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i was wondering whether AES-256, with a strong key, becomes less secure when some part of the solution is known. as an example scenario, the string i want to encrypt is “my secret password is: du420rfashud”. the attacker who tries to get my password knows that the string starts with “my secret password is: “. is it possible to decrypt the rest of the string in this case? also my second question is, if the attacker knows the complete solution, can he derive the key from that?
thanks
The scenario that you describe is called a known plaintext attack (or even a chosen plaintext attack). Good cryptographic ciphers are not vulnerable against those attacks, and that includes the AES family of ciphers.
Basically, no matter how much information you have except the key, finding the key is no easier than brute-force trying.
(Practically, such attacks are quite easy to mount, since all sorts of encrypted communication contains predictable standard headers. Therefore, such a vulnerability would make a cipher essentially entirely useless.)