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International Data Encryption
Algorithm
By
Rajabrata Ganguly
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Overview
• DES algorithm has been a popular secret key
encryption algorithm and is used in many
commercial and financial applications. However,
its key size is too small by current standards and
its entire 56 bit key space can be searched in
approximately 22 hours
• IDEA is a block cipher designed by Xuejia Lai
and James L. Massey in 1991
• It is a minor revision of an earlier cipher, PES
(Proposed Encryption Standard)
• IDEA was originally called IPES (Improved PES)
and was developed to replace DES
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Overview (cont’)
• It entirely avoids the use of any lookup
tables or S-boxes
• IDEA was used as the symmetric cipher in
early versions of the Pretty Good Privacy
cryptosystem
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Detailed description of IDEA
• IDEA operates with 64-bit plaintext and
cipher text blocks and is controlled by a
128-bit key
• Completely avoid substitution boxes and
table lookups used in the block ciphers
• The algorithm structure has been chosen
such that when different key sub-blocks
are used, the encryption process is
identical to the decryption process
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Key generation
• The 64-bit plaintext block
is partitioned into four 16-
bit sub-blocks
• six 16-bit key are
generated from the 128-
bit key. Since a further
four 16-bit key-sub-blocks
are required for the
subsequent output
transformation, a total of
52 (= 8 x 6 + 4) different
16-bit sub-blocks have to
be generated from the
128-bit key.
