RSA - Does knowledge of part of the ciphertext decrease encryption security for the rest of the ciphertext?
No known, modern cipher should be vulnerable against known plaintext attacks. Now just RSA 2048 is not likely to be used on all the text, and RSA encryption can be implemented in different ways, but it is unlikely to the extreme that the protocol is vulnerable against known plaintext attacks. It may of course well be vulnerable against other attacks, such as padding oracle attacks on PKCS#1 v1.5 padding, if that is used.

So you can create a string as large as you want and still be secure against known plaintext attacks, no matter how much of it is guessed or gets known otherwise.



Hah, and I looked up the standard after I wrote this answer and got this in return. It doesn't contain known plaintext attacks of course, that would be too easy. But it does use PKCS#1 v1.5 - which may be vulnerable and AES-CBC without authentication which is even more likely to be vulnerable (ugh).

The paper by the way has been "withdrawn for non scientific reasons" - probably to hide it under the carpet, but you can still download it if you know how to retrieve old internet content (deliberately not linked). Here is the security advisory that links to the missing paper.