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Given some input plaintext and output ciphertext, is there a group of crypto algorithms that mean that a change in the ending x bytes of the input plaintext makes some y change at the end of the ciphertext as well? The point is to conserve the visual pattern, but to still be encrypted.

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    $\begingroup$ This normally is not a desirable property, see en.wikipedia.org/wiki/Block_cipher_mode_of_operation#ECB $\endgroup$
    – bmm6o
    Dec 15, 2020 at 16:57
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    $\begingroup$ Can you elaborate on what you mean by conserve the visual pattern? $\endgroup$
    – Modal Nest
    Dec 15, 2020 at 16:59
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    $\begingroup$ no one wants that. If that happens then your encryption algorithm is weak and can be broken easily! $\endgroup$
    – Aven Desta
    Dec 15, 2020 at 17:24
  • $\begingroup$ Given something like a 156 byte string of encrypted data, if the plaintext was to change the last 20 bytes of the string then the ciphertext will have changed the last 20/25/32/etc bytes, but always at the end and if its a 20 byte change the output change is always the same amount. $\endgroup$
    – Monteven
    Dec 15, 2020 at 21:35

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Name for crypto algorithm that preserves visual pattern

I'm not sure exactly what you're looking for, but one interpretation of your question is that you're looking for a way to 'hide' a message within a picture in a way that the picture isn't obviously changed.

The general technique to do this is known as "steganography"; that has the goal of hiding a message into something innocuous (such as a JPEG) in such a way that it's hard to discern (without the key) that there's a message there. Steganography isn't an algorithm; instead, it is the general term for any algorithm that attempts to achieve this.

One common (and rather simple-minded) technique is to take the various bits of the message, and hide them within lsbits of various pixels the picture (tweaking the pixels only slightly); which pixels you use would depend on the key.

Standard joke: for more information, see the steganography stack exchange - just be warned, it is very well hidden...

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