Calculate a 3 byte checksum in configuration file

Question

I have 3 different encrypted configuration files.

The first line of the files are always the same (M1.28...).

Following lines are as follow for exmaple:

• 1N].1f01f ("1N]" is the checksum for the previous line, "." is the same in each line, "1f01f" is different per line, but always the same in the configuration files.)
• 256 charactes long string (Algorithm for encrypting / decrypting this content I alredy figured out)

So we need to find out how the checksum (for example here "1N]").

• The checksum changes if any changes in previous line string.
• The checksum always changes if line number changes

So the checksum is calculated based on these 2 things.

I also figured out that only following alphabet is used for all content of each configuration file:

0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[]^_`abcdefghijklmnopqrstuvwxy

And the first character of the checksum has only following alphabet:

0123456789:;

Can anyone please help me and find out the calculation algorithm ?

Configuration files:

https://anonfiles.com/Pfx6x0h3zc/File1_lng

https://anonfiles.com/Oex1x8hbz2/File2_lng

https://anonfiles.com/a5y1x2h8z0/File3_lng

See Screenshot:

https://ibb.co/Pt8YrnV

Answer 1

I found two example line in another file:

1f0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000

The checksum for this is:

7U>

Another example:

1f01f0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000

Checksum:

:Ih

Any idea?