On Tuesday, September 26, 2017, 8:14:35 AM PDT, \0xDynamite <dreamingforward@gmail.com> wrote:





>>AFAICT, it is UNBREAKABLE if the keysize is at least half the size of
> your plaintext.
>
>> My understanding is that the keysize ought to be as long as the message to
>> be encrypted.

>Well if they don't know the key length, then using somewhere between
1/3 and 2/3 should make it unbreakable.

No, using the full length is necessary.  Using a key length less than the full length of the message is very dangerous.

>> And yes, it is unbreakable...  UNLESS you try to reuse the keys!!!

>Hmm, but THEY don't know if you re-used them.  So it would take how
many ciphertexts messages to figure that out?

You presumably haven't Google-searched "Venona" yet.  I believe it took a few thousand.  
See:  https://en.wikipedia.org/wiki/Venona_project

                    Jim Bell

From that article:

"Decryption[edit]
This message traffic, which was encrypted with a one-time pad system, was stored and analyzed in relative secrecy by hundreds of cryptanalysts over a 40-year period starting in the early 1940s. Due to a serious blunder on the part of the Soviets, some of this traffic was vulnerable to cryptanalysis. The Soviet company that manufactured the one-time pads produced around 35,000 pages of duplicate key numbers, as a result of pressures brought about by the German advance on Moscow during World War II. The duplication—which undermines the security of a one-time system—was discovered and attempts to lessen its impact were made by sending the duplicates to widely separated users.[16] Despite this, the reuse was detected by cryptologists in the US."