CRYPTOSYSTEM DESIGN AND AES

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CRYPTOSYSTEM DESIGN AND AES
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Cryptosystem Design

• With cryptosystems, we desire perfect secrecy
• the probability that the contents of some
  intercepted data corresponds to some plaintext
  message is unaltered by knowledge of the
  ciphertext for that message.
• Measuring the strength for cryptosystem by what
  is known as its work factor
• the amount of time needed to decipher a message
  without knowledge of the key.
• A cryptosystem is considered secure when its
  workfactor is exponential in the length of the
  key 2.

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Cryptosystem Design

• General goals for designing secure encryption
  algorithms
• Confusion
• Diffusion
• A good encryption algorithm would satisfy the
  following two criteria
• No output bit should be a linear function of the
  input bits. In other words, the algorithm must
  induce non-linearity. This ensures confusion.
• Avalanche Criteria the probability of changing a
  given bit in the output is ½ when any subset of
  the input bits are complemented

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Cryptosystem Design

• Types of Cryptographic Functions
• Secret key (symmetric) involves 1 key, known as
  the secret key
• Public key (asymmetric)involves 2 keys, known as
  the private public keys
• hash involves 0 keys

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Advanced Encryption Standard (AES)

• the US "standard" secret key cryptosystem,
  replacing DES (Data Encryption Standard, adopted
  in 1977)
• AES is the result of a three year competition.
  This competition was announced in September 1997
  and had entries from 12 different countries
• The one submission that eventually won was called
  "Rijndael" and was invented by two Belgians, Joan
  Daemen and Vincent Rijmen.

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A Brief History of DES

• In 1974, IBM proposed "Lucifer", an encryption
  algorithm that uses 64-bit keys. Two years later,
  NBS (in consultation with NSA) made a modified
  version of that algorithm into a standard.
• DES takes in 64 bits of data, employs a 56-bit
  key, and executes 16 cycles of substitution and
  permutation before outputting 64 bits of
  encrypted data.

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A Brief History of DES
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A Brief History of DES

• In the summer of 1998, the Electronic Frontier
  Foundation (EFF) built a DES cracker machine at a
  cost of 250,000
• It had 1536 chips, worked at a rate of 88 billion
  keys per second, and was able to break a DES
  encrypted message in 56 hours
• One year later, with the cracker working in
  tandem with 100,000 PCs over the Internet, a DES
  encrypted message was cracked in only 22 hours.
• One common way to make DES more secure today is
  to encrypt three times using DES.
• triple-DES (3DES).
• 3DES is extremely slow, so a better algorithm was
  needed.

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Requirements for AES

• AES had to be a private key algorithm. It had to
  use a shared secret key.
• It had to support the following key sizes
• 128 bits ( 3.4 x 10 keys, equivalent to
  2560-bit RSA)
• 192 bits ( 6.2 x 10 keys)
• 256 bits ( 1.1 x 10 keys)
• DES uses only 56-bit keys, giving a key space of
  7.2 x 10 keys
• If you were able to search half the DES key space
  in 1 second, then on average, it would take 149
  trillion years to crack a 128-bit AES key.

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Requirements for AES

• It had to satisfy certain engineering criteria
• performance, efficiency, implementability, and
  flexibility.
• Rijndael can be implemented easily in both
  hardware and software,
• has realizations that require little memory (so
  the algorithm can be used in smartcards).

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Requirements for AES

• It had to be a block cipher
• an encryption algorithm structured in terms of an
  internal function and runs that function
  repeatedly on the input.
• Each iteration is called a round
• AES uses 10 rounds.

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Requirements for AES

• AES is also an instance of a Feistel cipher, a
  special case of a block cipher.
• The input to such a cipher consists of 2t bits.
• The input is first divided into 2 parts
• L and R
• The cipher then proceeds in rounds.
• In the i-th round,
• Li Ri-1
• Ri Li-1 XOR f(Ri-1, ki),
• where f is some function, and k is some number
  derived from the key, to be used in round i.

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IDEA (International Data Encryption Algorithm)

• IDEA, originally named the Improved Proposed
  Encryption Standard (IPES),
• Designed to be efficient in software.
• It was developed by Xuejia Lai and James Massey
  in 1991.
• It operates on a 64-bit plaintext data block and
  uses a 128-bit key.
• IDEA is used in PGP to encrypt messages.