Diffie Hellman -the Mathematics bit- Computerphile

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  • เผยแพร่เมื่อ 30 ต.ค. 2024

ความคิดเห็น • 810

  • @qm3ster
    @qm3ster 2 ปีที่แล้ว +356

    I think the fact that addition, multiplication, and exponentiation maintain their properties under modulo arithmetics was worth mentioning.

    • @znefas
      @znefas 2 ปีที่แล้ว +18

      could you please link an explanation to this? i've been trying to understand how g^a and g^b were made public when in actuality it was those numbers _mod n_ that were made public.
      also don't understand how you can calculate (g^a)b mod n without first having g^a, which again, wasn't made public because of the modulo term

    • @qm3ster
      @qm3ster 2 ปีที่แล้ว +21

      @@znefas I never saw an explanation myself, but consider the following: imagine we are working in base N, and not binary or decimal. Then, modN just keeps the last digit.
      So, since we know that it's possible to start by calculating the last digit when doing long addition/multiplication, as the carry only travels left, we can discard the left digits at any intermediate steps and still have the same result as if we only did it at the end.
      Signed addition and negative powers (division) break this, but wrapping subtraction of unsigned integers actually survives afaik.
      This is the same reason why we can know the last digit of 7^44444 will be 1, without having to calculate the whole number.

    • @znefas
      @znefas 2 ปีที่แล้ว +4

      @@qm3ster the only thing i understood is how the carry only traveling left may be useful when trying to calculate a number smaller than the entire expression; i don't understand how the whole mod N thing works
      i also hate modular arithmetic notation because "13 = 5 mod 8" is so much more annoying than "13 % 8 = 5" for me as a programmer

    • @qm3ster
      @qm3ster 2 ปีที่แล้ว +48

      ​@@znefas for all unsigned integers a, b, c, n:
      (a + b)%n == (a%n + b%n)%n
      (a * b)%n == ((a%n)*(b%n))%n
      ((a ^ b) ^c)%n == (((a%n)^b)%n)^c)%n
      As long as at the end you were dropping the stuff left of n, the result won't be changed by dropping it earlier, as it cannot affect what is on the right.
      The intermediate values will in fact be different, which is what makes this a computational saving.
      But outside cryptography and adjacent subjects like checksumming you usually want the most significant bits, not the least significant, they're called that for a reason, so this doesn't get used *too* often.

    • @wybird666
      @wybird666 2 ปีที่แล้ว +16

      @@znefas g^a and g^b wasn't made public, only g^a%n and g^b%n - Mike got a bit sloppy when presenting (just as we do when we write down the maths). If we knew g^a, and since we know g, it would be trivial to work out a (and similarly for b).

  • @peter62709
    @peter62709 6 ปีที่แล้ว +101

    I just took a crypto class in university and my professor got so wrapped up in the specifics of group theory that I didn't even understand how Diffie-Hellman worked, but this made everything so clear without really being any less mathematical.

  • @kusharora1435
    @kusharora1435 3 ปีที่แล้ว +86

    love the way this guy explains.. never imagined i could binge watch cryptography videos

  • @Ry____
    @Ry____ 5 ปีที่แล้ว +151

    It’s amazing how beautifully elegant and simple cryptography really is.

  • @rabreu08
    @rabreu08 6 ปีที่แล้ว +191

    Great clock analogy. This should be the main video

    • @F1ghteR41
      @F1ghteR41 6 ปีที่แล้ว +7

      A Parker clock, one might say!

    • @AndersJackson
      @AndersJackson 6 ปีที่แล้ว +3

      It should not, as they used the colour analogy there.

    • @mungflesh199
      @mungflesh199 5 ปีที่แล้ว +3

      The clock is not strictly an analogy, it's a common learning technique for modular arithmetic

    • @Cysecsg
      @Cysecsg 4 ปีที่แล้ว +2

      It explains why n needs to be a big value.

    • @skeetabomb
      @skeetabomb 3 ปีที่แล้ว

      The clock analogy I think is one of the best ways to explain modulo arithmetic...

  • @babel_
    @babel_ 6 ปีที่แล้ว +312

    Alice and Bob only communicate in Pub. Alice and Bob are probably undergrads.

  • @YouTubist666
    @YouTubist666 6 ปีที่แล้ว +82

    Wow. I've always thought D-H was some esoteric math magic. It is extremely clever and elegant.

    • @MrHaggyy
      @MrHaggyy 2 ปีที่แล้ว +2

      There is quite some math behind it about rings, fields and other group theory. But the plain formulas in crypto are pretty nice and easy. The computation is quite a mess as numbers get pretty big, often even bigger than your memory and power and multiply are not the chips best friend.

    • @Tasarran
      @Tasarran ปีที่แล้ว

      I find there are a lot of these types of algorithms; Radix Sort comes to mind... It seems like voodoo, then you look at the algorithm, and probably at first think 'that can't possibly work...?' but viola...

    • @justalonelypoteto
      @justalonelypoteto ปีที่แล้ว +1

      ​@@MrHaggyythat's still somewhat elegant, square-multiply-modulo is a genius and simple solution to easily calculate such an otherwise gargantuant number. I find the truly hard (and amazing) part is the theory behind which numbers are safe to use, even more fascinatingly and terrifyingly genius are the ways people find to crack it

    • @MrHaggyy
      @MrHaggyy ปีที่แล้ว +1

      @@justalonelypoteto square-multiply is a genius algorithm as it has view operations,. Lattice and Russian Peasant are awesome too. A lot more operations but most of them are cheap additions that can be done in parallel. My interest is hard-realtime systems. We want algorithms with fixed calculation times. Square-multiply can vary depending on the numbers. And on cheap and small hardware you might be able to eavedrop with an oscilloscope.

