[General] subtracting ascii's hex value in a 64 bit hex value

i have some ascii characters in a char variable.ex - char buf[100]="ab*(z&";
So each of the char in that char array will have some hex value.ex- *'s hex value is '2a'.
And i have a long long int variable in which there is a 64 bit hex value.
ex- long long int k=0x0000888888888888;
Now i want to subtract the char buff's hex value in k.How to accomplish it.please help.
That is 0x0000888888888888
- 0x000061622a287a26

= 0x000027265e600e62 (final result in a buffer or a long variable is okay)

Later i want to reverse the result as 26e006e56272.It should be stored in a variable such that i should be able to access each byte from it(ex- '72'(last 2 hex digits)).
Am stuck in this part in a very big project from Harman technologies.Please do help.
Thank You in advance.

Some points missing in your specification. How should substraction be performed for length(buf) > 8? Also the unused buf space beyond terminating 0 has to be zero filled.

You can convert the buffer content (first 8 characters) to a long log variable like this

Thank you so much ...It helped a lot ..

It was very helpfull,there one more query,now i have subtracted result in a long long variable,
long long int k=0x61626364.
now i want to reverse the data in the variable k.that is ex- i want 'k' to be= 46362616.
thank you in advance....
am very greatful for you

Do want to reverse the number nibble (4-Bit entity)-wise? Or reverse the byte order? The example is somehow unclear because you showed a 32 Bit value for the long long (64-Bit) variable.

i want to reverse the number by nibble wise.for example -i have
unsigned long long int k=0x0000888804eaad0e; - I want to reverse it and need result in another long long int variable as-

result=q=0x0000e0daae408888 , if the result is like below is also okay

i,e- r=0xe0daae4088880000 , I need any of the above two results in unsigned long long it,How can i accomplish it,Thank you....Your response means a lot to me....