modExp( b, e, n )

Modular exponentiation. Very efficient (until you overflow).

function y = modExp( b, e, n )
% Fast modular exponentiation e.g.: y = b^e mod n
% Applied Cryptography: Protocols, Algorithms, and Source Code in C,
% page 245
if (n == 1)
y = 0;
return
end
y = uint64(1);
b = mod(b,n);
while ( e > 0 )
if ( mod(e,2) == 1)
y = mod((y * b),n);
end
e = bitshift(e,-1);
b = mod( (b * b),n ) ;
end
end