Block Ciphers - Introduction to Cryptography - Lecture Slides, Slides of Network security

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Block Ciphers

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Ciphers

• Block cipher
• DES
• 3DES
• AES
• IDEA
• Twofish

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Block Cipher

• Block cipher is an encryption function that works on fixed size blocks
• Current block size is 128 bits
• Encrypting a 128-bit plaintext block produces a 128-bit ciphertext block
• The encryption key is also a series of bits, usually 128 or 256 bits
• A secure block cipher is one that keeps the plaintext secret

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Block Cipher

• Trivial distinguisher involves creating all possible encryptions of 0 with all the keys in the range 1, 2, …, 2 32
• A round is a collection of repetitions of a weak block cipher

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DES

• 64 bits are represented in 8 bytes. The rightmost bit in each byte is used as an odd parity bit. Since there are only 7 effective bits then, the encryption key strength is limited to 7 x 8 = 56 bits
• Lucifer takes a 64-bit block and divides it into a left and right block of 32-bits each
• Some operation is performed to permute the bits and then interchange the permuted 32-bit blocks. This is called round 1.

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DES

• Repeat this process for 16 rounds
• Each round uses a 48-bit key chosen from the 56-bit key
• Key for each round is different

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DES

• DES has complementation property
• X’ denotes the complement of X where all the 0s of X are replaced by 1s and vice versa
• E(K’,P’) = (E(K,P))’. Here E denotes encryption of plaintext P with key K. This is the complementation property.

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DES

• XOR has the property that if you XOR twice, then you get the original back: C = A xor B C xor B yields A back
• E.g., Consider 10111010 xor 11001011 -------------- 01110001 xor 11001011 -------------- 10111010

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Triple DES

• 3DES was developed to address the key size problem
• Modern computers have enough power to break the DES key in a short time (3 days or less)
• With three keys, 3DES makes it very difficult to break the keys
• NIST’s goal was to develop a strong encryption standard. NIST approved 3DES as an interim standard.

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AES

• Advanced Encryption Standard had two goals:
  • 128-bit or higher encryption
  • Usability with portable devices such as cell phones and PDAs
  • Developed through an international competition for proposals
  • Dutch scientists Joan Daemon and Vincent Rijmen developed the Rijndael (pronounced rain doll) cipher
  • Rijndael was adopted as the AES standard in 2001
  • AES provides 128, 192, or 256-bit encryption using blocks of size 128, 192, or 256-bits
  • All 9 combinations of key size and block size are valid

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IDEA

• International Data Encryption Algorithm is a 64-bit block cipher
• Developed in 1991 by Lai and Massey in Switzerland
• Uses 128-bit key
• Has 8 and 1/2 rounds in the algorithm
• Software is patented in U.S. but is free for non-commercial use

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Twofish

• Developed by Ferguson and Schneier
• Twofish was a finalist for AES
• Uses Feistel structure like DES
• Provides 128-bit security
• Uses 16 rounds
• Twofish is more expensive to implement than Rijndael