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fixed typos in picasso 

Signed-off-by: Julien CLEMENT <julien.clement@epita.fr>
This commit is contained in:
Julien CLEMENT 2023-05-04 11:24:30 +02:00
parent a680b32e79
commit bb81fd917e
1 changed files with 7 additions and 4 deletions
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jujure/content/writeups/fcsc_2023/picasso.md
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@ -91,15 +91,18 @@ So anyway we recover the index of the character in the alphabet and divide it
by 4. The remainder is kept for later and the quotient is used to index
a permutations matrix, which holds 6 permutation tables of 0x36 elements each.
The permutation matrix is linearized though so I already redefined the dimesions
so that's why it's showing really nicely in the code below. But to know the
dimensions I basically saw that the permutation matrix was indexed by `(pos / 4) * 0x36`, since our alphabet holds 24 characters, that gives us `24 / 4 == 6` arrays of size `0x36` in the matrix. We can confirm the 0x36 is we will iterate over `0x36` elements of the permutation array right after this.
The permutation matrix is linearized though so I already redefined the
dimesions so that's why it's showing really nicely in the code below. But to
know the dimensions I basically saw that the permutation matrix was indexed by
`(pos / 4) * 0x36`, since our alphabet holds 24 characters, that gives us `24 /
4 == 6` arrays of size `0x36` in the matrix. We can confirm the 0x36 because we
will iterate over `0x36` elements of the permutation array right after this.
We then copy the state and apply the permutations given by the fetched
permutation array. We repeat this process N times, N being `pos % 4`. We do not
actually really care about the copy, it is simply there as temporary buffer so
that permutations don't cancel each other or erase data in the state, it is
juste an implementation detail.
just an implementation detail.
So to sum this up, each character from our input will chose a permutation array
to apply to our state and how many times to apply the permutation. They are 6