If the message is tiny, does it matter … by tomxor

function u(t) {

c.width=1920 // clear the canvas
for(i=0;i<9;i++)
x.fillRect(400+i*100+S(t)*300,400,50,200) // draw 50x200 rects

} // 122/140

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    u(t) is called 60 times per second.
    t: Elapsed time in seconds.
    S: Shorthand for Math.sin.
    C: Shorthand for Math.cos.
    T: Shorthand for Math.tan.
    R: Function that generates rgba-strings, usage ex.: R(255, 255, 255, 0.5)
    c: A 1920x1080 canvas.
    x: A 2D context for that canvas.

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tomxor

function u(t) {

c.outerHTML='<code style="font-size:64px">uofuwe`</code>'

Aug 1 2020 9:29 PM UTC

}// 57/140

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If the message is tiny, does it matter that my cypher sucks?

Show more comments…
u/tomxor

#codebreaking
u/DataMeta

heheh
u/tomxor

I guess I should give some hints... You can guess the message and the key, so how is it encrypted?
u/tomxor

It's not standard, it's very insecure, but very easy to implement. done in JS
u/DataMeta

Hmm, I'm curious but honestly don't completely follow. Care to give another nudge without giving it away totally?
u/tomxor

sure. tbh even though it's simple, it will never be obvious unless you have at least some experience with cryptanalysis (that also exludes me).
u/tomxor

The cipher is the simplest way I could think of making a stream cipher, where the keystream is just one 32bit LFSR seeded with the "key". The key is the ascii encoding of a string truncated to 32bits. This is combined (ok it's XORed) with the plaintext, but something else is done to preserve the printability in ASCII encoding. So the unknowns here are message and key (but you can guess them, they are on the page) and the 32bit LFSR used and how it's combined with the plaintext.
u/tomxor

As far as I understand this is fundamentally insecure because the PRNG is a single LFSR, which makes the entire keystream linear and susceptible to cryptanalysis methods i'm not familiar with.
u/tomxor

I'll post the exact method here in a week.
u/DataMeta

Interesting. Thanks for laying it out - learned some things looking stuff up. Will be giving it a thonk.
u/tomxor

Will, add bits of solution occasionally, LMK if you just want it all.
u/tomxor

Converting key from string to 32bit int for LFSR PRNG: x = parseInt([...key.slice(0,4)].map(c => c.charCodeAt(0).toString(2).padStart(8, 0)).join(''), 2)
u/tomxor

Combining keystream (x) with plaintext to get ciphertext: ciphertxt = [...plaintxt].map(c => {c = String.fromCharCode(x & 31 ^ c.charCodeAt(0));x = LFSR(x);return c;}).join('')
u/tomxor

Remaining pieces: keytext, plaintext, LFSR function.
u/tomxor

key = 'dwitter'; plaintxt = 'awesome'
u/tomxor

Only piece left is which 32bit LFSR is used to generate keystream x.


      u(t) is called 60 times per second.
      t: elapsed time in seconds.
      c: A 1920x1080 canvas.
      x: A 2D context for that canvas.
      S: Math.sin
      C: Math.cos
      T: Math.tan
      R: Generates rgba-strings, ex.: R(255, 255, 255, 0.5)

dwitter.net