ivan pineda i mean that is because of the bitwise addition (xor). It actually makes quite a lot of sense.

@@martanvanderstraaten8448 how is bitwise xor going to find coprime factors?! that's madness

therealquade that’s not how it works. It would add the hash bitwise to the thing you’d want to encrypt. If you then calculate the hash again and add it bitwise again (x-or) you added the same thing bitwise. Bitwise Adding is mod 2, so adding something 2x is adding something 2 mod 2 = 0 times. That’s why F can be anything.

therealquade Did you watch the video? The reason is because a ⊕ b ⊕ b = a (since XOR is associative and b ⊕ b = 0)

@@martanvanderstraaten8448 if I have 2 different 1 way hash functions, which rely on prime numbers and common factors for N-Root, because exponents are faster to calculate than roots, How is, in any system, Running that hash function twice, Ever going to reverse itself? The XOR's are going to be reversed, but the hash functions won't be.

Actually, this made me extremely appreciative of the sociopolitical constructs in the late 16th century historical context of agriculture in central Europe.... Dunnu why, but it did!

Don’t pull that “agree” bs please.

Damien Townsend agree

ilovepudding 🤬🤪😖🤮

Have I had this guy on uni, I might have masters right now. Instead, we were learning f@cking wordpress 😱.

Thank you!

Wow, now it makes sense from a theoretical standpoint as well. Great explanation.

Absoulotly lovly thx for sharing

As a native English speaker, I need subtitles as well! It's honestly unacceptable for such a large channel to have no subtitles.

As a non-native speaker I understand everything. It's just a little accent just get better at english then you'll understand it with ease

@@MaxDiscere Yes I get that, but it's the content that featured in the videos have some words that you have to listen twice to understand sometimes.

As an Alien, I need subtitles as well !

@@Jamie-st6of Subtitles are automatically generated by KZfaq. Can uploaders choose whether or not to have them?

It doesn't reverse them, it eliminates them via xor. There's a difference, but for some reason many people in the comments don't seam to get it.

@@ehtuanK The thing that surprises people is that the cipher is reversable, even if the hashes themselves are one-way.

Dont we all

All you need to do is reverse the order of applied functions like the part of the key (k1-F1, k2- F2) => cipher => (k2-F2, k1-F1) right ?

@@Arwahanoth Exactly. Not sure how useful it would be, but it should be possible, at least. Heck, even the key is optional. Not that you'd want to leave _that_ out; otherwise, it's no longer a proper cipher, just an obfuscation function, but again, it can be done.

Man love is real

Handwaving when explaining scientific concepts is mandatory. They won't give you a PhD if you can't handwave in a sufficiently cryptic yet empathetic manner.

They HAND you your Ph. D. after all

I've always seen XOR as selective bit inversion. That makes it very easy to see why it's reversible.

@MichaelKingsfordGray Yes. And what of it? We're proud Pound-groupies! 😂

add a thumb for yourself, Leonhard

I loved it too - I've come back just to hear it again

Be careful tho - that function isn't CPA secure!

This method only works if they're all the same size, unfortunately he didn't get into how to handle them being different sizes

@@romajimamulo Why wouldn't it work. You can pad or truncate the output of F. F can be anything. Even F=0 works. (Not very secure, but it works.)

@@misterhat5823 don't ask me, I don't know

No, in DES, the function (an XOR with a rotation of a subkey, then a sub-block substitution box of the output of that, followed by a permutation of the entire block) is the same for each round. The only difference between rounds would be the initial expansion of the data block from 32 bits to 48 bits on encryption (prior to round 1), and the collapse back to 32 bits from 48 bits on decryption (after round 16).

Because F can be anything. Go back through the analysis with F=0 as a function. It still works just fine.

I think it might make the cypher more complex, but might be possible. One thing you might be able to do is use a feistel cipher as the functions in the larger feistel cipher, so do it recursively.

As long as each step retains what goes in to the round functions then it is reversible in theory. if you want the same network to reverse it you have to think about which block goes where very carefully so that correct blocks get xorred.

The XOR is bitwise, not a logical XOR of the "truthiness" of the entire value.

Xor takes each bit and compares it to another, if they are not the same, it outputs the bit 1, otherwise 0. So the truth table is something like: 0 xor 0 => 0 0 xor 1 => 1 1 xor 0 => 1 1 xor 1 => 0

You can remember xor stand for "exclusive or" mean one or the other, but not both. That's how I do it.

@@ITR You have it the other way around: if they are *not* the same, it outputs True (or 1), otherwise False (0)

@@alimanski7941 Thanks, forgot the not, fixed now.

"or but not both"

Yes it is, not because it uses the same algorithm, but because it uses the same key (or rather set of keys).

@@aoe9857 Bad semantics on my part. I said "algorithm" when what I really meant was "function" in this case "F". Which indeed isn't the same. Thank you for the clarification. :)

Not quite; it is if you use the same subkey for each stage, but if you use different subkeys you have to turn the order around, effectively having a different key for decription and encryption. Even if it's trivial to derive each one from the other, they are different keys. For example: You use a 32-bit key 0xDEADBEEF for encrypting 16-bit blocks, with two rounds. K1 would be 0xDEAD and K2 would be 0xBEEF. For decryption, you would use the same algorith but with 0xBEEF for K1 and 0xDEAD for K2. The decryption key would be 0xBEEFDEAD, different to the encryption key, so it would be an asymmetric key. It could be argued that the encryption and decryption keys can be derived from each other trivially, but the key is that they are different keys. This comment clearly not sponsored by PETA.

@@fchurca I… I really can't focus without my adhd meds but I'm sure this makes sense. " The decryption key would be 0xBEEFDEAD, different to the encryption key, so it would be an asymmetric key." I mean… That's not what I mean. If A encrypts "Hello" with that function F, B can only decrypt it using the same function F and if either A or B were to encrypt "Hello" the output of the encryption scheme would be the same, wouldn't it?

@@LemonChieff if the rounds had the exact same F and the exact same key then it would be symmetrical. I think.

I assume it is needed so that the output can be reversed. If the last step wasn't there you couldn't decrypt by running it again.

Not necessarily, but then you'd need to flip a few things around in the decryption algorithm, hence breaking the symmetry.

@@TheJayCZ yeah that's what I meant. It is needed if you want to directly take the output and put it in the input to decrypt

This happened because he only did two rounds. Normally ones does many rounds of this, each with a different subkey, which would not expose it like this.

In addition to Markus said, the message would be split into blocks. So you'd know every other block (assuming just two passes) and that might not be very helpful. Especially if blocks are scrambled before being encrypted. Or if the block size was extremely small. Like one ASCII character. Knowing 4 bits of the 8 that make up one character would be useless.

You would still need the keys, for every step

Depends on the function F. If F is F=0, it still works, but not all that secure. If F was say... An MD5 hash of a key and the input, it would look like noise and be more secure.