The symmetric ciphers

[Robert J. Hansen]

> Well, here's a (rough, and maybe naive) explanation of why I assumed
> that the effort is at least max(a, b):

If you first encrypt with ROT10 and then with ROT16, the final  
strength is not the maximum of (ROT10, ROT16).  You may think that's a  
silly example, and I grant that it is, but it illuminates the point  
pretty well and avoids a lot of difficult math.

Cryptographic algorithms are extremely subtle and interact with each  
other in subtle ways.  As a general rule they should not be stacked  
unless there is (a) a clear necessity and (b) the particular stacking  
has been formally proven to not diminish the overall security of the  
system.  Otherwise, much as how ROT10+ROT16 has really awful security  
characteristics, your stacking may be similarly awful.