International Data Encryption Algorithm (IDEA)

 International Data Encryption Algorithm is one of the strongest cryptographic algorithms launched
in 1990.

 IDEA is not as popular as DES for two primary reasons:
                 
                        o It is patented unlike DES and therefore must be licensed before it can be used in
                            commercial applications.
                 
                         o DES has a long history as compared to IDEA.

 One popular e-mail privacy technology known as pretty good privacy (PGP) is based on
IDEA.

 BASIC PRINCIPLES
                 o IDEA is a block cipher
                 o Works on 64 bit plain text blocks.
                 o Key consists of 128 bits
                 o Same algorithm is used for encryption and decryption

o 64 bit plain text block is divided into four portions of plain text each of size 16 bits(say P1to P4).

o P1 to P4 are the inputs to the first round of the algorithm.

o There are eight such rounds.

o The key consists of 128 bits. In each round 6 subkeys are used and after the round

operation an output transformation is performed using subkeys( K49 to K52).

o The final output produced is the output produced by the output transformation step which

is the four blocks of cipher text named C1 to C4( 16 bits each).

o These are combined to form the final 64 bit cipher text block.

 SUB KEY GENERATION

o Each rounds makes use of 6 subkeys (8X6=48) and the final output transformation uses

four subkeys( 48+4 =52 subkeys).

o 128 bit key is divided into 8 parts K1 to K8 each of 16 bits.

o Of which K1 to K6 is used for ROUND 1 operation.

o For the remaining round subkeys are generated by performing key shifting.

o The orginal key is shifted left circularly by 25 bits.

o And hence 52 subkeys are generated.

 OPERATIONS

o Operations performed are:

                              Addition,

                              Multiplication,

                              XOR

 ROUNDS

o Each round involves a series of operation on the four data blocks using six keys.

1.   Multiply P1and K1                   i.e,   S1=P1 x K1

2.   Add P2 and K2                         i.e,   S2=P2 + K2

3.   Add P3 and K3                         i.e,   S3=P3 + K3

4.   Multiply P4 and K4                  i.e,   S4=P4 x K4

5.   XOR step1 and step3                i.e,   S5=S1⊕S2

6.   XOR step2 and step4                i.e,   S6=S3⊕S4

7.   Multiply step5 and K5              i.e,   S7=S5 x K5

8.   Add step6 and step7                  i.e,   S8=S6 + S7

9.   Multiply step8 with K6             i.e,   S9=S8 x K6

10. Add step7 and step9                  i.e,   S10=S7 + S9      

11. XOR step1 and step9                 i.e,  S11=S1 ⊕ S9

12. XOR step3 and step9                 i.e,  S12=S3 ⊕ S9

13. XOR step2 and step10               i.e,  S13=S2 ⊕ S10

14. XOR step4 and step10               i.e,  S14=S4 ⊕ S10

 OUTPUT TRANSFORMATION

o It is a one time operation

o Takes place at the end of 8th round.

o The output of this process is the final 64 bit cipher text which is the combination of four

four cipher text blocks C1 to C4.

 DECRYPTION

o The process is exactly same as that of encryption process.

o There are some alterations in the generation and patterns of subkeys.

o The decryption subkeys are actually an inverse of the encryption subkeys. That is while

subkey generation an right circular shift is performed.

 STRENGTH OF IDEA

o IDEA uses 128 bit key which is double than the size of DES.

o Thus to break IDEA 2 ^ 128 encryption operations would be required.

o Thus IDEA is more secured.