs around 50KiB of RAM, between static and dynamic memory. We are using a security layer that doesn’t invent anything new: it’s an implementation of the widely used TLS.

Posted in: Embedded