    • @justalonelypoteto
      @justalonelypoteto ปีที่แล้ว +1

      @@MrHaggyy Montgomery's ladder might be up your alley, it performs both operations each time with the only distinguishable difference being a possible timing variation because the needed operation affects which memory address is read, apparently this is fixed by scattering the data around to ensure cache misses

  • @fsmoura
    @fsmoura 5 ปีที่แล้ว +80

    5:02 THIS! IS! CRYPTOGRAPHY! *_*kicks you down a 4096-bit deep well*_*

  • @VoilaTadaOfficial
    @VoilaTadaOfficial 6 ปีที่แล้ว +7

    I love these math versions. They help me understand it so much better than just the computer science bits. Please do more of these!

  • @drewad0
    @drewad0 ปีที่แล้ว +3

    All these years of hearing how "diffie helman is extremely complex" yet such a simple and easily understandable algorithm. Thank you so much for this!

    • @EannaButler
      @EannaButler 15 วันที่ผ่านมา +1

      As soon as someone says "It's extremely complex", usually means they don't understand it, and therefore don't want to be asked to explain it...

  • @Syldar
    @Syldar ปีที่แล้ว +4

    This is the most interesting channel about computer science. I’ll never be as much grateful as I would like to be. Thanks a lot for sharing these.

  • @Kolop315
    @Kolop315 6 ปีที่แล้ว +74

    Wow, that is way more simple than I was expecting.

  • @wildwest1832
    @wildwest1832 4 ปีที่แล้ว +13

    The idea of diffie hellman is actually really clever. Its actually pretty hard to come up with a way to do that, and not have anyone be able to reverse how they did it or needing to share something that can be reversed. I can see why we still use this today it would be very difficult to replace it, and its very ingenious how it works and its not incredibly complicated either.

  • @kennethcarvalho3684
    @kennethcarvalho3684 2 ปีที่แล้ว +5

    For some reason I understand all his videos but not many of the other tutors on Comphile.. Quite a teacher

  • @xcvsdxvsx
    @xcvsdxvsx 6 ปีที่แล้ว +968

    I actually understood the math for once!

    • @peppybocan
      @peppybocan 6 ปีที่แล้ว +10

      yeah, modular arithmetics is easy. :D ...

    • @jasonpatowsky6929
      @jasonpatowsky6929 6 ปีที่แล้ว +62

      And theeeeres the party pooper.

    • @BryanLeeWilliams
      @BryanLeeWilliams 6 ปีที่แล้ว

      Didn't work for me a=11 b=13 g=7 n=199

    • @grrr1351
      @grrr1351 6 ปีที่แล้ว +2

      The math is easier to understand, because it's more technical.

    • @musashi939
      @musashi939 6 ปีที่แล้ว

      Jyoti Das lol.

  • @outofahat9363
    @outofahat9363 2 ปีที่แล้ว +22

    Came in expecting some incomprehensible esoteric math but this was very easy to understand. Reminds me of that Doctor Strange quote "It's a simple spell but quite unbreakable"

    • @mackmenezes4912
      @mackmenezes4912 ปีที่แล้ว +1

      If you watched Attack on Titan ,one of the character used. Only his words and hacked the entire cosmos to make it seem real at that moment

  • @trevordobbertin6469
    @trevordobbertin6469 6 ปีที่แล้ว +4

    It would be awesome if you guys did some podcasts. I learn so much from you guys and it would be great to listen while in the car or at school. Thanks guys.

  • @emilmartens123
    @emilmartens123 6 ปีที่แล้ว +221

    awesome video, I see some comments point out its actually (((g^a) mod n) ^ g)) mod n, so here is why its the same as (g^a)^b mod n:
    lets say:
    g^a = k*n+x
    where x is an unknown number
    because: (u + v) mod n =(u mod n + v mod n) mod n
    and because: k*n mod n = 0
    we get:
    (g^a) mod n = (k*n+x) mod n = x
    ((g^a) mod n)^b mod n = x^b mod n
    i used the binomial theorem nCr to get:
    (g^a)^b = (k*n+x)^b = (k*n)^b + (nC1)*(k*n)^(b-1)*x + ... + (nC(b-1))*( (k*n)^(1)*x^(b-1) + x^b
    since n appears in every part except in x^b every other part is set to 0 when we take the modulo:
    (g^a)^b mod n = x^b mod n
    so therefore (g^a)^b mod n = ((g^a) mod n)^b mod n

    • @AhsimNreiziev
      @AhsimNreiziev 6 ปีที่แล้ว +1

      Yup.

    • @theguyoo1
      @theguyoo1 6 ปีที่แล้ว +39

      Thanks for this, felt like a logical leap without this proof.

    • @nelsblair2667
      @nelsblair2667 3 ปีที่แล้ว +11

      At the beginning there, did you put a g, where you meant a b?

    • @AhsimNreiziev
      @AhsimNreiziev 3 ปีที่แล้ว +7

      @@nelsblair2667
      He did, yes.

