B4R Code Snippet ESP32: HMAC with SHA256 (other md's will do, too) via inline c

Here's a short example how to generate a HMAC SHA256 hash (via passphrase). It's good to "sign" messages as a passphrase is used to hash a message,

Taken from: https://techtutorialsx.com/2018/01/25/esp32-arduino-applying-the-hmac-sha-256-mechanism/

Globals:

Public hmackey(255) As Byte
Public hmackey_len As ULong
Public hmacpayload(255) As Byte
Public hmacpayload_len As ULong
Public hmacbytes(32) As Byte

Payload ("the message") can be longer. I just have set it to 255.

Code:

Sub Hmac

    Dim s As String="secretKey"
    bc.ArrayCopy2(s,0,hmackey,0,s.Length)
    hmackey_len=s.Length
    
    Dim s As String="Hello HMAC SHA 256!"
    bc.ArrayCopy2(s,0,hmacpayload,0,s.Length)
    hmacpayload_len=s.Length
    
    
    RunNative("GenHmac", Null)
    Log(bc.HexFromBytes(hmacbytes))
    
End Sub

#if c

#include "mbedtls/md.h"

void GenHmac (B4R::Object* o) {
  mbedtls_md_context_t ctx;
  mbedtls_md_type_t md_type = MBEDTLS_MD_SHA256;

  mbedtls_md_init(&ctx);
  mbedtls_md_setup(&ctx, mbedtls_md_info_from_type(md_type), 1);
  mbedtls_md_hmac_starts(&ctx, (unsigned char *)b4r_main::_hmackey->data, b4r_main::_hmackey_len);
  mbedtls_md_hmac_update(&ctx, (unsigned char *)b4r_main::_hmacpayload->data, b4r_main::_hmacpayload_len);
  mbedtls_md_hmac_finish(&ctx, (unsigned char *)b4r_main::_hmacbytes->data);
  mbedtls_md_free(&ctx);
}
#End If

Helper to check the hash: https://www.freeformatter.com/hmac-generator.html#ad-output

Supported digest's:

Change the constant or use another varaible in Globals to use it by parameter. Change the size of the hashcode variable, too (e.g. 64 bytes for sha512)

MBEDTLS_MD_NONE     
None.

MBEDTLS_MD_MD2     
The MD2 message digest.

MBEDTLS_MD_MD4     
The MD4 message digest.

MBEDTLS_MD_MD5     
The MD5 message digest.

MBEDTLS_MD_SHA1     
The SHA-1 message digest.

MBEDTLS_MD_SHA224     
The SHA-224 message digest.

MBEDTLS_MD_SHA256     
The SHA-256 message digest.

MBEDTLS_MD_SHA384     
The SHA-384 message digest.

MBEDTLS_MD_SHA512     
The SHA-512 message digest.

MBEDTLS_MD_RIPEMD160     
The RIPEMD-160 message digest.