How to ignore spaces in Vigenere cipher code? - delphi

Question

I have two functions:
function EnCryptSymbolVij(text, key:char; abc:string):char;
var st: string;
positionText: word;
begin
if text=' ' then begin EnCryptSymbolVij:=' '; exit; end;
PositionText:=pos(text,abc);
if positionText=0 then exit;
st:=MoveVij(length(abc)-pos(Key,abc)+1, abc);
EnCryptSymbolVij:=st[positionText];
end;
function EnCryptVij(text, key, abc:string):string;
var i, n, j: longword;
st:string;
begin
setlength(st,length(text));
j:=1;
for i:=1 to length(text) do
begin
st[i]:=EnCryptSymbolVij(text[i], Key[j], abc);
inc(j);
if j>length(key) then j:=1;
end;
EnCryptVij:=st;
End;
In the first function, I add space support. But when I encrypt the text with spaces, I get a wrong encryption, because the second function counts spaces like a part of alphavit. How can I fix this and ignore encrypt spaces in the second function? Please help to resolve this trouble.

Answer 1

You only support alphabetic characters and spaces. If you want to do more you need to be a bit cleverer in you code. Spaces are not encrypted, and nothing encrypts to a space so you just need to add that test in your decryption. If you later support other non alphabetic characters you will need to do the same for these too, but the test is the same for both encryption and decryption. So for spaces, your decryption code for a symbol will start just as your encryption does, like this
function DeCryptSymbolVij(text, key:char; abc:string):char;
var st: string;
positionText: word;
begin
if text=' ' then begin DeCryptSymbolVij:=' '; exit; end;
Personally I prefer to use 'Result' rather than the function name, but that is just personal preference.
Edit
To cater for spaces in the second function, you could just surround the inc(j) with a test for spaces, like this
function EnCryptVij(text, key, abc:string):string;
var i, n, j: longword;
st:string;
begin
setlength(st,length(text));
j:=1;
for i:=1 to length(text) do
begin
st[i]:=EnCryptSymbolVij(text[i], Key[j], abc);
if st[ I ] <> ' ' then
begin
inc(j);
end;
if j>length(key) then j:=1;
end;
EnCryptVij:=st;
End;
but to 'future proof' it you might be better adding a Var boolean parameter to the first function and setting it if a substitution has occurred:
function EnCryptSymbolVij(text, key:char; abc:string; var changed : boolean):char;
var st: string;
positionText: word;
begin
changed := FALSE;
PositionText:=pos(text,abc);
if positionText=0 then exit;
// else
changed := TRUE;
st:=MoveVij(length(abc)-pos(Key,abc)+1, abc);
EnCryptSymbolVij:=st[positionText];
end;
function EnCryptVij(text, key, abc:string):string;
var i, n, j: longword;
iChanged : Boolean;
st:string;
begin
setlength(st,length(text));
j:=1;
for i:=1 to length(text) do
begin
st[i]:=EnCryptSymbolVij(text[i], Key[j], abc, iChanged);
if iChanged then
begin
inc(j);
end;
if j>length(key) then j:=1;
end;
EnCryptVij:=st;
End;
Note that the space test is not required in the first function (assuming there is not a space in 'abc')