Isaac Parenteau
January 15, 2011
Computer Security
Questions #2
2.2) Two basic functions used in an encryption algorithm are substitution and transposition
2.4) a block cipher processes the input one block of elements at a time producing an output block for each input block while a stream cipher
processes the input elements continuously, producing output one element at a time, as it goes along.
2.6) Cipher text only attack –Encryption algorithm and cipher text
Known plaintext attack- Encryption algorithm, cipher text and one or more plaintext
Chosen plaintext attack- Encryption algorithm, cipher text, plaintext message chosen by cryptanalyst, together with its corresponding cipher
text generated with the secret key
Chosen Cipher text attack- Encryption algorithm, cipher text, cipher text chosen by cryptanalyst, together with its corresponding decrypted
plaintext generated with the secret key
Chosen text attack- Encryption algorithm, chipper text, plaintext message chosen by cryptanalyst, together with its corresponding cipher
text generated with secret key and cipher text chosen by cryptanalyst, together with its corresponding decrypted plaintext generated with the
secret key
2.8) Caesar cipher involves replacing each letter of the alphabet with the letter standing three places further down the alphabet.
2.10) Play fair cypher treats diagrams in the plaintext as single units and translates these unites into cipher text diagrams. It is based on the use
of a 5x5 matrix of letters constructed using a keyword.
2.12) Two problems with the one-time pad is that there is the practical problem of making large quantities of random keys and problem of key
distribution and protection because for every message to be sent a key of equal length is needed by both sender and receiver.
2.14) Steganography conceals the existence of a message.
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Decipher:
A good glass in the bishops hostel in the devils seat twenty one degrees and thirteen minutes northeast and by north main branch seventh limb
east side shoot from the left eye of the deathshead a bee line from the tree through the shot fifty feet out
2.6) stenography cypher
2.9)
R O
N E
V B
H I/J
Q S
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P
X
PT BOAT ONE OWE NINE LOST IN ACTION IN BLACKETT
STRAIT TWO MILES SW MERESU COVE X CREW OF TWELVE
X REQUEST ANY INFORMATION.