If a message is sent using one-time pad scheme, which generates random bits in order to, for example, XOR them with the message's bits - how does the receiving side generate the exact series of bits ?

If the receiving side is able to generate the same series then how is it still random ?

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    $\begingroup$ The receiving side needs to receive an explicit copy of the random bits. $\endgroup$
    – SEJPM
    Feb 9 '19 at 14:45
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    $\begingroup$ And said random bits have to remain secret. In the rare practical cases when the OTP is used, the random bits are generated in advance of transmission, and sent by secure means such as trusted couriers. $\endgroup$
    – fgrieu
    Feb 9 '19 at 15:18

The one-time pad requires pre-shared key. Usually, one of the two parties generates the keystream, writes into a paper or CD/DVD, or USB, etc.. and sends the keystream to the other party in a secure way. The secure way is not encryption, it is using some men to carry it to the target. After the transmission, they can use the key to decipher.

Wikipedia lists historical uses of the one-time pad. One case is very important; the Venona Project. At some point, the Russians had reused some part of the key. So, the question is how one notices this? The USA and British Code-breakers always tried the crib-dragging and one day was successful!

Some of The Crib Draggin answers in CryptoSO;

  1. Taking advantage of one-time pad key reuse?
  2. How does one attack a two-time pad (i.e. one-time pad with key reuse)?
  3. Little problem with Vernam Cipher

And, an article: A Natural Language Approach to Automated Cryptanalysis of Two-time Pads

  • 1
    $\begingroup$ A coincidence test suffices to detect re-use. No need for crib-dragging for that. See the classic texts by Friedman etc. $\endgroup$ Feb 10 '19 at 22:19
  • $\begingroup$ @HennoBrandsma good point. When I see the coincidence test I remembered but needs to refresh. Thanks. $\endgroup$
    – kelalaka
    Feb 10 '19 at 22:31

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