•Details
of the Encryption Scheme
•The
design of a cryptosystem is based on the following two cryptography algorithms −
–Public
Algorithms −
With this option, all the details of the algorithm are in the public domain,
known to everyone.
–Proprietary/Private
algorithms −
The details of the algorithm are only known by the system designers and users.
•In
case of proprietary algorithms, security is ensured through
obscurity.
–Private
algorithms
may not be the strongest algorithms as they are developed in-house and may not
be extensively investigated for weakness.
–They allow
communication among closed group only. Hence they are not suitable for modern
communication where people communicate with large number of known or unknown
entities.
•According
to Kerckhoff’s
principle, the
algorithm is preferred to be public with strength of encryption lying in the
key.
•Thus,
the first assumption about security environment is that the encryption
algorithm is known to the attacker.
•Availability
of Ciphertext
•We
know that once the plaintext is encrypted into ciphertext, it is put on unsecure public channel
(say email) for transmission.
•Thus, the
attacker can obviously assume that it has access to the ciphertext
generated by the cryptosystem.
•Availability
of Plaintext and Ciphertext
•Not
that obvious…
because plaintext availability is what we are talking about.
•However,
there may be situations where an attacker can have access to plaintext and
corresponding ciphertext.
•Some such
possible circumstances are −
–The
attacker influences the sender to convert plaintext of his choice and obtains
the ciphertext.
–The
receiver may divulge the plaintext to the attacker inadvertently. The attacker
has access to corresponding ciphertext gathered from open channel.
–In a
public-key cryptosystem, the encryption key is in open domain and is known to
any potential attacker. Using this key, he can generate pairs of corresponding
plaintexts and ciphertexts.
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