Please help - a question on Ciphering...
From: Vincent (anonymous_at_discussions.microsoft.com)
Date: 02/20/04
- Next message: ipassmcse: "NO Exam!! Get your Cert directly!"
- Previous message: anonymous_at_discussions.microsoft.com: "Re: Good news"
- In reply to: Jay Walters: "Please help - a question on Ciphering..."
- Messages sorted by: [ date ] [ thread ]
Date: Thu, 19 Feb 2004 21:50:36 -0800
Hello Jay,
Thanks a lot for your valuable comments. It helps me a
lot. Thanks again! :-)
Vincent
>-----Original Message-----
>This feels like a game of boggle :-)
>
>How to crack: write a recursive function that iterates
>through the possibly. For a person to physically look at
>each variant would take forever. However writing another
>function to perform substring checks against a
dictionary
>to tag possible successes would cut this down.
>
>It's 8am ... I'm tired. So I'm shooting in the dark here.
>
>A real example would be better, do you have spaces in
>your text, or are all the words run together?
>
>number of possibilities / number of keys tried per
second
>= total number of seconds
>
>fixed key size of 26 ... 26 different letters something
>like : (26*25*24* ... *3*2*1) = total number of
>possibilities minus the math for a!=a, b!=b
>
>Interms of question 4 and 5 ... I smell a trick here.
>
>Decrypting a single word would be very hard because as
>you run through all of the cipher key possibilities,
>you'd basically create every 5 letter word in the
>dictionary. So yes, you could decrypt it, you just
>wouldn't know which word is right :-)
>
>The more words to decrypt, the greater chance you'll get
>a match.
>
>Bottom line: It would still take some time, but it's
very
>achievable.
>
>
>
>
>
>
>
>
>
>
>
>>-----Original Message-----
>>Dear experts,
>>
>>I am a college student and I was asked a question on
>>Cipher (network security). I am frustrated as I could
>not
>>solve this problem myself. If possible, please help.
>>
>>Scenario:
>>Consider the subsitution cipher for English text which
>>consists of A,B,...,Y,Z only (a total of 26 letters).
>The
>>encryption rule is to substitute a letter by another
>>letter which is different from itself. For instance,
>>subsitute A by W (but not A), B by H (but not
B),...etc.
>>The actual subsitution rule is governed by a key. Once
>>the key is chosen, the subsitution rule is fixed and
can
>>represented as follows:
>>
>>A-W; B-H; C-J; D-K; E-Y; .... ;X-B; Y-U; Z-L
>>
>>Questions:
>>1) Based on the above cipher system, determin the total
>>number of different keys
>>2) If an attacker uses a brute force attack to decrypt
a
>>particular message, and he try 1,000,000 keys in a
>>second. WHat is the average time that he can decrypt
the
>>message?
>>3) Is it possible to decrypt the ciphertext "WXEUV" by
>>this brute force attack? Why?
>>4) Is it possible to decrypt a ciphertext which
consists
>>of 100,000 letters? WHy?
>>
>>Please give me some ideas if possible.
>>Thanks a lot for your help. :-)
>>.
>>
>.
>
- Next message: ipassmcse: "NO Exam!! Get your Cert directly!"
- Previous message: anonymous_at_discussions.microsoft.com: "Re: Good news"
- In reply to: Jay Walters: "Please help - a question on Ciphering..."
- Messages sorted by: [ date ] [ thread ]
Relevant Pages
|
