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Assume that a malicious user knows what string is being encrypted (a user_id, for example, which the user can know) and can also see the result of the encryption (the encrypted user_id that is used to authenticate them to another site.)

If the user knows what is going into the encrypt function, as well as what is coming out, in theory it seems it could be possible for them to determine the encryption key as well.

Assuming that this is the case, what could be done to address this problem?

Background: The project I'm working on uses the code found below. I have concerns that the shared encryption key could become compromised http://grokbase.com/p/php/php-notes/1181cckm7e/note-105166-added-to-function-mcrypt-encrypt

The concern is simple: The key is shared, so if someone knows the key, they can trick the authentication mechanism and log in with whatever user ID they want.

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    $\begingroup$ You are afraid of a known-plaintext attack. Modern ciphers are designed to withstand this kind of attack. However, the linked code still has (at least) two issues: It uses DES (nowadays breakable by brute-force) in ECB mode (which is generally a bad idea). Also, note that encryption alone is not enough to prevent manipulation; you may want to add a message authentication code. $\endgroup$
    – yyyyyyy
    Mar 10 '15 at 19:21
  • $\begingroup$ It might not be possible but i would STRONGLY recommend you to use an other encryption method like AES. DES is extremely weak nowadays and some real effort should be put into replacing it by a better algorithm. $\endgroup$
    – Vincent
    Mar 11 '15 at 8:41