Please help - a question on Ciphering...

Discussion in 'MCSD' started by Vincent, Feb 19, 2004.

  1. Vincent

    Vincent Guest

    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. :)
     
    Vincent, Feb 19, 2004
    #1
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  2. Vincent

    Jay Walters Guest

    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. :)
    >.
    >
     
    Jay Walters, Feb 19, 2004
    #2
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  3. Vincent

    Vincent Guest

    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. :)
    >>.
    >>

    >.
    >
     
    Vincent, Feb 20, 2004
    #3
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