AES-128 - encrypted public domain text

Question

I work with Historical Geographic Informations Systems on the academic level. I note this to give you an idea of my technical knowledge when it comes to computing, but that most of my knowledge of cryptography is a little outdated I'm more familiar with say the caesar cipher, the Enigma Machine, VENONA, etc from reading about them history and much less with modern encryption systems like PGP or AES.

I came across a text file of an encrypted public domain work which use AES-128 (XML encryption standard). From my initial research on google a brute force attack is unfeasible at that bit length. My question is if I already know what large portions of the encrypted text says when decrypted, because its a public domain work, is that something that can be use to decrypt the file?

Answer 1

if I already know what large portions of the encrypted text says when decrypted (..can it be used..) to decrypt the file?

No, beyond validating a candidate key. With modern ciphers, knowing large portions of the plaintext does not help decipher the rest. They are designed to be immune to Known Plaintext Attack, and even to Choosen Plaintext Attack. And as far as we know, AES-128 in CBC mode (as prescribed in the XML encryption standard) achieves that goal well.