Suppose that an attacker intercepts a cryptogram and wants to try an exhaustive key search to find the message. What are his chances if:
(a)
he does not know the encryption
algorithm used
Without the knowledge of the plaintext
and algorithm, the attacker will have the highest level of difficulty in
figuring out the message. The attacker may not be able to retrieve the message.
This also known as Cipher-text Only Attack.
(b)
he knows the algorithm but has no
other information except that the plaintext is random
The attacker has chances of retrieving
the message. However, this may still be very time-consuming as the plaintext
could be in any language.
This attack probably still falls under
the Chosen Plaintext Attack.
(c)
he knows the algorithm used and that
the message is in English
The attacker has better chances of
successfully gaining the message. The knowledge that the plaintext is in
English can greatly reduce the time as all the other languages can be
eliminated.
(d) he
knows the algorithm and has some corresponding plaintext/ciphertext obtained by
using the same key
Plaintext | Algorithm | Key | Ciphertext
The attacker has very high chances of being able to retrieve the
message as he has the algorithm, has the pairs and the key. With all the
available information, the attacker will have the best opportunity to
successfully gain the message compared to the all the other attacks described
above.
This falls under the Chosen
Chiper-text Attack.
