I am working on an kernel for my Real mode OS in Asm. x86, of course.
I want to write a subroutine for encryption of text input.
It doesn't need to be complicated, I was thinking just shift the ASCII value by an arbitrary number. I tried, but am not having any luck.
Help!!!

A simple xor would do this for you, not sure what that is in x86 though

I would suggest you to get a hold of the Phalkon Skism tutorials in virii making. There is quite a lot about encryption/decryption routines in Assembly. Normally you would go for a XOR cycle, if you want to keep it simple, but you can also make it more interesting, using an encryption that supports private and public keys.

Hi,

the problematic thing with shifts is that they are 'lossy'. When you shift the value 65 (Character 'A', 01000001 Binary) by,
lets say, 4 places to the right, you get 00000100. The right shift operation places binary zeros to the at left-most positions.
But when you now do the oposite operation, a left shift, which places binary zeros at the right-most places, you get
01000000, which is a decimal 64 and not 65 which would be the value you expect !!!

So said that the shift-operations are lossy, you need a lossless operation, such as rotation and/or XORing.
You could, for example, combine rotation with XORing. In the first phase rotate a byte and in the second phase you could
XOR the rotated value by a constant (Encryption). In the Decryption phase do the operations you did in the Encyption
phase in the oposite order: Here first XORing than rotation

Encryption: ENC(x) = ROTATE-LEFT (x, 4) XOR 0x65
Decryption: DECR(x) = ROTATE-RIGHT (x XOR 0x65, 4)

That's all
With regards,
Martin