5. Suppose you have a language with only the 3 letters a, b, c, and they occur with frequencies .7, .2, .1, respectively. The following ciphertext was encrypted by the Vigenere method (shifts are mod 3 instead of mod 26, of course): ABCBABBBAC. Suppose you are told that the key length is 1, 2, or 3. Show that the key length is probably 2, and determine the most probable key.

5. Suppose you have a language with only the 3 letters a, b, c, and they occur with frequencies .7, .2, .1, respectively. The following ciphertext was encrypted by the Vigenere method (shifts are mod 3 instead of mod 26, of course): ABCBABBBAC. Suppose you are told that the key length is 1, 2, or 3. Show that the key length is probably 2, and determine the most probable key.

Elements Of Modern Algebra

8th Edition

ISBN:9781285463230

Author:Gilbert, Linda, Jimmie

Publisher:Gilbert, Linda, Jimmie

Chapter2: The Integers

Section2.8: Introduction To Cryptography (optional)

Problem 19E: Suppose that in an RSA Public Key Cryptosystem. Encrypt the message "algebra" using the -letter...

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Suppose that in an RSA Public Key Cryptosystem, the public key is. Encrypt the message "pay me later” using two-digit blocks and the -letter alphabet from Example 2. What is the secret key? Example 2 Translation Cipher Associate the letters of the "alphabet" with the integers. Let and define the mapping by where is the key, the number of positions from the plaintext to the ciphertext. If our alphabet consists of through, in natural order, followed by a blank, then we have "letters" that we associate with the integers as follows:
Suppose that in an RSA Public Key Cryptosystem, the public key is e=13,m=77. Encrypt the message "go for it" using two-digit blocks and the 27-letter alphabet A from Example 2. What is the secret key d? Example 2 Translation Cipher Associate the n letters of the "alphabet" with the integers 0,1,2,3.....n1. Let A={ 0,1,2,3.....n-1 } and define the mapping f:AA by f(x)=x+kmodn where k is the key, the number of positions from the plaintext to the ciphertext. If our alphabet consists of a through z, in natural order, followed by a blank, then we have 27 "letters" that we associate with the integers 0,1,2,...,26 as follows: Alphabet:abcdef...vwxyzblankA:012345212223242526
Rework Example 5 by breaking the message into two-digit blocks instead of three-digit blocks. What is the enciphered message using the two-digit blocks? Example 5: RSA Public Key Cryptosystem We first choose two primes (which are to be kept secret): p=17, and q=43. Then we compute m (which is to be made public): m=pq=1743=731. Next we choose e (to be made public), where e must be relatively prime to (p1)(q1)=1642=672. Suppose we take e=205. The Euclidean Algorithm can be used to verify that (205,672)=1. Then d is determined by the equation 1=205dmod672 Using the Euclidean Algorithm, we find d=613 (which is kept secret). The mapping f:AA, where A=0,1,2,...,730, defined by f(x)=x205mod731 is used to encrypt a message. Then the inverse mapping g:AA, defined by g(x)=x613mod731 can be used to recover the original message. Using the 27-letter alphabet as in Examples 2 and 3, the plaintext message no problem is translated into the message as follows: plaintext:noproblemmessage:13142615171401110412 The message becomes 13142615171401110412. This message must be broken into blocks mi, each of which is contained in A. If we choose three-digit blocks, each block mim=731. mi:13142615171401110412f(mi)=mi205mod731=ci:082715376459551593320 The enciphered message becomes 082715376459551593320 where we choose to report each ci with three digits by appending any leading zeros as necessary. To decipher the message, one must know the secret key d=613 and apply the inverse mapping g to each enciphered message block ci=f(mi): ci:082715376459551593320g(ci)=ci613mod731:13142615171401110412 Finally, by re-breaking the message back into two-digit blocks, one can translate it back into plaintext. Three-digitblockmessage:13142615171401110412Two-digitblockmessage:13142615171401110412Plaintext:noproblem The RSA Public Key Cipher is an example of an exponentiation cipher.
Suppose the alphabet consists of a through z, in natural order, followed by a blank and then the digits 0 through 9, in natural order. Associate these "letters" with the numbers 0,1,2,...,36, respectively, thus forming a 37-letter alphabet, D. Use the affine cipher to decipher the message X01916R916546M9CN1L6B1LL6X0RZ6UII if you know that the plaintext message begins with "t" followed by "h". Write out the affine mapping f and its inverse.
Suppose that in an RSA Public Key Cryptosystem. Encrypt the message "algebra" using the -letter alphabet from Example 4. Use two-digit blocks. Use three-digit blocks. What is the secret key?
a. Excluding the identity cipher, how many different translation ciphers are there using an alphabet of n "letters"? b. Excluding the identity cipher, how many different affine ciphers are there using an alphabet of n "letters," where n is a prime?
Suppose that the check digit is computed as described in Example . Prove that transposition errors of adjacent digits will not be detected unless one of the digits is the check digit. Example Using Check Digits Many companies use check digits for security purposes or for error detection. For example, an the digit may be appended to a -bit identification number to obtain the -digit invoice number of the form where the th bit, , is the check digit, computed as . If congruence modulo is used, then the check digit for an identification number . Thus the complete correct invoice number would appear as . If the invoice number were used instead and checked, an error would be detected, since .
Suppose a codding scheme is devised that maps -bit words onto -bit code words. The efficiency of the code is the ratio . Compute the efficiency of the coding scheme described in each of the following examples. Example 1 Example 2 Example 3 Example 4 Example 1: Parity check – Consider -bit words of the form . One coding scheme maps onto , where is called the parity check digit. Example 2: Repetition Codes – Multiple errors can be detected (but not corrected) in a scheme in which a -bit word is mapped onto a -bit code word according to the following scheme: Example 3: Maximum Likelihood Decoding- Multiple errors can be detected and corrected if each -bit word is mapped onto a - bit code word according to the following scheme (called a triple repetition code): Example 4: Error Detection and Correction – Suppose -bit words are mapped onto -bit code words using the scheme , Where is the parity check digit .
Use the alphabet C from the preceding problem and the affine cipher with key a=11andb=7 to decipher the message RRROAWFPHPWSUHIFOAQXZC:Q.ZIFLW/O:NXM and state the inverse mapping that deciphers this ciphertext. Exercise 7: Suppose the alphabet consists of a through z, in natural order, followed by a colon, a period, and then a forward slash. Associate these "letters" with the numbers 0,1,2,...,28, respectively, thus forming a 29-letter alphabet, C. Use the affine cipher with key a=3andb=22 to decipher the message OVVJNTTBBBQ/FDLWLFQ/GATYST and state the inverse mapping that deciphers this ciphertext.
Suppose that the most common letter and the second most common letter in a long ciphertext produced by encrypting a plaintext using an affine cipher f (p) = (ap + b) mod 26 are Z and J, respectively. What are the most likely values of a and b?
Suppose that when a string of English text is encrypted using a shift cipher f (p) = (p + k) mod 26, the resulting ciphertext is DY CVOOZ ZOBMRKXMO DY NBOKW. What was the original plaintext string?
Consider the affine cipher with key k = (k1, k2) whose encryption and decryption functions are given by (1.11) on page 43. Alice and Bob decide to use the prime p = 601 for their affine cipher. The value of p is public knowledge, and Eve intercepts the ciphertexts c1 = 324 and c2 = 381 and also manages to find out that the corresponding plaintexts are m1 = 387 and m2 = 491. Determine the private key and then use it to encrypt the message m3 = 173.