invmodn.m

vigenere密码解密的matlab实现

function y = invmodn( b,n);
% This function calculates the inverse of an element b mod n
% It uses the extended euclidean algorithm

n0=n;
b0=b;
t0=0;
t=1;

q=floor(n0/b0);
r=n0-q*b0;
while r>0,
   temp=t0-q*t;
   if (temp >=0),
      temp=mod(temp,n);
   end;
   if (temp < 0),
      temp= n - ( mod(-temp,n));
   end;
   t0=t;
   t=temp;
   n0=b0;
   b0=r;
   q=floor(n0/b0);
   r=n0-q*b0;
end;

if b0 ~=1,
   y=[];
   disp('No inverse');
else
   y=mod(t,n);
end;