    • @deckard5pegasus673
      @deckard5pegasus673 3 ปีที่แล้ว +9

      Interesting proof. But a few corrections:
      First what you are trying to prove is that (g^a)^b mod n == (g^a mod n)^b
      You made a mistake, in (((g^a) mod n) ^ g)) mod n . You put an extra "g", it's a "b"
      Also to simplify...or clarify your proof, and make it more understandable, organized:
      1) supposing: g^a = n*k+x
      ( *WHERE "x" IS NOT A MULTIPLE OF "n"* , in other words "x" is the "remainder" of the division by "n" )
      This is an important purposeful "setting up" so that: n*k mod n == 0 and x mod n == x
      2) mod n of any number which is a multiple of n will equal zero --> n*k mod n == 0
      3) Proof that: (g^a mod n)^b == x^b
      a) Which would mean --> (g^a mod n) == x
      a) substitute "n*k+x" for "g^a", from step "1"
      (n*k + x) mod n == x --> given that: n*k mod n == 0 (any multiple of "n" will be zero)
      b) If (n*k + x) mod n == x, then (g^a mod n) == x,
      c) meaning --> (g^a mod n)^b == x^b
      4) (g^a)^b mod n
      a) substitute (n*k+x) for g^a, given in step "1" --> (n*k+x)^b mod n

      b) Expanding (n*k+x)^b, using binomial theorem,
      *IMPORTANT MAGIC* : *all terms will be multiples of "n"* except the last term which is x^b
      c) Mod n of all terms that are multiples of "n" will be zero, leaving only the last term: x^b
      d) Thus (g^a)^b mod n = x^b
      5) (g^a)^b mod n == (g^a mod n)^b , as both equal to x^b
      .

  • @nalbertcerqueira6079
    @nalbertcerqueira6079 ปีที่แล้ว +1

    The simplicity of this algorithm is so amazing!

  • @xZise
    @xZise 6 ปีที่แล้ว +268

    It looks really simple, but it unfortunately doesn't explain whether "(g^a mod n)^b" and "(g^a)^b mod n" are the same. I mean obviously it must be the same for it to work.

    • @bgroks1
      @bgroks1 6 ปีที่แล้ว +48

      Fabian Neundorf (g^a mod n)^b mod n is the same as (g^a)^b mod n, I believe he also mentioned it briefly in the video.

    • @jonathanseamon9864
      @jonathanseamon9864 6 ปีที่แล้ว +63

      The simple answer as to why they're the same is that Modulo arithmetic=Finite fields, and Finite fields are consistent. Proving that finite fields are consistent would probably be another couple videos...

    • @TheUnclepecos
      @TheUnclepecos 6 ปีที่แล้ว +174

      Short answer: It's because the product in modulo/clock arithmetic is defined that way. If you wanna know more I'll explain it in more detail:
      Maybe it's easier if you think of g^a mod n just as a number between 0 and n-1 (actually if we use this notation we should start from 1 and go up to n-1, but it's not extremely important so let's forget about it). Think of the multiplication between two such numbers as a standard multiplication, but if the result is bigger than n-1 we take the remainder.
      So basically the product is just:
      (a mod n)*(b mod n) = a*b mod n (THIS IS THE DEFINITION OF PRODUCT IN MODULO ARITHMETIC)
      Elevating g to the power a is just repeated multiplication, so what you do is apply the definition of product:
      (g mod n)^a = (g mod n)*(g mod n)*...*(g mod n) = g*g*g*...*g mod n
      You can easly see that we get a similar thing if we elevate g^a to the power b:
      (g^a mod n)*(g^a mod n)*...*(g^a mod n) = (g^a)*(g^a)*...*(g^a) mod n = (g^a)^b mod n
      So yeah, (g^a mod n)^b = (g^a)^b mod n
      Hope it's clear now ;)

    • @FelkCraft
      @FelkCraft 6 ปีที่แล้ว +25

      The "mod n" isn't an operation in this case. Appending "mod n" to a term just signals that you're calculating in clock arithmetic, meaning you always stay between 0 and n-1. If you do 2+3 in "mod 4", the answer is 5, or 1, or even 9, because in "mod 4" 1 is equal to 5 by definition. Calculating a modulo is not an operation that needs to be performed if you define yourself to be working in clock arithmetic

    • @danieljensen2626
      @danieljensen2626 6 ปีที่แล้ว +43

      Actually g^(ab) mod n = (g^a mod n)^ b mod n, but yes, it is the same. The proof is just long and kinda gross which is why he didn't get into it. Here it is if you want to see it though.
      Proof:
      Any number m mod n is defined as the remainder left after dividing m by n, so we have m=q*n+r where q is the whole number of times n goes into m, and m mod n=r, which is the remainder.
      So g^a=q_a*n+r_a, and g^a mod n=r_a. (I'm using the underscore to denote a subscript.)
      (g^a mod n)^b=r_a^b=q_1*n+r_1, and (g^a mod n)^b mod n=r_1.
      g^(a*b)=q_(ab)*n+r_(ab), and g^(a*b) mod n =r_(ab).
      But we also know g^(a*b)=(g^a)^b, and g^a=q_a*n+r_a, so g^(a*b)=(q_a*n+r_a)^b.
      Expanding that out into a sum we get
      (q_a*n+r_a)^b=sum((q_a*n)^(b-j)*r_a^j,j=0..b)
      The last term in this sum is r_a^b and every other term has at least one factor of n, so we can rewrite it as
      (q_a*n+r_a)^b=n*sum(q_a^(b-j)*n^(b-j-1)*r_a^j , j=0..b-1)+r_a^b.
      Now let q_2=sum(q_a^(b-j)*n^(b-j-1)*r_a^j , j=0..b-1), so we have
      (q_a*n+r_a)^b=q_2*n+r_a^b.
      Remember that this is equal to g^(a*b). So we have
      g^(a*b)=q_2*n+r_a^b.
      We already have r_a^b=q_1*n+r_1, so we have
      g^(a*b)=(q_1+q_2)*n+r_1
      and g^(a*b) mod n = r_1.
      We already had that (g^a mod n)^b mod n = r_1 so we have now proved that
      g^(a*b) mod n = (g^a mod n)^b mod n.

  • @CJBurkey
    @CJBurkey 6 ปีที่แล้ว +485

    Wouldn't a mod n only return values from 0 to _n_-1? You wrote _n_ on the clock.

