ICS 454: Principles of Cryptography
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Transcript of ICS 454: Principles of Cryptography
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ICS 454:Principles of Cryptography
Block Ciphers & DES
Sultan Almuhammadi
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Outline
(Stallings 3.1-3.2) Stream cipher vs. Block cipher Motivation Reversible vs. Irreversible mapping Ideal Block Cipher DES
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Stream Cipher & Block Cipher stream cipher: encrypts data stream one bit
or one byte at a time. E.g.: Caesar shift cipher (one letter at time) XOR-scheme (one bit at a time)
block cipher: a block of plaintext is treated as a whole and used to produce a ciphertext block of equal length. Block size (typically): 64 or 128 bits e.g. Feistel cipher and DES
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Motivation A block cipher operates on a block of n bits. It produces a ciphertext block of n bits. There are 2n possible different
plaintext/ciphertext blocks. The encryption must be reversible. i.e.
decryption to be possible. each plaintext must produce a unique ciphertext
block. (one-to-one correspondence)
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Reversible vs. Irreversible
Reversible MappingPlaintext Ciphertext
00 1101 1010 0011 01
Irreversible MappingPlaintext Ciphertext
00 1101 1010 0111 01
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Ideal Block Cipher(a general substitution cipher)
0 0 1 1
0 0 0 1
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Encryption/Decryption Table for Substitution Cipher
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Problems with Ideal Cipher If a small block size, such as n = 4, is used,
then the system is equivalent to a classical substitution cipher Easy attack (statistical analysis of the plaintext)
If large block size is usednot practical (for implementation and performance) Huge encryption/decryption tables Huge key:
for n = 4, key size = 4 bits x 16 rows = 64 bits for n = 64, key size = 64 x 264 = 270 = 1021 bits
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Data Encryption Standard (DES) Widely used encryption scheme. Adopted by National Bureau of Standards in
1977. The algorithm itself is called Data Encryption
Algorithm (DEA). Data are encrypted in 64-bit blocks using a
56-bit key.
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DES Encryption
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DES: Initial Permutation (IP)
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DES: Inverse Initial Permutation
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Single Round of DES Algorithm
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DES: Expansion Permutation (E)
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DES: S-Boxes
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DES Encryption
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DES Decryption Decryption uses the same algorithm as
encryption, except that the application of the subkeys is reversed.