Suppose I have a random number (eg
625 467 921). I use it once to encrypt a message OTP style, and when I want to use them again, I reverse them (eg to
129 764 526) and use to encrypt the other message. What is the weakness of this scheme?
Again, suppose I have some random numbers (again, eg
625 467 921). When I want to use them again, I add a static value to each number (eg adding $4$ modulo $10$, the numbers will be
069 801 365) and use these new values for the second message. For the 3rd message, I add to the numbers $6$ or $7$ etc. I keep using the same base numbers for each message, and all I need to encrypt is the number to add to the last used numbers. Will these numbers be random? What is the weakness in the scheme?
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A standard assumption in cryptography is that an attacker knows everything about your cryptosystem, apart from the key. This is known as Kerckhoffs's principle. Taking a look at your construction under this assumption, shows that it's extremely weak.
What you are essentally describing is a Pseudo-Random Generator in which the base numbers act as the seed. This does not make a good method for generating OTPs because if you know the seed then you just work from there and crack the message.