    • @seraphina985
      @seraphina985 6 ปีที่แล้ว +80

      Correct that was an error on their part since of course if a divides by n exactly the remainder is 0 not n.

    • @VoilaTadaOfficial
      @VoilaTadaOfficial 6 ปีที่แล้ว +47

      Technically you could have n on the clock but you'd have to omit 0. You can have n or you can have 0, but not both. Having n on the clock would actually be more like counting units of 1 up to n and then starting back at 1 again for the next unit until you run out of units. Same result (specifically in this case because the number itself doesn't matter), but a different perspective on it.

    • @Friek555
      @Friek555 6 ปีที่แล้ว +23

      Having a 0 instead of the n is very useful when you work with such a construct though. This clock face is what mathematicians call a cyclic group, and its "neutral element" is 0 (or n, they are the same in the cyclical group). That means that (a+0)mod n=a mod n, so the 0 doesn't change an element by being added.
      That is very obvious if you actually write it as a 0 instead of an n.

    • @AndersJackson
      @AndersJackson 6 ปีที่แล้ว +3

      And multiply by zero also give nice properties. :-)

    • @tohopes
      @tohopes 6 ปีที่แล้ว

      But clocks are 1-based.

  • @KraylusGames
    @KraylusGames 6 ปีที่แล้ว +225

    Lmao everyone calling him out for not stopping at n-1. Give the dude a break!

    • @tohopes
      @tohopes 6 ปีที่แล้ว +8

      ☞ nope.avi

    • @tedbaltz2164
      @tedbaltz2164 4 ปีที่แล้ว +17

      they need to feel smart

    • @martinkunev9911
      @martinkunev9911 3 ปีที่แล้ว +13

      If you're not familiar with what he's explaining, this is enough to confuse you and this is not the only thing he says that can create confusion.

    • @realcryptc
      @realcryptc 3 ปีที่แล้ว +1

      he kinda of implicitly stopped at n-1 when he mentioned there is the element 0 in mod operations

    • @skeetabomb
      @skeetabomb 3 ปีที่แล้ว

      😂

  • @ianpatrick23
    @ianpatrick23 2 ปีที่แล้ว +1

    Fantastic explanation of the mathematics behind this encryption!

  • @flockenlp1
    @flockenlp1 5 หลายเดือนก่อน

    You are why I am confident I won't fail my Safety and Security module. Mike is exceptional at communicating these concepts!

  • @AdamMPick
    @AdamMPick 6 ปีที่แล้ว +42

    The whole time I am thinking about the clock-drawing test, after seeing those clockface numbers beeing squished together.
    PS. I find this video even simpler to understand than the original "simple" version with the food colouring.

    • @kayinnasaki
      @kayinnasaki 6 ปีที่แล้ว +6

      Same. With the color one I was like "what do you mean I can't figure out what the other color was? We just subtract the public color from it!"

    • @lambertbrother1628
      @lambertbrother1628 6 ปีที่แล้ว +1

      The actual process is more difficult as instead of adding, it's multiplication to the power of another colour. Imagine Yellow^red x blue.

    • @xcvsdxvsx
      @xcvsdxvsx 6 ปีที่แล้ว +3

      Me too. It's so easy to understand math when it doesn't have weird symbols that I have no idea what the heck they mean.

    • @AndersJackson
      @AndersJackson 6 ปีที่แล้ว +3

      Yes, the thing with math is that it is a language which you need to learn. Once you do, the concepts behind the maths become not that complicated, when you know them. Implementation could still be complicated though. Devils in the details...

  • @desiassassin3268
    @desiassassin3268 ปีที่แล้ว +1

    This made so much sense and was super easy to grasp. Mike pound really is a great teacher. Also props to Diffie Hellman for creating such an easy but quite unbreakable algorithm.

  • @vibhavsharma9093
    @vibhavsharma9093 ปีที่แล้ว +2

    great video, just to clarify modulo math, ((g^a)^b) mod n = ( ( (g^a) mod n )^b )mod n,
    by using this we can generate same value from both side

    • @HarishNarayanan
      @HarishNarayanan ปีที่แล้ว

      This is the most important fact that makes this explanation work.

  • @Starfuchs
    @Starfuchs 6 ปีที่แล้ว +144

    The clockface only goes up to n-1, (n mod n = 0). He forgot to change that too when he added the zero afterwards

    • @kpan
      @kpan 6 ปีที่แล้ว +5

      Thomas Wigele Yeah I came down to point it out as well :)

    • @666Tomato666
      @666Tomato666 6 ปีที่แล้ว +1

      not only that, but the primality of g is completely irrelevant - the n needs to be prime if you want the n in the 2 to 4k range (and DH to remain secure), best if n is actually a safe prime (then g can be any number but 0, 1 and n-1) - a prime for which (n-1)/2 is also prime

    • @kpan
      @kpan 6 ปีที่แล้ว

      666Tomato666 I don't get why n would have to be prime at all?

    • @aleksandarsavev
      @aleksandarsavev 6 ปีที่แล้ว +2

      +Π. Καράπας This way you are 100% sure that the remainder is in [0, n-1].

    • @kpan
      @kpan 6 ปีที่แล้ว +4

      Александър Савев but doesn't modulo always return something in the range of [0,n-1]?

  • @skarnl
    @skarnl 14 วันที่ผ่านมา +1

    I finally understand how this public-private key system works 🎉
    Thanks!

  • @xdyps
    @xdyps 6 ปีที่แล้ว +24

    Please do a video on Elliptic-curve cryptography and Elliptic-curve Diffie-Hellman , pretty please :D

  • @davidsanders9426
    @davidsanders9426 6 ปีที่แล้ว +200

    I thought "solving the discrete log problem" was what happens when you clog the toilet at your in-laws' house?
    ...Sorry, I'll show myself out.

