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Say i'm given a cipher text to decrypt, however i also have access to a server that performed the encryption; where i could enter any plaintext and observe the result of encryption. would it be classed as a known plain text attack or cipher text? I'm abit confused

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Neither, as you've described the prerequisites or - in a theoretical model - the setting for an attack to take place, but not the attack itself.

What you seem to have is a ciphertext oracle which can be used within a Chosen Plaintext Attack (CPA). As you can "enter any plaintext" I presume you can investigate the results and the request encryption of another plaintext from the oracle. If you can use this to attack the cipher then it is known as an Adaptive Plaintext Attack.

If the cipher resists such attacks and the ciphertext doesn't give you any information about the key (or any other unkonwn plaintext) then the cipher is IND-CPA2 secure.

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If you can encrypt any text of your choosing, that'd be a chosen plaintext attack.

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  • $\begingroup$ yes but i only have access to the cipher text. not the plain text. $\endgroup$
    – hithere
    May 18, 2020 at 21:30
  • $\begingroup$ @hithere: but you can 'chose' 'plaintext' to encrypt; this makes it a chosen plaintext attack (even if you cannot immediately access the plaintext to the encrypted message in question) $\endgroup$
    – poncho
    May 18, 2020 at 21:32

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