Linker error when trying to use openssl library

paddlix · July 27, 2023, 2:57pm

Hey,
I’m trying to use the openssl library for SHA256 encryption within my platformio project. Including the header files by using the “build_flags” option in the platformio.ini file worked fine (“-I/usr/local/include”) and the project compiles without any errors. But I’m facing the following error when linking all the files:
src/senddata.cpp:99: undefined reference to 'SHA256'.

I tried adding linkflags “-L/usr/local/lib -lcrypto” as described in the openssl doc, but it did not do anything.
I also tried adding the path of the entire openssl library to “lib_deps” in platformio.ini, but this just caused more errors.

Does anybody know a similar problem and can help me finding a solution for this?

Thanks a lot in advice.
Paddlix

Some information:
openssl installed as described in the openssl doc.
IDE: VSCode
OS: macOS 13.4.1

Complete Error

Linking .pio/build/usb/firmware_ttgov21new_v3.6.1.elf

/Users/../.platformio/packages/toolchain-xtensa-esp32@8.4.0+2021r2-patch5/bin/../lib/gcc/xtensa-esp32-elf/8.4.0/../../../../xtensa-esp32-elf/bin/ld: .pio/build/usb/src/senddata.cpp.o:(.literal._Z8sendDatav+0x4): undefined reference to 'SHA256'

/Users/../.platformio/packages/toolchain-xtensa-esp32@8.4.0+2021r2-patch5/bin/../lib/gcc/xtensa-esp32-elf/8.4.0/../../../../xtensa-esp32-elf/bin/ld: .pio/build/usb/src/senddata.cpp.o: in function 'sendData()':

/Users/../Library/Mobile Documents/com~apple~CloudDocs/Documents/../ESP32-Paxcounter-master/src/senddata.cpp:99: undefined reference to 'SHA256'

collect2: error: ld returned 1 exit status
*** [.pio/build/usb/firmware_ttgov21new_v3.6.1.elf] Error 1

maxgerhardt · July 27, 2023, 5:55pm

SHA256 is a hash, not encryption.

You’re compiling for Xtensa32, aka an ESP32. You cannot link in anything from your /usr/local/lib, as that will have the precompiled library for your computer, most likely x86_64. Those are binary incompatible.

If you don’t want the troubles of compiling OpenSSL from source for the ESP32, just use mbedTLS. That is already built into Arduino-ESP32 (and ESP-IDF) with the integration for the hardware accelators. Calculating a SHA256 hash is as simple as

github.com

espressif/arduino-esp32/blob/21b88659b9ded3fcc1082f23f498bc2a04cd4f1b/libraries/WiFiClientSecure/src/ssl_client.cpp#L498-L502


      
          mbedtls_sha256_context sha256_ctx;
          mbedtls_sha256_init(&sha256_ctx);
          mbedtls_sha256_starts(&sha256_ctx, false);
          mbedtls_sha256_update(&sha256_ctx, crt->raw.p, crt->raw.len);
          mbedtls_sha256_finish(&sha256_ctx, sha256);

with uint8_t sha256[32] that holds the output hash, crt->raw.p being the data pointer, crt->raw.len being the length of the data to hash. And of course you’ll need

#include <mbedtls/sha256.h>

paddlix · July 28, 2023, 6:55pm

Thanks, your explanation helped me a lot. That construct I was trying to create obviously cannot work.
And also thanks for the recommendation of using mbedtls. The library works just fine.