Encrypting files with known headers If I encrypt a file of a known format that has a lengthy header (e.g. an XML Excel file), does that render the encrypted file susceptible to a "known plain text" attack?  In other words, if the first X bytes of the decrypted text are known, does that weaken the encryption for the remaining data?
If a popular encryption scheme is being used: No.

The typical solution is that symmetric stream/block ciphers generate a constant stream of new pseudo-random bits which are merged/XOR'd with the plaintext to produce the ciphertext.

The pseudo-random stream is seeded indirectly by the private key - so as long as the previous or future bits of the PRNG can not be infered from known bits (or infer the PRNG seed for that matter), the cipher is protected from classical known-plaintext attacks.