What is double-length derivation key?
What is double-length derivation key ?


My reading is that is the Base Derivation Key (BDK), used to derive (produce) an Initial PIN Entry device Key (IPEK). Both BDK and IPEK are 16-byte keys for 3DES; otherwise said for TDEA keying option 2 per the (obsolete) FIPS&nbsp;SP-800-67-Rev1 section 3.2, item 2.

The 16-byte BDK or IPEK is split into two 8-byte halves $K_1$ and $K_2$, and 8-byte $P$ is enciphered into $C=E_{K_1}(D_{K_2}(E_{K_1}(P)))$. That's the usual Encrypt-Decrypt-Encrypt of 3DES/TDEA, as explained in step 3.

Step 5 complements the high-order two bits of the four left/high order bytes of each of the 8-byte halves of BDK, to form a different BDK'. The same 8-byte value obtained in the first half of step 3 is enciphered per 3DES/TDEA using BDK (resp. BDK') to form the left/high order (resp. right/low order) half of IPEK.