[frankmch]

puedes probar este algoritmo.
yo no lo he probado, asi que no conosco muy bien cuan seguro sea.
espero te sirva

program Crypt;


uses WinCRT;

const
C1 = 52845;
C2 = 22719;

function Encrypt(const S: String; Key: Word): String;
var
I: byte;
begin
Result[0] := S[0];
for I := 1 to Length(S) do begin
Result[i] := char(byte(S[i]) xor (Key shr 8));
Key := (byte(Result[i]) + Key) * C1 + C2;
end;
end;

function Decrypt(const S: String; Key: Word): String;
var
I: byte;
begin
Result[0] := S[0];
for I := 1 to Length(S) do begin
Result[i] := char(byte(S[i]) xor (Key shr 8));
Key := (byte(S[i]) + Key) * C1 + C2;
end;
end;

var
S: string;
begin
Write('>');
ReadLn(S);
S := Encrypt(S,12345);
WriteLn(S);
S := Decrypt(S,12345);
WriteLn(S);
end.