user56157: I understand the HMAC with SHA256 example provides a set of keys:
(https://github.com/Cathedrow/Cryptosuite/blob/master/Sha/examples/hmacsha256test/hmacsha256test.pde)
#include "sha256.h"
uint8_t hmacKey1[]={
  0x0b,0x0b,0x0b,0x0b,0x0b,0x0b,0x0b,0x0b,0x0b,0x0b,0x0b,0x0b,0x0b,
0x0b,0x0b,0x0b,0x0b,0x0b,0x0b,0x0b
};
uint8_t hmacKey2[]={
  0x01,0x02,0x03,0x04,0x05,0x06,0x07,0x08,0x09,0x0a,0x0b,0x0c,0x0d,
0x0e,0x0f,0x10,0x11,0x12,0x13,0x14,0x15,0x16,0x17,0x18,0x19
};
...

I did a bit research and have a sense that these keys are in hexadecimal form with 0x added as prefix. If I like to use my own key, like a string, abece123*, do I just convert the key string to hex?
I tried to use this site below to convert above hmacKey2 to see what the key may look like. But the result is not recognizable. Here's what I did: copy and past
0x01,0x02,0x03,0x04,0x05,0x06,0x07,0x08,0x09,0x0a,0x0b,0x0c,0x0d,0x0e,
0x0f,0x10,0x11,0x12,0x13,0x14, 0x15,0x16,0x17,0x18,0x19

to the site below to convert to string. 
https://codebeautify.org/hex-string-converter
I am very new to this field. Please help me understand how these key works and and the steps needed to replace one of the preset hmac keys with my own key string (e.g. abece123*). Thanks.

user30198: Those are examples for keys. You need one key and you can choose any text with any length.
Did you see this line in the example:
sha.initHmac((uint8_t*)"Jefe",4);

It uses the key "Jefe" with a length of 4.
You can use your own key, but you also need the length.
char myKey[] = "abece123*";
int length = strlen(myKey);
sha.initHmac((uint8_t*)myKey, length);


56157: <p>I understand the HMAC with SHA256 example provides a set of keys:
(<a href="https://github.com/Cathedrow/Cryptosuite/blob/master/Sha/examples/hmacsha256test/hmacsha256test.pde" rel="nofollow noreferrer">https://github.com/Cathedrow/Cryptosuite/blob/master/Sha/examples/hmacsha256test/hmacsha256test.pde</a>)</p>

<pre><code>#include "sha256.h"
uint8_t hmacKey1[]={
  0x0b,0x0b,0x0b,0x0b,0x0b,0x0b,0x0b,0x0b,0x0b,0x0b,0x0b,0x0b,0x0b,
0x0b,0x0b,0x0b,0x0b,0x0b,0x0b,0x0b
};
uint8_t hmacKey2[]={
  0x01,0x02,0x03,0x04,0x05,0x06,0x07,0x08,0x09,0x0a,0x0b,0x0c,0x0d,
0x0e,0x0f,0x10,0x11,0x12,0x13,0x14,0x15,0x16,0x17,0x18,0x19
};
...
</code></pre>

<p>I did a bit research and have a sense that these keys are in hexadecimal form with 0x added as prefix. If I like to use my own key, like a string, abece123*, do I just convert the key string to hex?</p>

<p>I tried to use this site below to convert above hmacKey2 to see what the key may look like. But the result is not recognizable. Here's what I did: copy and past</p>

<pre><code>0x01,0x02,0x03,0x04,0x05,0x06,0x07,0x08,0x09,0x0a,0x0b,0x0c,0x0d,0x0e,
0x0f,0x10,0x11,0x12,0x13,0x14, 0x15,0x16,0x17,0x18,0x19
</code></pre>

<p>to the site below to convert to string. 
<a href="https://codebeautify.org/hex-string-converter" rel="nofollow noreferrer">https://codebeautify.org/hex-string-converter</a></p>

<p>I am very new to this field. Please help me understand how these key works and and the steps needed to replace one of the preset hmac keys with my own key string (e.g. abece123*). Thanks.</p>
 30198: <p>Those are examples for keys. You need one key and you can choose any text with any length.</p>

<p>Did you see this line in the example:</p>

<pre><code>sha.initHmac((uint8_t*)"Jefe",4);
</code></pre>

<p>It uses the key "Jefe" with a length of 4.<br>
You can use your own key, but you also need the length.</p>

<pre><code>char myKey[] = "abece123*";
int length = strlen(myKey);
sha.initHmac((uint8_t*)myKey, length);
</code></pre>
