Page 51 - DCAP516_COMPUTER_SECURITY

Unit 4: Cryptography

signature by applying the non-secret key. Secret, signed messages can be obtained by Notes
digitally signing with your secret key, then encrypting using the recipient’s non-secret
key.
Stream Encryption: Some encryption schemes increase security by varying the key for
separate packets of a long message. Often, the key is computed from previous packets. As
long as all packets ultimately arrive, this works, but if packets are lost, subsequent packages
are not decryptable. Various synchronizations can be used to minimize the loss. This is
particularly an issue for potential encryption of audio or video where the underlying
transport will drop packets when load gets high.

File Encryption: Various encryption algorithms have been applied to files and databases.
The main issue here is one of packaging the encryption naturally into normal file access
and managing keys when a key may need to be used for a long time after it was originally
used to encrypt.

Electronic Cash: Cryptography is used to create unforgeable “electronic cash” tokens.
Tokens include a serial number that can be decrypted (and saved) by the bank accepting
the token. Reuse (illegitimate) of the token allows the user to be identified because the
serial number will have already been seen in a previous transaction.

Self Assessment

State whether the following statements are true or false:
7. A set of reverse rounds are applied to transform ciphertext back into the original plaintext
using the same encryption key.
8. The DES cipher is specified as a number of repetitions of transformation rounds that
convert the input plaintext into the final output of ciphertext.
9. Null ciphers are waste of bandwidth but they are very easy to break.
10. The MixColumns function takes four bytes as input and outputs four bytes.
11. RSA Algorithm is based on some principles from number theory.

4.7 Summary

The message is encrypted at the sender end and decrypted at the receiving end to maintain
privacy with the help of the encryption and decryption techniques. The secret key and
public key techniques are the available techniques with their advantages and disadvantages.
Substitution and transposition ciphers are two categories of ciphers used in classical
cryptography. Substitution and transposition differ in how chunks of the message are
handled by the encryption process.
In January 1977, the US government adopted a product cipher developed by IBM as its
official standard for unclassified information. A number of manufacturers implemented
this encryption algorithm known as the ‘Data Encryption Standard’ (National Bureau of
Standards, 1977) in hardware, thus making it fast.
AES is based on a design principle known as a Substitution permutation network. It is fast
in both software and hardware. Unlike its predecessor, DES, AES does not use a Feistel

network. AES operates on a 4×4 column-major order matrix of bytes, termed the state
(versions of Rijndael with a larger block size have additional columns in the state). Most
AES calculations are done in a special finite field.

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