caesar.html

> # a little bit of cryptography:

> with(StringTools):

Warning, the assigned name Group now has a global binding

> Map( Capitalize, "This is a test." );

> Map( LowerCase, %);

> convert("Test string", 'bytes');

> convert([65,66,67], 'bytes');

> convert("abcdefghijklmnopqrstuvwxyz", 'bytes');

> convert("ABCDEFGHIJKLMNOPQRSTUVWXYZ", 'bytes');

> # irem(a, b) gives the remainder r of a upon division by b,
# i.e. the r in a = q * b + r with 0 <= r < b.
# For example,

> irem(17,5);

> # but

> irem(-1,5);

> # the Caesar cipher is the following:
# it takes the plaintext and shifts the letters
# alphabetically by some fixed amount
# For instance, a shift by 3 amounts to
# a -> d
# b -> e
# c -> f
# ...
# w -> z
# x -> a
# y -> b
# z -> d
#
# So, abc becomes def, for instance.
# to decipher, oneft back the other direction by the same amount.

> # now we implement the Caesar cipher,
# which is shifting the letters of the alphabet by some fixed amount

> caesar := proc(s::string, shift::integer)
local S, l, i, c;
S := convert(s,'bytes');
l := nops(S);
for i to l do
   c := S[i];
   if c >= 97 and c <= 122 then
     c := c - 97 + shift;
     c := irem(c, 26);
     if c < 0 then c := c + 26; end;
     S[i] := 97 + c;
   end;
end;
return convert(S,'bytes');
end;