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I need to know there is any algorithm that crypts data in same size,

Any knowledge appriciated!

Thank you!

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It's possible to do that, but there are downsides. You should describe your additional requirements and say how you plan to use it. For example CTR mode encrypts to the same size, but it's quite likely that it'll be totally insecure in your scenario. – CodesInChaos Jan 24 at 12:00
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Your question is very vague. Same length encryption can't offer semantic security, but that doesn't mean its insecure for every situation. There are different approaches, with different downsides. So you need to describe your scenario so we can find one that fits for your situation. – CodesInChaos Jan 24 at 12:22
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Search for format preserving encryption. I'd guess the correct security model for length preserving encryption is that it's a variable length pseudo-random permutation (PRP). – CodesInChaos Jan 24 at 12:30
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To 100% clarify, you are looking for an encryption algorithm that maps every possible binary input with an integral number of bytes to a encrypted output of the same length. Is that correct? And you understand the inherent security problems with such an algorithm? (No defense against corruption, replay attacks, and so on.) – David Schwartz Jan 29 at 1:10
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-1, this question is so vague and poorly written that, even with the "clarifying" comments, it's hard for me to tell what's really being asked. Sorry, but it really is. Considering voting to close as "not a real question". – Ilmari Karonen Feb 27 at 11:37
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1 Answer

up vote 2 down vote

Your are looking for a encryption scheme that supports length preserving encryption. I recommend to use an authenticated encryption scheme like OCB or McOE. There are two common techniques to achieve this goal:

  1. Ciphertext Stealing
  2. Tag splitting

Note that you need at least either a nonce or authentication tag -- or better both -- to preserve data privacy, i.e., security against chosen plaintext attacks (CPA).

Maybe I got you wrong. Maybe you are looking for format-preserving encryption or something else. Hence, a more detailed problem description would be great.

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I see no way for authenticated encryption to be length preserving. Length preserving encryption is obviously a bit weaker than what we can achieve by adding a nonce/IV and authentication tag, but it's still useful sometimes. – CodesInChaos Jan 28 at 10:27
@CodesInChaos I am not familiar with cryptography dictionary, however I am a porgrammer somehow. I am not in a needing of a length preserving algorithm, I am trying to understand the situation of my algorithm in real world of cryptography with your precious assists with your understanding, thank you! – Digerkam Jan 29 at 10:33

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