Please help - a question on Ciphering...
From: Jay Walters (anonymous_at_discussions.microsoft.com)
Date: 02/19/04
- Next message: Hayato Iriumi: "Re: 70-300 Exam"
- Previous message: Brunswick Lowe: "Re: MCSD or MCSD .Net?"
- In reply to: Vincent: "Please help - a question on Ciphering..."
- Next in thread: Vincent: "Please help - a question on Ciphering..."
- Reply: Vincent: "Please help - a question on Ciphering..."
- Messages sorted by: [ date ] [ thread ]
Date: Thu, 19 Feb 2004 08:13:14 -0800
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: Hayato Iriumi: "Re: 70-300 Exam"
- Previous message: Brunswick Lowe: "Re: MCSD or MCSD .Net?"
- In reply to: Vincent: "Please help - a question on Ciphering..."
- Next in thread: Vincent: "Please help - a question on Ciphering..."
- Reply: Vincent: "Please help - a question on Ciphering..."
- Messages sorted by: [ date ] [ thread ]
Relevant Pages
|
