CSC 308

• Encryption


  The process of converting plain text, which can be seen by anyone, into an incomprehensible form of ciphertext
• Decryption


  The process of converting ciphertext back into plaintext, so that it can be understood
• Cryptography


  The two processes of encryption and decryption
• Cryptographic algorithm


  The mathematical function used in the encryption and decryption processes
• Cryptographic key


  The key used by the cryptographic algorithm to perform encryption and decryption
• Encryption is an important means to obtain confidentiality, which is one of the three main goals of information security
• Encryption


  1. Convert plain text into ciphertext

  2. Ciphertext cannot be recognized by a third party
• Decryption


  Convert ciphertext back into plaintext
• Classification of cryptographic techniques

  • Cryptographic techniques
  • Cryptographic protocols
• Cryptographic techniques


  • Symmetric key cryptosystem
  • Public key cryptosystem
• Symmetric key cryptosystem


  Encryption key and decryption key are the same
• Public key cryptosystem


  Encryption key and decryption key are different
• Cryptographic protocol


  Protocols that use cryptographic techniques to achieve a certain purpose (authentication, confidentiality, integrity, non-repudiation)
• Cryptographic algorithms use an encryption key to perform encryption and decryption
• Ciphertext is generally transmitted over communication lines or networks that are not secure
• Computational impossibility means that we can understand the contents of the plaintext, even though we don't know the decryption key, if infinite time is used
• Cryptosystem


  All the elements needed for a series of encryption and decryption processes
• Requirements for a cryptosystem

  • Encryption and decryption should be effectively performed by the encryption key
  • A cryptosystem should be easy to use
  • Security should be performed by the encryption key, rather than by a cryptographic algorithm
• Private key cryptographic algorithm

  Encryption key and decryption key are the same, key length may be short, encryption and decryption operations are fast
• Private key cryptographic algorithm


  • Difficult to share the key if the distance between sender and receiver is long
  • Burdensome to generate and maintain different keys for many parties
• Types of private key cryptographic algorithms

  • Block cipher
  • Stream cipher
• Block cipher


  • Plaintext is divided into fixed size inputs, called blocks, and each block is encrypted
  • Low risk of error spread
  • Easy to implement
• Stream cipher


  • Plaintext is encrypted in bits
  • Easy to implement in a mobile communication environment
  • Slow execution time
  • Malicious attackers can easily modify the content
  • Initial values should be set
• Block cipher attack techniques

  • Differential attack
  • Linear attack
  • Brute force attack
  • Statistical analysis
  • Mathematical analysis
• Public key cryptographic algorithm

  Sender and receiver use different keys to establish secret communication
• Public key cryptographic algorithm

  1. Sender encrypts data using receiver's public key and sends ciphertext

  2. Receiver decrypts ciphertext using their private key to restore plaintext
• Public key cryptographic algorithm

  • Secure communication can be established without sharing the key
  • Each user discloses their public key and holds their private key
  • Anyone can encrypt information, but only those with the private key can decrypt
• Public key cryptographic algorithms

  • RSA
  • ElGamal
  • ECC
• RSA


  Used for encryption and authentication, safety based on difficulty of factoring large integers
• ElGamal


  First public key cryptographic algorithm based on difficulty of discrete algebra problem, ciphertext length is doubled, different ciphertext created each time due to use of random numbers
• Public key cryptography


  The sender knows their private key corresponding to the public key, which can be checked by everyone
• Representative public key cryptographic algorithms

  • RSA (Rivest, Shamir and Adleman)
  • ElGamal
  • ECC (Elliptic Curve Cryptosystem)
• RSA public key cryptosystem


  • Used for encryption and authentication
  • Safety based on the difficulty of factoring a large integer
  • Ciphertext for the same message is always the same as no random numbers are used in the encryption process
  • Safety depends on prime number p and q, so prime number selection conditions are important
• ElGamal


  • Proposed in 1984 by Stanford University cryptographer, T. ElGamal
  • First public key cryptographic algorithm, based on the difficulty of the discrete algebra problem
  • When a message is encrypted, its length is doubled
  • Different ciphertext is created each time, even for the same message, because random numbers are used for encryption
• ECC (Elliptic Curve Cryptosystem)


  • Based on the discrete logarithm problem on the elliptic curve
  • High security and fast speed
  • A 1024-bit key of RSA and a 160-bit key of elliptic curve cipher have the same level of security
  • Suitable for encryption of mobile communication with limited power supply
• Hash function


  Mathematical function that outputs a short hash value (hash code) with a fixed length from random data of various sizes
• Properties of hash function

  • Pre-image resistance: Given y, it is difficult to find x where h(x)=y
  • 2nd pre-image resistance: Given h(x)=y, it is difficult to find x' where h(x')=y
  • Collision resistance: It is difficult to find x and x' where h(x)=h(x')
• MD5


  • 128-bit hash function used for testing the integrity of the program or file
  • Recommended to use a safer algorithm like SHA-2 for hashing purposes
• SHA


  • Set of interlinked hash functions developed by the National Security Agency (NSA)
  • SHA-0, SHA-1, SHA-2, SHA-3
  • SHA-256 or higher recommended for safety
• Salt


  • Arbitrary bit string added when a hash function generates a hash value
  • Salting generates a hash value by adding a bit string to the original message
  • Improves the problem of the possibility of recognition