    • @Ping727
      @Ping727 3 ปีที่แล้ว +18

      ...yeah you should really log off

    • @ivanarabome4172
      @ivanarabome4172 3 ปีที่แล้ว

      You didn’t get the video.... tbh he actually did explain it well

    • @jamhamtime1878
      @jamhamtime1878 3 ปีที่แล้ว +13

      @@ivanarabome4172 and you didn't get the joke

    • @jadielrhys3156
      @jadielrhys3156 3 ปีที่แล้ว

      @Jesse Duncan yup, been watching on instaflixxer for since november myself =)

    • @RussTeeTrombone
      @RussTeeTrombone 3 ปีที่แล้ว

      Nice

  • @kegelsknight
    @kegelsknight 6 ปีที่แล้ว +208

    It shouldn't be 'n' on the clock but 'n-1' because 'n'==0 in modolo

    • @sebastianheinrich8683
      @sebastianheinrich8683 6 ปีที่แล้ว +22

      I'm not the only one who is botherd by this ^^

    • @AhsimNreiziev
      @AhsimNreiziev 6 ปีที่แล้ว +5

      This is very true. Easy mistake to make, though.

    • @Majenga
      @Majenga 6 ปีที่แล้ว +3

      same thought here - was about to write a comment.

    • @remuladgryta
      @remuladgryta 6 ปีที่แล้ว +42

      There are 3 hard things in programming:
      1 off by one errors and
      2 naming things

    • @breadnoodle
      @breadnoodle 6 ปีที่แล้ว

      I was about to write the same xd

  • @sammonger4002
    @sammonger4002 6 ปีที่แล้ว +10

    It's interesting that he doesn't mention that n should be prime in addition of being large. That is an aspect equally as important as its size which could easily break the encryption (if n isn't prime). There was even a defcon panel (2016 I think) that discussed this very problem and how it is often ignored.

    • @Legendarior
      @Legendarior 6 ปีที่แล้ว +1

      In the previous video regarding this topic, with the color mixing, he mentions that n is a prime number.

    • @tissuepaper9962
      @tissuepaper9962 4 ปีที่แล้ว

      I really was hoping he would explain why n has to be prime. I'm thinking the reason is that the search space of [numbers] mod n would be much smaller than n if it weren't a prime number, but that's just my intuition and I'm not sure if I'm right.

    • @MarioFanGamer659
      @MarioFanGamer659 2 ปีที่แล้ว

      @@tissuepaper9962 The goal is to have the denominator and numerator being coprime i.e. their greatest common divisor is 1 as otherwise, you'd end up getting 0 as the result. The only way to guarantee that is with a prime number which is naturally coprime to any number.

  • @Pilbaran00b
    @Pilbaran00b 6 ปีที่แล้ว

    It's beautiful how simple and elegant it is.

  • @edsonrocks
    @edsonrocks 4 ปีที่แล้ว +1

    This is the best explanation I've ever found about it. Incredible concise and very illustrative. Thank you guys.

  • @bariswheel
    @bariswheel 6 ปีที่แล้ว +1

    Great explanations folks, keep pumping these videos out!

  • @wybird666
    @wybird666 2 ปีที่แล้ว

    The clock-face analogy is really powerful as it really helps explain why it is not very reversible. "Undoing" g^a%n is effectively going backwards in steps of n and checking to see if g factors into this number an integer number of times. Since a is generally (very) large, we'd have to go backwards an awful lot of steps. There can be more efficient ways of doing it, but conceptually it's the same.
    Note 2048 bits --> N < 2^2048 ~ 10^616. The world's best supercomputer has a power of ~1exaflop or 10^18 floating point calculations per second. Assuming every attempt could be achieved in 1 operation, that would take 10^598 sec = *10^591 years*
    I always find it difficult to understand how "difficult" something is when we say p is a really big number, until it is put into a time-to-calculate number.

  • @johnpugh24
    @johnpugh24 6 ปีที่แล้ว +2

    Thanks for explaining, you do such a great job. Always my favourite computerphile presenter.

  • @Snyper20
    @Snyper20 6 ปีที่แล้ว

    Finally a decent video on computerphile... all of them should be AT LEAST this caliber instead of resorting to oversimplified (and hence incorrect) analogies.

  • @programmercouple
    @programmercouple 3 ปีที่แล้ว +1

    There is no better explanation of the "Diffie-Hellman key exchange".

  • @TheEndOfMadness
    @TheEndOfMadness 4 ปีที่แล้ว

    This video is a paradox, in which your head embodies the problem and the solution at the same time.

  • @SarveshParakh
    @SarveshParakh 3 ปีที่แล้ว

    After watching all three videos, its much easier to understand and glad you used 'n' here that you didn't use in the last video with colors. This was an amazing series but if you label them as part of a series or make 1 full video combining all three or the first and last video, more viewers might decide to stick to the end. Anyway, Thanks a lot for this.

  • @allien5329
    @allien5329 5 ปีที่แล้ว

    beautiful and simplified explanation

  • @vladpuha
    @vladpuha 6 ปีที่แล้ว +1

    thank you for putting this video up. Such a good teacher and elegant explanation of public private key at its core. There are 100s of videos super complicated.. this is very nice, one can implement themselves right off the this video..

  • @kevinnio
    @kevinnio 2 ปีที่แล้ว

    I was wondering why Diffie-Hellman hasn't been deprecated in favor of a better algorithm since it's almost 50 years old now. Now I know why. You could always increase the bit count of N to compensate for faster computers and still get a shared secret in as low as 2 messages. Neat!

  • @mladenkaorlic
    @mladenkaorlic 3 ปีที่แล้ว

    Amazing explanation with the circle!

