I'm a freshmen in cryptography and want to know more about IV reversing. There's a lot of posts about finding the passphrase with a new IV. But what about the other situation? This is for educational purposes.

I have a cypher text :

cY1Y1VPXbhUqzYLIOVR0RhUXD5l+dmymBfr1vIKlyqD8KqHUUp2I3dhFXgASdGWzRhOdTj8WWFTJ PK0k/GDEVUBDCk1MiB8rCmTZluVHImczlOXEwJSUEgwDHA6AbiCwyAU58e9j9QbN+HwEm1TPKHQ6 JrIOpdFWoYjS+cUCZfo/85Lqi26Gj7JJxCDF8PrBp/EtHLmmTmaAVWS0ID2cJpdmNDl54N7tg5TF TrdtcIplc1tDvoCLFPEomNa5booC

The corresponding plain text :

Marvin: "I am at a rough estimate thirty billion times more intelligent than you. Let me give you an example. Think of a number, any number."
Zem: "Er, five."
Marvin: "Wrong. You see?"

And the passphrase :


I looked here and here but haven't found what I need.

How can I use those to find the IV? The IV length is unknown.

  • Do you mean the key or IV?
    – RoraΖ
    Sep 30, 2015 at 15:56
  • I mean the IV. But if you can got both easily it's ok too !
    – Platonium
    Sep 30, 2015 at 16:03
  • 1
    Usually the algorithm and mode of operation is known in advance. Was AES / CBC mode used for this encryption (don't let us try all posibilities!)? Note that your passphrase does not seem to be a passphrase, it looks quite a lot like a base 64 encoded key value. Keys are binary, passphrases are text. Sep 30, 2015 at 16:31
  • For CBC mode, the IV length is the same as the block length. Since the block length of AES is always 16 bytes (128 bits) that is your IV length,
    – Xander
    Sep 30, 2015 at 16:32
  • Come on man, for real ? Asking for other peoples to solve the chall for you ? At least, link the credits ! --" <br><br> root-me.org/en/Challenges/Cryptanalysis/Initialisation-Vector Btw, the value given isn't the flag, pfew ! (well, not hardcoded at least...)
    – Laluka
    Jan 10, 2018 at 7:03

1 Answer 1


Its AES in CBC mode with PKCS#7 padding.

The IV in CBC is XOR'ed with the plaintext. This results in the following value in hexadecimals:


and "Marvin: \"I am at" is:


so the IV will be the two values XOR'ed:


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