The only way to view this method of generating unit keys is as a back-door.
What else can you call a key deterministically generated from the serial
number?


 To generate the unit key for a serial number N, the 30-bit value N is
   first padded with a fixed 34-bit block to produce a 64-bit block N1.
   S1 and S2 are then used as keys to triple-encrypt N1, producing a
   64-bit block R1:

	   R1 = E[D[E[N1; S1]; S2]; S1] .

   Similarly, N is padded with two other 34-bit blocks to produce N2 and
   N3, and two additional 64-bit blocks R2 and R3 are computed:  

	   R2 = E[D[E[N2; S1]; S2]; S1] 
	   R3 = E[D[E[N3; S1]; S2]; S1] .

   R1, R2, and R3 are then concatenated together, giving 192 bits. The
   first 80 bits are assigned to U1 and the second 80 bits to U2.  The
   rest are discarded.  The unit key U is the XOR of U1 and U2.  U1 and U2
   are the key parts that are separately escrowed with the two escrow
   agencies.
