Return to Video

01ps-09 One Time Pad Solution

  • 0:00 - 0:03
    [Narrator] The answer is 1011001,
  • 0:03 - 0:07
    and the way to get that is to look at the 2 possible messages.
  • 0:07 - 0:12
    We had for Y 1011001.
  • 0:12 - 0:22
    For N, we had 1001110, and what we want to do is XOR them to figure out the difference between those 2.
  • 0:22 - 0:30
    The XOR of Y and N is equal to 0010111,
  • 0:30 - 0:36
    and so this is the value that will flip a Y answer into an N answer.
  • 0:36 - 0:39
    Now we want to XOR that with the actual message that was sent.
  • 0:39 - 0:41
    We don't know the key, but we know that
  • 0:41 - 0:46
    that message is the result of XORing Alice's message with the key,
  • 0:46 - 0:53
    and so by XORing those, we get this value 1011001,
  • 0:53 - 0:57
    and if that's what Mallory sends, when Bob XORs that with the key,
  • 0:57 - 1:01
    what Bob will get is the XOR of Alice's message.
  • 1:01 - 1:04
    Alice's message XOR'd with this value,
  • 1:04 - 1:07
    which will flip Alice's message to be the opposite.
  • 1:07 - 1:11
    The point of this question is to illustrate the malleability of the one-time pad.
  • 1:11 - 1:16
    Someone who intercepts the transmission can alter it and change the meaning in predictable ways.
  • 1:16 -
    This is a very dangerous property for a cipher to have.
Title:
01ps-09 One Time Pad Solution
Description:

dummy description

more » « less
Team:
Udacity
Project:
CS387 - Applied Cryptography
Duration:
01:20
Amara Bot added a translation

English subtitles

Revisions