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SENDER: Sender transfer message to other party through some sort of internet. RECEIVER: Receiver receives message send by sender through some sort of internet. Trusted third party: trusted third party is needed o secure the message transmits from one party to another. Third party distributes secret information to both the parties. Third party actually provides authenticity of message transmission.
Opponent: An opponent can be a human or a virus that can harm the transmitted message going through some sort of internet.
Secure message: It is a message to which a secret key is applied which is not in readable by opponent.
Secret information: Secret information is a key applied to message.
Ques2:Why is polyalphabetic substitution superior to monoalphatic substitution.
Ans: Polyalphabetic ciphers use multiple substitution alphabetic.
A monoalphabetic substitution replaces each letter with another letter according to cipher alphabet. Polyalphabetic substitution is superior than monoalphabetic because polyalphabetic substitutions are apparently more secure than monoalphabetic substitution, because frequency distribution reflects the distribution of the underlying alphabet .
Ques3:Why are conventional encryption /decryption method not suitable for bank.
Ans: Conventional encryption/decryption method is not suitable for bank because it is less secure.
If conventional encryption/decryption is using, then the limitation is that the keys must be shared between participants before decryption can occur.
If an asymmetric cipher is used instead, only public keys need to be shared -- private keys do not need to be share.
Symmetric encryption is the oldest and best-known technique. A secret key, which can be a number, a word, or just a string of random letters, is applied to the text of a message to change the content in a particular way. This might be as simple as shifting each letter by a number of places in the alphabet. As long as both sender and recipient know the secret key, they can encrypt and decrypt all messages that use this key.
Asymmetric Encryption: The problem with secret keys is exchanging them over the Internet or a large network while preventing them from falling into the wrong hands. Anyone who knows the secret key can decrypt the message. One answer is asymmetric encryption, in which there are two related keys--a key pair. A public key is made freely available to anyone who might want to send you a message. A second, private key is kept secret, so that only you know it.Any message that are encrypted by using the public key can only be decrypted by applying the same algorithm, but by using the matching private key. Any message that is encrypted by using the private key can only be decrypted by using the matching public key.
This means that you do not have to worry about passing public keys over the Internet . A problem with asymmetric encryption, however, is that it is slower than symmetric encryption. It requires far more processing power to both encrypt and decrypt the content of the message.
Ques4:Define types of attack based on what is known to attacker.
Ans: The following are the cryptographic attacks usually performed by an attacker:
Known plaintext attack: In a known plaintext attack, an attacker should have both the plaintext and ciphertext of one or more messages. These two items are used to extract the cryptographic key and recover the encrypted text.
Ciphertext only attack: In this attack, an attacker obtains encrypted messages that have been encrypted using the same encryption algorithm. For example, the original version of WEP used RC4, and if sniffed long enough, the repetitions would allow a hacker to extract the WEP key. Such types of attacks do not require the attacker to have the plaintext because the statistical analysis of the sniffed log is enough.
Man-in-the-middle attack: In this form of attack, an attacker places himself in the middle of the communications flow between two parties. Once an attacker enters the communications flow, he is able to perform a ciphertext only attack, exchange bogus keys, etc.
Replay attack: In this type of attack, an attacker tries to repeat or delay a cryptographic transmission. A replay attack can be prevented using session tokens.
Chosen plaintext attack: In a chosen plaintext attack, an attacker somehow picks up the information to be encrypted and takes a copy of it with the encrypted data. This is used to find patterns in the cryptographic output that might uncover a vulnerability or reveal a cryptographic key.
Chosen ciphertext attack: In this type of attack, an attacker can choose the ciphertext to be decrypted and can then analyze the plaintext output of the event. The early versions of RSA used in SSL were actually vulnerable to this attack.
1:Cipher text only attack
Given cipher encrypted with same key
C1=Ek (p1) ,C2=Ek(p2)
Attackers find key.
2:Known plain text attack
Plain text is corresponding to the cipher text.
Eg: P1,C1=Ek(P1) P2,C2=Ek(P2)
3:Chosen plain text attack
Plain text can be chosen that gets encrypted.
4:Adaptive chosen plain text attack
Can modify the plain text choice depending on results of previous encryption.
5 :Chosen cipher text attack
Can choose different cipher texts to be decrypted.
Ques6:Encrypt the following plaintext by playfair substitution "wearediscoveredsaveyourself"
Ans:Playfair is a multi-letter encryption which treats diagrams in the plaintext as a single units and translates these units into ciphertext diagrams.
The Playfair algo is based on use of 5*5 matrix of letters constructed using a key.
Choosen key is "WONDERS"
The matrix is constructed by filling in the letters of the keyword "WONDERS" from left to right and top to bottom and then filling in the remainder of the matrix with the remaining letters in alphabetic order. The letters I and J count as one letter.
Rules will be : 1:Repeating plaintext letters that are in same pair are separated with a filler letter .
2: Two plaintext letters that fall in the same of matrix are each replaced by the letter to the right circularly .
3:Two plaintext letters that fall in the same column are each replaced by the letter beneath to it.
4:Otherwise ,each plaintext letter in a pair is replaced by the letter that lies in its own row and the column occupied by the plaintext letter.
Given string is "WEAREDISCOVEREDSAVE YOURSELF"
Playfair substitution is 'OWBSWEHGABEBCGKNASXDZWFCOUL"gtttttttttfygjhvhghjqqQQCh