# Partial encryption of a file?

I want to partially encrypt a large block of data using AES. Meaning: given plaintext[0:1100], I would like to encrypt plaintext[0:1023]. The plaintext is generally machine code, but might have snippets of text.

Does this expose what I encrypted to any realistic challenges in the context of my application (for example: copy protection)?

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The answer depends on the nature of the plaintext, and how that makes it possible to deduce something useful about the plaintext from plaintext[1024:1100] (which if I get the question correctly is not encrypted). –  fgrieu Oct 17 '13 at 11:57
It depends largely on the mode of operation used and if you want to be able to decrypt parts with random access. If you use ECB (not recommended), then an attacker can identify two identical blocks easily. –  tylo Oct 17 '13 at 12:30
@tylo No need to decrypt with random access. –  hulkingtickets Oct 18 '13 at 22:16