Python binary opposite operations

Question

I have two numbers:

num1 = 1
num2 = 1

And then someone applied a secret operation on them:

mask = 2**8 -1
secret = (num1 << 8) | (num2 & mask)
>>> 257

How can I do the opposite of the secret operation? to get from 257 to 1 1

You can't recover the original value of of num2 if it is greater than 255, as the mask will erase the high bits in the original. — chepner
– chepner, Commented Nov 13, 2021 at 21:57

Joran Beasley · Accepted Answer · 2021-11-13 22:02:12Z

2

this is a standard way of encoding 2 small integers into a larger one

its 16bits of data representing 2 8 bit integers

to get the first one (upper 8 bits) back you shift it the other way

256 >> 8

to get the lower 8 bits just and it with the original mask

256 & (2**8-1)

as mentioned in the comments most ints are int32 (32 bits, not 8) and you likely cannot recover num2 if its value was greater than 255, because 8 bits can only represent 0-255

as an aside I imagine most people would use (1<<8)-1 instead of 2**8-1

answered Nov 13, 2021 at 22:02

Joran Beasley

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1 Comment

Frank Yellin Over a year ago

You shouldn't have done this guy's homework. :-(

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