use the same pad reversed

Question

1. lets say I have this random numbers ' 625 467 921 ' and use them once to encrypt a message otp style and then I want to use them again so I reverse them to be ' 129 764 526' and use to encrypt the other message what 'll be the weakness of that.

2. same thing I have this numbers ' 625 467 921 ' and want to use again so I add static value to each number let it be '4' module10 the numbers will be ' 069 801 365 ' and use them for the second message and for the 3rd message I add to the numbers '6' or '7' and keep use the same numbers for each message and all I need to encrypt is the number to add to the last used numbers.

Will this numbers be random or not and what its weakness.

Answer 1

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.

1. is unkeyed and can be trivially reversed by an attacker. So you end up with something that amounts to a two-time-pad, and that's not very secure.
2. The second one uses a keyed transformation, essentially a caesar cipher. Apart from this transform you're once again reusing a one-time-pad. Since the caesar cipher has a very small key, your security gain over plain one-time-pad reuse is just as small. Still extremely weak.