CRYPTOSYSTEM DESIGN AND AES

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CRYPTOSYSTEM DESIGN AND AES. 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.

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CRYPTOSYSTEM DESIGN AND AES

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
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
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.
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.
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.
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).
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.
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.