Separate data encryption -

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i store sensitive data. data divded parts , want have separate accees each part. let's assume have 1000 files. want encrypt each file same symetric encryption algorithm. guess breaking key easier when hacker has got 1000 cryptogram has 1 cryptogram, think should use separate key each file. question following:

should use separate key each file?

if should, there problem storing 1000 keys. want have 1 secret key , use own algorithm calculate separate key each file secret key. idea?

if consider passive adversary , use cpa-strong cipher (like aes), sufficient use 1 key files. supposing adversary knows cipher use, , knows plaintexts, cannot reconstruct key non-negligible probability. here more detailed answer.

if consider active adversary (which can replace ciphertexts) should use authenticated encryption. understand not case.

so want have 1 secret key , use own algorithm calculate separate key each file secret key. idea?

in general, developing own algorithm or scheme bad idea. can make unseen mistake in algorithm or implementation , data vulnerable. better use well-known algorithms , implementations peer-reviewed lots of people , proved secure.


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