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CRYPTOSYSTEM DESIGN AND AES

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

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  1. CRYPTOSYSTEM DESIGN AND AES

  2. 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. keylen

  3. 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

  4. 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

  5. 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.

  6. 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.

  7. A Brief History of DES

  8. 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.

  9. 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. 38 57 77 16

  10. 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).

  11. 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.

  12. 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. 0 0 i i

  13. 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.

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