Is it possible to find key to decrypt ciphertext by having the original plain text?

This is called a known-plaintext attack Modern ciphers such as AES are not vulnerable to these attacks.

This is called a known-plaintext attack. Modern ciphers such as AES are not vulnerable to these attacks.

– Tao May 18 at 12:37 Not on any practical timescale, unless the attacker is supernaturally lucky. – crazyscot May 18 at 12:38.

Depends how you define "Possible". It is generally understood, for the moment, to be an impractical task. That said, storing plaintext values with their encrypted counterparts is never recommended.

Especially if you do this for many sets of plaintext/ciphertext pairs, you increase the amount of information available to the attacker and weaken the encryption.

I don't agree with the last sentence. The reason you don't store the plaintext with the ciphertext is the obvious one: the whole point of encrypting is to prevent knowledge of the plaintext by unauthorized parties. – GregS May 18 at 22:56 @GregS: yes, clearly that is a more obvious reason.

Assuming that that particular plaintext is NOT a secret, however, and its ciphertext could exist in a related location for some other reason (eg to be handled by a process that would normally expect ciphertexts), setting things up this way is bad practice. Security standards such as the Payment Card Industry Data Security Standard (PCI-DSS) explicitly disallow this type of setup: you cannot store the last 4 digits of a cardnumber in the same table that stores the encrypted cardnumber. – Tao May 19 at 7:02 I don't like "well known facts" and other similar types of statements, including declaring things as "bad practice".

I say, prove it. – GregS May 19 at 22:18 1 @GregS: prove that the PCI-DSS disallows storing plaintext last 4 digits in the same table as full cardnumber? All I have to go on is our auditor's word.

The phrase "well known facts" is your own, not mine. That storing plaintext with ciphertext is considered bad practice more generally? Apparently the debate rages on with more vehemence than I expected: ciphersbyritter.Com/NEWS6/KNOWNPLN.

HTM – Tao May 19 at 23:31.

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