plaintextciphertext encryption algorithmdecryption algorithm plaintext
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plaintext ciphertext
encryption algorithm decryption algorithm
plaintext
Round 1
Round 2
Round N
XYWJLRAM block of ciphertext
••••••
• Horst Feistel (IBM) invented the basic algorithm in 1973.
• Feistel ciphers use symmetric block encryption relying upon product transformations.
HI Mom S block of plaintext Key
f
+
f
+
f
+
subkey1
subkey2
subkeyN
•••
f some function
+ exclusive OR (bitwise)
• Encryption & decryption use the same algorithm.
DESData Encryption Standard - an algorithm selected by the National Institute ofStandards Technology for the encryption of non-classified data.
The current DES algorithm was adopted by the U.S. Nat. Bureau of Standards in 1977.Block size: ____ bitsKey size: ____ bits + 8-bit parity16-round Feistel encryption
(preceded by one permutation and followed by the permutation’s inverse)
SubkeysThe key (56-bits) is split in two and each half is rotated left by 1 or 2 bits. The resultingtwo 28-bit values index into a table to produce a 48-bit subkey. The rotated values are alsoforwarded to compute the next subkey.
f function1) input to f is expanded from 32 to 48 bits via table lookup2) 48-bit value from (1) is XORed with subkey3) 48-bit value from (2) is partitioned into eight 6-bit values4) 6-bit values from (3) are separated into outside 2 bits and center 4 bits (outside bits select row and inside bits select column from S-box table)5) eight S-box lookup values (each 4 bits long) are concatenated
The DES cipher substitutes via eight different S-boxes; two are below.
Brute Force
• 1977 - brute force attacks might be possible in the future [Diffie & Hellman IEEE Computer, June]
Time to Produce All Possible Encodings
Key Size (in bits) Number of Keys (1 encrypt./s) (106 encrypt./s)
32 232 ≈ 4.3 X 109 36 min. 2 msec.
56 256 ≈ 7.2 X 1016 1142 years 10 hr.
128 2128 ≈ 3.4 X 1038 5.4 X 1024 years 5.4 X 1018 years
26-char permutation 26! ≈ 4 X 1026 6.4 X 1012 years 6.4 X 106 years
• 1997 - a distributed collection of 3500 research computers discover DES key in ___ months
• 1998 - Electronic Frontier Foundation builds ___________________________________.
REF: www.tropsoft.com/strongenc/des.htm
• 1999 - Net uses DES Cracker plus 100,000 PCs to crack DES in under ______________.
______ - 1979 • also called triple DES • use two keys and three DES encryptions cipher = Ekey1( Dkey2( Ekey1( plaintext ) ) )• 3DES has an effective key size of 112 bits.
• developed by Bruce Schneier• compact, efficient, algorithm w/ key of 128 bits
• International Data Encryption Algorithm • developers from Swiss Federal Institute of Technology• 128-bit key, but differs from DES in round function and subkey generation.
______ - 1991
___________ - 1993
• Advanced Encryption Standard• selected algorithm: Rijndael • a product cipher using key sizes of 128, 192 and 256 bits (128-bit block size).
______ - 1999
• developed by Ron Rivest• efficient algorithm with variable length keys
______ - 1994
Advantages • only one key to generate • relatively well-tested and established algorithms• cipher algorithms are efficient
• potentially large number of keys
Disadvantages
• ______________________________ (How can this be accomplished securely?)
number of private keys
number of communicators 2 3 4 5 6 n
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