November 15th, 2004


Crypto Task #3

вот, сделал еще один таск. хороший повод попрактиковаться :)

Welcome to the Crypto Task #3!

Here is some story:
There are three virtual heroes: Alice, Bob and Charley who wants
to know all interesting (especially confidential) information
regarding Alice and Bob.

You are cryptoanalyst who helps Charley. He asked you to decrypt
the last chat of Alice and Bob. You don't know the algorithm of
crypto but Charley provided you with their some previous chat
session with both pairs of plain text / cipher text, and moreover
with the key used in the crypto algorithm!

Here is previous log:

//Secure session: Setting key to 2C5B64F0AE9738D1
//Secure session: Established
Bob: Hi!
Bob^: D5B512
Alice: Halo Bob!
Alice^: D5053697B45ACA7B78
Bob: do you have SP2 installed ?
Bob^: 547592DEA870D64B7CD4CE2B8E4FEA6FA4A5ED8E38697A9BDC0906
Alice: yes i have, but i don't recommend it :)
Alice^: AB35EDA3E8C53A6BC3BDBA2BC0F8396FA4817697D805B1E783BD6E1FA0B102835481C68EB4D106
Bob: I am deciding now... k, I won't install it
Bob^: A5811617B4A98A3B2CED4E4F704D7253BBA1B2C3B48976E72D09551F508D396FA4A5ED8E38697AE72CE4
Alice: :)
Alice^: 8CB1
Bob: :) thank for tip
Bob^: 8CB1928E1849FAABF8DD0EB63C28826A
Alice: np
Alice^: 948C
//Secure session: Closed

( Legend:
( // - just comments
( ^ - crypted message
( without ^ - plain text

But now Charley has only Bob's plain text messages (maybe keylogged?) and wants to know Alice's! So, the main goal of this crypto task is to decrypt Alice's messages in the next chat log after they have established a secure tunnel. The key is unknown too. You have to reveal it.

Here is the last log:

Alice: Hi Bob!
Bob: ohh Alice! Halo!
Alice: how are you doing Bob?
Bob: I am fine! and you?
Alice: everything is cool! I have some news ;)
Bob: ohh, what news? :)
Alice: is this connection secure? did you establish a secure session?
Bob: ahh, no... just a second, I'll create it
//Secure session: Setting key to: *censored*
//Secure session: Established
Alice^: AD26EA7BC24D7B5FA067BFF3748506ED38EF37BB6CA37B9F76E7BFE3B4BBF3B728CFF0DB7C3D7BFF56A7DFF3C47596E74849FA8B6CAD9B2FA0F1EF
Alice^: EF6FCA013C6A3E9F66C1BF8984759C6A151E4A1BEC8DD18CC5914263B41A16C75D397A385964AE253677C2099448F3BA58B9F01EC1D2E3
Alice^: 7BB6
Bob: cool!!!
Bob^: B8394A3B899AAB
Alice^: BE0F702B59E00B
Bob: :)
Bob^: 7BB6
Alice^: EE092A3B496A149FD6E748C019F536F415BE37FB8CD35E8C56B1BF93E4D2F3ADF80937D16C437B45A0F1C2091AF5CC34
Bob: c u
Bob^: B82610
//Secure session: Closed

What interesting news has Alice said to Bob? And why this algorithm is weak?

If you have answers on this questions please send them to me!

My email: [ funbit woof ]

P.S> It is really solvable! Just spend 1 or 2 hours...

Good Luck!

Dated: 15.11.2004

Удачи! Пока существует только одно решение - моё :)
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