  • @nomad_geek
    @nomad_geek 6 ปีที่แล้ว

    This is the video I've been waiting for!
    Thanks Guys, awesome job!

  • @benjaminshropshire2900
    @benjaminshropshire2900 7 หลายเดือนก่อน

    The missing part is why (g^a mod n) is fast to compute even though the reverse is not.
    Tl;dr; g^a can be computed by starting with x_0=1 and and defining x_1=a_0^2 or x_1=x_0^2 * g and then doing the same for x_2 and so on. Making the right choices at each step (which is basically reading out the bits of a) you get g^a in at most log2(a)+1 steps.

  • @abhinavsixfaces
    @abhinavsixfaces 6 ปีที่แล้ว

    Watched both videos. Beautifully explained.

  • @aanesijr
    @aanesijr 6 ปีที่แล้ว +1

    Is it weird that I found this easier to understand than the one with the colors?

  • @rikwisselink-bijker
    @rikwisselink-bijker 6 ปีที่แล้ว

    Another thing that I noticed (but doesn't matter), is that actually, Bob can't compute (g^a)^b mod n, because he has to calculate (g^a mod n)^b mod n. The beauty of modulo arithmetic is that the result is the same.

  • @karimbarakat7732
    @karimbarakat7732 4 ปีที่แล้ว +1

    Thank you so very much for these videos. I find them invaluable in understanding the ins and outs of cryptography.

  • @elultimopujilense
    @elultimopujilense 4 ปีที่แล้ว +2

    At 4:00 that exponentiation is flawed, because you need to calculate (g^a mod n)^b, not (g^a)^b. Those are not the same, and the explanation he gave is not correct. He should have said that because of the modulus exponentiation property (g^a mod n)^b mod n is the same as (g^b mod n)^a mod n. He omitted a really important step when he jumped to (g^a)^b is equal to (g^b)^a. That is something that you arrive at when you apply the modulus exponentiaion property.

  • @agrawalyogesh
    @agrawalyogesh 2 ปีที่แล้ว +7

    Hi Mike, Alice or Bob never shared "g to the power of a or b" but they shared "g to the power of a (b) mod n". But as we continued the example we said they shared the power of a or b. Can you please help explain that?

    • @cliftonfestusmalvea9777
      @cliftonfestusmalvea9777 ปีที่แล้ว +2

      For anyone who got confused with this, we start with the left-hand side of the equation:
      (g^a mod p)^b mod p
      = (g^ab mod p) mod p [by the property of modular exponentiation]
      Now, we move on to the right-hand side of the equation:
      (g^b mod p)^a mod p
      = (g^ba mod p) mod p [by the property of modular exponentiation]
      Since (g^ab mod p) mod p = (g^ba mod p) mod p, we can say that:
      (g^a mod p)^b mod p = (g^b mod p)^a mod p
      Hope this helps.

  • @Inritus618
    @Inritus618 6 ปีที่แล้ว

    That really is a beautiful solution to exchanging keys. I really love that. Fantastic video as always!

  • @shanedk
    @shanedk 6 ปีที่แล้ว

    4:05 - To be more accurate, it's the same as long as g, a, and b are all positive. Throw in negative numbers and it doesn't always hold.

  • @AbhishekSharma0318
    @AbhishekSharma0318 6 ปีที่แล้ว +27

    This was not that difficult!

  • @salerio61
    @salerio61 6 ปีที่แล้ว +34

    The clock-face numbers should go from 0 to (n-1)

    • @OXXOI77777
      @OXXOI77777 6 ปีที่แล้ว +3

      You are technically correct, the best kind of correct

    • @almightyhydra
      @almightyhydra 6 ปีที่แล้ว +1

      Martin O'Donnell Or even 1..n-1, as this is a multiplicative group.

    • @CJBurkey
      @CJBurkey 6 ปีที่แล้ว

      I'm glad a lot of people saw it.

    • @potatopassingby
      @potatopassingby 6 ปีที่แล้ว

      thank you. i kept thinking about it and it was so hard to concentrate on the video haha

    • @xXThePr0Xx
      @xXThePr0Xx 4 ปีที่แล้ว

      And it should start at 1 because 0 isn't an element of the multiplicative group Z*_n

  • @syrix5914
    @syrix5914 3 ปีที่แล้ว

    This is extremely elegant.

  • @supdawg7811
    @supdawg7811 6 ปีที่แล้ว +8

    Yeah but you never talked about if (g^{a} mod n)^{b} is the same as (g^{b} mod n)^{a}. As in, does g^{a^{b}} = g^{b^{a}} hold in the modulo field.

    • @Aidiakapi
      @Aidiakapi 6 ปีที่แล้ว +4

      Yeah, I wanted to hear that too, but considering the technique wouldn't work otherwise, it probably is. It also kind of makes intuitive sense in the terms of significant bits. Modulo tosses away all most-significant bits, and only keeps the least significant bits. Multiplication will push the least significant bits into the most significant bits (two 32 bit numbers multiplied for example, gives a 64 bit number). So tossing away those most significant bits (which would've been moved to even higher significant bits) shouldn't matter.

    • @Dsiefus
      @Dsiefus 6 ปีที่แล้ว +2

      The modulo just takes the remainder of the division: being the multiplication commutative, g^(ab) = g^(ba), you'll get the same number. You can then do modulo n, but the number will be the same anyways.

    • @yondaime500
      @yondaime500 6 ปีที่แล้ว +2

      Let's say g^{a} = X + Y, where X is divisible by n and Y < n (therefore g^{a} mod n = Y). Then
      g^{ab} is (X^b + c1*X{b-1}Y + c2*X{b-2}Y^{2} + ... + Y^b). Since X is a multiple of n, all but the last term will be zero mod n, so we are left with Y^b mod n = ( g^{a} mod n)^b mod n after we take the modulo. Therefore g^{ab} mod n = g^{a} mod n)^b mod n. I hope that's clear enough.

