Data Encryption Standard (DES)
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22-02-2011, 11:47 AM

.ppt   crypto6.ppt (Size: 54 KB / Downloads: 123)
Data Encryption Standard (DES)
• Financial companies found the need for a cryptographic algorithm that would have the blessing of the US government (=NSA)
• First call for candidates in May 73, followed by a new call in August 74
• Not very many submissions (Why?)
– IBM submitted Lucifer
• NSA worked with IBM in redesigning the algorithm
• DES became a federal standard in November 76
– NBS (NIST) hardware standard in January 77
– ANSI X3.92-1981 (hardware + software)
– ANSI X3.106-1983 (modes of operation)
– Australia AS2805.5-1985
• Used in most EFT and EFTPOS from banking industry
– It was reconfirmed as a standard for 5 years twice
– Currently 3DES is recommended
• The standard is public, the design criteria is classified
• One of the biggest controversies is the key size (56 bits)
– W Diffie, M Hellman "Exhaustive Cryptanalysis of the NBS Data Encryption Standard" IEEE Computer 10(6), June 1977, pp74-84
– M Hellman "DES will be totally insecure within ten years" IEEE Spectrum 16(7), Jul 1979, pp 31-41
• Another controversy: is there a back door?
• DES has proven a well designed code
• 56 bits has been proven inadequate
– EFF built a cracker for around $200,000
– Increase the key to 112 bits?
• The best way known to cryptanalyze DES is (after brute force) the differential analysis
– NSA new this from the design??
• Uses Feistel principle
• Many similarities with Lucifer
• Improves on the S-Boxes
Simple DES
• 8 bits block with a 10 bits key
• The encryption process is :
– Initial Permutation
– Function fk1
– Switch of the key halves
– Function fk2
– Final Permutation (inverse of initial permutation)
• Key generation
– Initial permutation P10
– Divide in left and right parts
– Left shift and Merge
– An 8 bits permutation, resulting in a 8 bits K1
– Divide in left and right parts
– Double left shift and Merge
– An 8 bits permutation, resulting in a 8 bits K2
• Structure of S-P boxes
– S-Boxes
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25-08-2012, 11:42 AM

Data Encryption Standard - DES

.pptx   DES presentation.pptx (Size: 103.81 KB / Downloads: 27)

What is Encryption? And why needed?

A message in its original form (plaintext) is converted (encrypted) into an unintelligible form (cipher text) by a set of procedures known as an encryption algorithm (cipher) and a variable, called a key.
The cipher text is transformed (decrypted) back into plaintext using the encryption algorithm and a key.
Encryption has long been used by militaries and governments to facilitate secret communication. It is now commonly used in protecting information within many kinds of civilian systems. Just for example password protection.


DES is a system of encryption that takes a fixed-length string (64 bits) of plaintext bits and transforms it through a series of complicated operations into another cipher text of the same length (64 bits).
Its purpose is to provide a standard method for protecting sensitive commercial and unclassified data.
IBM created the first draft of the algorithm, calling it LUCIFER. DES officially became a federal standard in November of 1976.


for example I have the plain text “What is your name”
The hexadecimal conversion for this is "596F7572206C6970 732061726520736D 6F“ a total of 136 bits.
Now this is converted to a code that contains 64x bits number, this is done by adding that number of zeros.
Now each block of 64 bits block is separated such that we get x number of blocks and then each block is individually encrypted.
Here we 64*3=192. i.e. add 192-136=56 zeros.

DES Modes of Operation

The DES algorithm turns a 64-bit message block M into a 64-bit cipher block C. If each 64-bit block is encrypted individually, then the mode of encryption is called Electronic Code Book (ECB) mode. There are two other modes of DES encryption, namely Chain Block Coding (CBC) and Cipher Feedback (CFB), which make each cipher block dependent on all the previous messages blocks through an initial XOR operation.


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