Can someone please check my code (Yenc encrytion) Here the formula i used
The encoding process represents each octet of input data with a single corresponding encoded output character. The ASCII value of each output character is derived by the following simple formula:
O = (I+42) % 256
That is, the output value is equal to the ASCII value of each input character plus 42, all modulo 256. This reduces overhead by reducing the number of NULL characters (ASCII 00) that would otherwise have had needed to be escaped, since many binaries contain a disproportionately large number of NULLs).
And the code:
xor eax,eax
mov al, ;src ascii
add al,42
shr eax, 8 ;division of 256
mov ,al
Is this correct? its the phrases 'octet ' + 'modulo ' that have me asking....
thx. :)
The encoding process represents each octet of input data with a single corresponding encoded output character. The ASCII value of each output character is derived by the following simple formula:
O = (I+42) % 256
That is, the output value is equal to the ASCII value of each input character plus 42, all modulo 256. This reduces overhead by reducing the number of NULL characters (ASCII 00) that would otherwise have had needed to be escaped, since many binaries contain a disproportionately large number of NULLs).
And the code:
xor eax,eax
mov al, ;src ascii
add al,42
shr eax, 8 ;division of 256
mov ,al
Is this correct? its the phrases 'octet ' + 'modulo ' that have me asking....
thx. :)
modulo means taking the remainder left after division not a quotient.
movzx eax, byte ptr
add eax, 42
xor edx, edx
mov ecx, 256
div ecx
;now edx contains the needed remainder. 256 = 100h, so no div needed:
Ossa
movzx eax, byte ptr
add eax, 42
mov ,alOssa
It is pointless to bother with the modulus if you are dealing with byte operations as every addition has already a modulo 256 in it.
so just discard ah? :)
ie.
and eax, 0ffh
ie.
and eax, 0ffh
Your original equation is O = (I+42) % 256
If you want O to be in al and I is initally in al,
add al, 42
will give you (I+42) % 256
If you want O to be in al and I is initally in al,
add al, 42
will give you (I+42) % 256
Btw this is "encoding", not "encryption" :)