Multipling by 10 ain't so hard!
Add the number to itself & save the result. Continue with the original number, adding it to itself 2 more time (X*8). Add the number you saved earlier to get 10* the original value. Now add the new key value. Do all this as often as you want.

An example in 8085 assembler (same difference, except I know it by heart!):

;accumulated value is in HL (16 bits)
;new key value in c

dad h ;X*2
push h ;save it
dad h ;X*4
dad h ;X*8
pop d ;recover X*2
dad d ;add it to the rest
mvi b,0
dad c ;new key value


Regards,

Richard