Explain working of DES

Mumbai University > Computer Engineering > Sem 7 > Cryptography and System Security

Marks: 10 Marks

Year: May 2016

1 Answer
  • The key size used in DES is of 56 bit length .According to modern day computational powers, a brute-force attack on DES can be completed in 22 hours.
  • IDEA is an alternative developed in place of DES. It is also a symmetric-key cipher.
  • IDEA stands for International Digital Encryption Algorithm
  • It operates with 64-bit plaintext and 128-bit key.
  • The basic IDEA cipher structure is shown in the figure below.

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  • Initially the Plain text of 64-bits is divided into four blocks of 16-bits each.
  • The 128 bit key is divided into 8 sub-keys of 16 bits each. The keys which are not used is passed on to the next round. For generating a new set of sub-keys, the 128-bit key is cyclically shifted left by 25 bits.
  • A total of 9 rounds (8 +1 output transformation round) comprising of 52 sub-keys are used.


  • The first four 16-bit key sub-blocks are combined with two of the 16-bit plaintext blocks using addition modulo 216, and with the other two plaintext blocks using multiplication modulo 216 + 1 (see figure)

Modes of Operation: IDEA supports all modes of operation such as ECB, CBC, CFB, OFB

Applications: It is currently being used in highly secure applications for manufacturing, broadband Internet services, telecommunications, financial institutions, digital television and the U.S. government. Since 2012, IDEA has remain patent-free

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