Step 3d: S-box substitution

• The nonlinear step

• Group 48-bit data as 8 blocks of 6 bits each

  • Kn ? E(Rn–1) =B1B2B3B4B5B6B7B8

• Each 6-bit block is an index into an S-box

  • First and last bits form a row address
  • Middle 4 bits form a column address
  • Each S-box entry gives a 4-bit result
  • There are 8 S-boxes

• Form S1(B1)S2(B2)S3(B3)S4(B4)S5(B5)S6(B6)S7(B7)S8(B8)

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