    • @AndersJackson
      @AndersJackson 6 ปีที่แล้ว +1

      This is why he put this in the Maths video and not the colour video. As you should know this basic thing to know the theory. And he mentioned it in the video, very short.

  • @abigicic
    @abigicic 6 ปีที่แล้ว

    This is a really nice explanation and thank you for talking about it. I wish you talked a bit about man in the middle weakness and the ElGamal improvement.

  • @EquationHub
    @EquationHub 8 หลายเดือนก่อน

    Just to caveat, in modulo we deal with numbers from 0 to n-1. We exclude n since the remainder can't be equal n - it must be less than n, otherwise when we're dividing by n and we see that there's a remainder n it means that n could fit one more time in that number leaving us with the remainder 0.

  • @natedsamuelson
    @natedsamuelson 6 ปีที่แล้ว

    Somehow this is the first time I've seen the clock representation of modulo. Really helpful to conceptualize when you're rushing through a problem!

    • @grasweg3
      @grasweg3 5 ปีที่แล้ว +1

      Really? I've never seen it explained without this clock representation.

  • @DimMyPrp
    @DimMyPrp 6 ปีที่แล้ว +2

    Yeah! Best explanation on key exchange ever. Thanks to both videos I understood it instantaneously. A test on paper and mental arithmetic worked like a charm.
    Thanks :-)

  • @RineezAhmed
    @RineezAhmed 2 ปีที่แล้ว +1

    In previous video Mike said the math is very complex so separated out to another video. Now by the end of this video Mike says the math is not very complex it's simple. 🙃
    Actually it was hard for me to understand how (g^a mod n)^b became (g^ab mod n) . It felt so counter intuitive to me that I had to test the math with several sample values before I could convince my brain it is true.

  • @Nwoknu
    @Nwoknu 6 ปีที่แล้ว

    I know it's not very scientific, but I still wish a merry Christmas to all you computerphiles out there :)

  • @nauthic3p0
    @nauthic3p0 6 ปีที่แล้ว

    I like the videos with that guy, he knows his stuff and knows how to explain it!

  • @robfielding8566
    @robfielding8566 2 ปีที่แล้ว

    This is explained well with a pair of Rubiks Cubes, and dice. Rubiks Cube notation is identical to the group notation.

  • @zazzy7681
    @zazzy7681 6 ปีที่แล้ว +3

    The Mike Pound Show

  • @YousifNael
    @YousifNael 5 ปีที่แล้ว +11

    g needs to be a primitive root of n for this to work.

    • @trogdorstrngbd
      @trogdorstrngbd 5 ปีที่แล้ว +4

      Is that true? After thinking about this (admittedly only for a few minutes), it is obvious to me that for a given size of n, selecting g to be a primitive root is the most secure because you'll get the maximal cycle length of n-1. But choosing a non-(primitive root) doesn't seem to actually break the algorithm.

    • @drewkavi6327
      @drewkavi6327 3 ปีที่แล้ว +2

      @@trogdorstrngbd yes is doesn't break the algorithm but you will need to exhaust fewer values

  • @jursamaj
    @jursamaj 3 ปีที่แล้ว

    I think this was back in the 128-bit key era, but I recall an article explaining that it *was* practical to break the key, if you were willing and able to spend a few million dollars on custom chips. No doubt that price tag has gone up with larger keys, but it's reasonable to assume state actors are capable.

  • @shijovarghese95
    @shijovarghese95 3 ปีที่แล้ว

    Thank you so much, this really helped me get to grips with SSH!

  • @lynx-titan
    @lynx-titan 6 ปีที่แล้ว +1

    g must be the generator of cyclic group of n (the set 0..n-1) by repeatedly applying the group operation (exponent, modulo)

  • @hassananwer3674
    @hassananwer3674 3 ปีที่แล้ว

    beautifully explained!!! I love this channel. Thank you!

  • @dilipchander2540
    @dilipchander2540 6 ปีที่แล้ว

    I will never forget Diffie-Hellman math again in my life|

  • @avonstar8893
    @avonstar8893 6 ปีที่แล้ว

    Superb explanation!

  • @macigli
    @macigli 6 ปีที่แล้ว

    I really loved this vid. Great work guys!

  • @lherfel
    @lherfel 2 ปีที่แล้ว

    4:45 when he writes g^a and g^b as being made public, he should have said and written g^a mod N, and g^b mod N, were made public. thanks for explanation.

  • @S3b1Videos
    @S3b1Videos 6 ปีที่แล้ว

    Thank you, this explained the matter (at least to me) a lot better than wikipedia.

  • @yah3136
    @yah3136 ปีที่แล้ว

    Wonderful explanation, thank you

  • @vorniy
    @vorniy 6 ปีที่แล้ว +43

    But isn't the public value "g^a mod n" instead of "g^a" or is that irrelevant, because "(g^a mod n)^b mod n" is the same as "(g^b mod n)^a mod n"?

    • @evge
      @evge 6 ปีที่แล้ว +9

      was wondering the same thing, can someone answer

    • @ChenfengBao
      @ChenfengBao 6 ปีที่แล้ว +8

      You can put "mod n" anywhere or nowhere. It doesn't make any difference as long as the end result is mod n.

