Cryptography Report for MITS5004: Public Key and Secret Key Systems

This report provides a comprehensive overview of cryptography, focusing on public key and secret key encryption methods. It details the differences between these approaches, explaining how data is encrypted and decrypted using either public or private keys. The report then delves into secret key cryptography, exploring the use of substitution and transposition ciphers as core techniques. Furthermore, it examines block ciphers and stream ciphers, highlighting their distinct characteristics and applications. The report also discusses the hybrid algorithm PGP (Pretty Good Privacy), which combines the advantages of both public and secret key systems. References to academic papers are included to support the information presented. This assignment is a report for MITS5004 IT Security.

CRYPTOGRAPHY

PUBLIC KEY AND SECRET KEY CRYPTOGRAPHY
Two major ideas used for encryptions are cryptography of public key and secret key.
Data encrypted using public key can be decrypted only with private key and data
encrypted using private key can be decrypted only with public key.
In secret key system, one single key is used only for decryption and encryption.
Transmitters and receivers have same secret key.

SUBSTITUTION CIPHERS AND TRANSPOSITION CIPHERS
In encryption algorithm, two major techniques are used in implementation of secret key
cryptography: Transposition cipher and substitution cipher.
Characters or bits are rearranged in data by transposition ciphers.
Transposition technique can be achieved by applying permutation on letters of plaintext.
Characters, bits or characters block are replaced with substation by substitution ciphers.
Substitution ciphers use single mapping which means same plaintext would have similar
ciphertext. This problem can be solved by polyalphabetic cipher with the use of multiple
substitutions.

BLOCK CIPHER
Block cipher is deterministic algorithm which operates on bits with fixed-length.
Typical block size of block cipher is 128 bits, 64 bits or larger.
Each single bit of output is influenced by every bits in original data.
Block cipher’s disadvantage is block size must be fitted or encryption of block cipher
cannot be done.
Sometimes additional redundant is added for fitting block size for encryption.

STREAM CIPHER
Stream cipher is symmetric key cipher in which pseudorandom cipher keystream is
combined with plaintext.
Pseudorandom keystream is serially generated from arbitrary seed value by use of digital
shift registers.
Each digit of plaintext is encrypted with corresponding keystream digit one by one .
XOR operation is performed on plaintext by using key stream and output could be used
for encryption.

HYBRID ALGORITHM, PGP
PGP is an algorithm composed by combining all advantages of public key and
secret key algorithms.
Symmetric cipher system and temporary key encrypts original plaintext.
Temporary key is produced randomly.
As symmetric cipher is the mechanism used, key is to be passed to each parties
who are part of communication system.
The key should be encrypted with mechanism of public key before transmission
takes place.
Therefore, there would be x versions of symmetric key hidden for x number of
receivers.

REFERENCES
[1] Y.K Apoorva, “Comparative study of different symmetric key cryptography algorithms”. Internafional
Journal of Applicafion or Innovafion in Engineering and Management, 2(7), pp.204-6, 2013
[2] J. Buchmann, “Introduction to cryptography”. Springer Science & Business Media, 2013
[3] E. Barker and N. Mouha, “Recommendation for the triple data encryption algorithm (TDEA) block
cipher (No. NIST Special Publication (SP) 800-67 Rev. 2 (Draft))”. National Institute of Standards and
Technology, 2017
[4] S.Chandra, S. Paira, S.S Alam and G. Sanyal, “A comparative survey of symmetric and asymmetric key
cryptography”. In 2014 International Conference on Electronics, Communication and Computational
Engineering (ICECCE) (pp. 83-93). IEEE, 2014

THANK YOU
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