    • @johnmiller8884
      @johnmiller8884 6 ปีที่แล้ว +32

      That "mod n" is important for keeping things secret. Remember that it is the *discrete* logarithm problem that is hard. If I give you `g` and `g^a` you can find `a` you could find `a` by log_g (g^a). That is what a logarithm is: the inverse of exponentiation. The 'mod n' piece is what make the problem hard because you don't know how many times you have looped back to 0 again. For any given g and n there actually might be a lot of values for a that would work, *but* if g and n are relatively prime (they have no common factors) then there is exactly one value a less than n that will result in g^ mod n. That is why the subtle mention that g and n are large *prime* numbers. The math still works if they are not, but it makes the problem of finding a much easier.

    • @efeyzee
      @efeyzee 6 ปีที่แล้ว +13

      Tested with a couple values, (g^a mod n)^b mod n seems to equal (g^b mod n)^a mod n .

    • @philippetrov4881
      @philippetrov4881 6 ปีที่แล้ว +9

      Yes, it is.
      And yes, "(g^a mod n)^b mod n" is the same as "(g^b mod n)^a mod n"

  • @KDOERAK
    @KDOERAK ปีที่แล้ว

    Excellent explanation!👍

  • @SteveMacSticky
    @SteveMacSticky 6 ปีที่แล้ว

    Your videos are excellent. Thank you very much guys.

  • @SkylerGhostly
    @SkylerGhostly 4 ปีที่แล้ว +1

    G TO THE A TO THE B TO THE C, COMPUTERPHILE IS THE BEST YOU SEE

  • @spuddo123
    @spuddo123 2 ปีที่แล้ว +4

    I love how this all works on the fact that (((a^b)%x)^c)%x=(((a^c)%x)^b)%x

  • @kynan7412
    @kynan7412 2 ปีที่แล้ว

    At 1:51 you don't need the n as you have 0. n (mod n) = 0, so the clock would only go from 0, to n-1

  • @Stilgarsan
    @Stilgarsan 5 ปีที่แล้ว

    The DF Key exchange being simple reminds me of Columbus' egg. The gist of it was "Of course it is simple, once someone showed you how to do it"

    • @wildwest1832
      @wildwest1832 4 ปีที่แล้ว

      IDK if I would say its simple. its actually kind of complicated, and figuring out that all this works is not at all easy. its actually quite clever. I can see why we still use it. Finding another way to do this would be very hard to do.

  • @danybeam
    @danybeam 6 ปีที่แล้ว

    I imagine there are not enoguh cases to make a whole new channel
    but as a maths fan and a programmer I really did enjoy... I actually I understood it a little better than my cryptography profesor explanation

  • @samgregg7
    @samgregg7 6 ปีที่แล้ว

    I'm pretty sure a and b (the private keys) should be between 2 and n-2 inclusive, as a^1 (mod n) is just the value of a itself (mod n), and a^n-1 (mod n) will be 1 by Fermat's Little Theorem, so this would be easy to spot for an attacker.

  • @Mark1Mach2
    @Mark1Mach2 3 ปีที่แล้ว

    Great explanation!

  • @daboyz6106
    @daboyz6106 ปีที่แล้ว

    Very well explained video.

  • @anderslarsen4100
    @anderslarsen4100 6 ปีที่แล้ว

    So damn clever. Diffie and Hellman should get an award.

    • @jaspermooren5883
      @jaspermooren5883 5 ปีที่แล้ว

      They both got several, according to Wikipedia.

    • @wildwest1832
      @wildwest1832 4 ปีที่แล้ว

      It is actually super clever. I mean we are still using this how many years later? This would be very very difficult to come up with, and there are not a lot of other ways to come up with a reliable way to do this that is also fast to do.

  • @rutvaydhami367
    @rutvaydhami367 3 ปีที่แล้ว

    Absolutely amazing explanation! Keep it up

  • @jamilxt
    @jamilxt 4 ปีที่แล้ว

    Such an amazing teacher he is! 🤩

  • @LFSPharaoh
    @LFSPharaoh 2 ปีที่แล้ว +1

    Amazing explanation. I can almost picture the whole thing in my head, albeit still a rough understanding but this video really helped a lot. Every time I revisit the D/H exchange, it makes me want to start obsessing over prime numbers for some reason.

  • @isaiah20171
    @isaiah20171 4 ปีที่แล้ว

    N-1. Great video. Thank you!

  • @mohammadamiri8847
    @mohammadamiri8847 3 ปีที่แล้ว

    Best explanation ever

  • @suparthghimire1644
    @suparthghimire1644 4 ปีที่แล้ว

    So simple yet so beautiful!!

  • @regdenied3306
    @regdenied3306 6 ปีที่แล้ว +10

    It is quite misleading that after they exchange (g^a)mod n and (g^b)mod n Mike goes on as if alice knew the value of g^b and bob g^a and even saying that those values would now be public, which is simply wrong.

    • @ChrisStavros
      @ChrisStavros 4 ปีที่แล้ว

      I'm glad I'm not the only one I thought this, I literally couldn't go on watching the video, this bothered me so much. At 3:47 He says Bob sends Alice g^b, well now everyone knows b. If he meant to say Bob sends Alice g^b mod n, then he doesn't explains how Alice is able to extract g^b from that (why can't people listening in do this then??) in order to raise it to a.
      Very frustrating.

  • @jlamothe2
    @jlamothe2 4 ปีที่แล้ว +1

    X mod n will be in the range of 0 to n-1, not 0 to n.
    I'm also a little unclear as to how we get g^(a*b) mod n. Bob, for instance would have g, n, b, and g^a mod n. I think it might be the same as (g^a mod n)^b mod n, which Bob could compute, but I'm not sure if that's correct, or if so, why.

    • @xiezhenhaoa
      @xiezhenhaoa ปีที่แล้ว

      According to Congruence modulo, (a mod n) * (b mod n) = a*b mod n, so (g^a mod n)^b mod n = (g^a)^b mod n