hello how would I create an Ada program to take an encrypted string
from a txt file and decrypt it. i have a so so program but it doesnt
work i cant get the algorithm right.
an example is this
Mother would be Mtoehr encrypted
so its just like cutting the string in half and rearranging the
chaacters to fix the decryption
my example uses a txt file which the contents are
encrypted version
aaabbb
real code
ababab
here is the program i have so far
with Ada.Text_Io;
use Ada.Text_Io;
procedure Assignment_4 is
--|This program takes in a railfence encryption and deciphers it
--|Author: Isaac Levin
subtype Message is String (11024);
Codedtext : Message; - string of coded text
Plaintext : Message; - plain text message
Indata : File_Type;
Length : Natural := 1; size of the encrypted message
J : Integer := 0; midpoint for odd and even strings
K : Integer := 2; for the encrypted string
begin -- Assingment_4
the proper text file and read the code into the string
CodedText
(
File =Indata,
Mode =In_File,
Name ="test.txt");
while not EFile(File =Indata) loop
Get(
File =Indata,
Item =Codedtext (Length));
Length := Length + 1;
end loop;
the length to account for odd or even string lengths
if Length REM 2 = 0 then
J := 0;
else
J := 1;
end if;
the first charcter of the coded message into the decoded
one
if Length 1 then
Plaintext(1) := Codedtext(1);
end if;
every character of the encoded string up to the halfway
point
every odd position in the decoded one
for I in 2(Length - 1) loop
if I REM 2 = 0 then
Plaintext(I + 1) := Codedtext(K);
K := K + 1;
else
null;
end if;
end loop;
K := 1;
every character of the encoded string past the halfway
point
every even position in the decoded one
for I in 2(Length - 1) loop
if I REM 2 = 0 then
Plaintext(I) := Codedtext((Length/2) + J + K);
K := K + 1;
else
null;
end if;
end loop;
the string PlainText to the screen
for I in 1(Length - 1) loop
Put(Item =Plaintext (I));
end loop;
Close(File =Indata);
end;
the program just cuts off the last letter in the string. i think this
has something to do with my splitting of even number strings. any help
would be greatly appreciated.
Isaac

isaac2004 wrote:
hello how would I create an Ada program to take an encrypted string
from a txt file and decrypt it. i have a so so program but it doesnt
work i cant get the algorithm right.

an example is this
Mother would be Mtoehr encrypted

so its just like cutting the string in half and rearranging the
chaacters to fix the decryption

my example uses a txt file which the contents are

encrypted version
aaabbb

real code
ababab

Your two examples above do not agree.
Following the encrypted example, Mother would encrypt to
Mteohr. I suspect that may have been a slip of the fingers during
typing, but it does add confusion to your requirements.

here is the program i have so far

with Ada.Text_Io;
use Ada.Text_Io;
procedure Assignment_4 is
--|This program takes in a railfence encryption and deciphers it
--|Author: Isaac Levin

subtype Message is String (11024);

Codedtext : Message; - string of coded text
Plaintext : Message; - plain text message
Indata : File_Type;

Length : Natural := 1; size of the encrypted message
J : Integer := 0; midpoint for odd and even strings
K : Integer := 2; for the encrypted string

begin -- Assingment_4

the proper text file and read the code into the string
CodedText

(
File =Indata,
Mode =In_File,
Name ="test.txt");

while not EFile(File =Indata) loop

Get(
File =Indata,
Item =Codedtext (Length));
Length := Length + 1;

end loop;

the length to account for odd or even string lengths

if Length REM 2 = 0 then
J := 0;
else
J := 1;
end if;

the first charcter of the coded message into the decoded
one

if Length 1 then
Plaintext(1) := Codedtext(1);
end if;

every character of the encoded string up to the halfway
point
every odd position in the decoded one

for I in 2(Length - 1) loop

if I REM 2 = 0 then
Plaintext(I + 1) := Codedtext(K);
K := K + 1;
else
null;
end if;

end loop;

K := 1;

every character of the encoded string past the halfway
point
every even position in the decoded one

for I in 2(Length - 1) loop

if I REM 2 = 0 then
Plaintext(I) := Codedtext((Length/2) + J + K);
K := K + 1;
else
null;
end if;

end loop;

In both loops you are placing characters only into
even positions in Plaintext. What about the odd positions?

It also appears that you can do all the work in a single loop,
cutting the time in half during which you traverse the strings.

First_Half_Index : Positive := 1;
Second_Half_Index : Positive := 2;

for Code_Index in Codetext'Length loop
if Code_Index <= Codetext'Length / 2 then
Plaintext(First_Half_index) := Codetext(Code_Index);
First_Half_Index := First_Half_Index + 2;
else
Plaintext(Second_Half_Index) := Codetext(Code_Index);
Second_Half_Index := Second_Half_Index + 2;
end if;
end loop;

I am not sure how the encryption deals with odd sized strings.
Does the First Half or the Second Half get the extra character?
Adjust the algorithm above accordingly.

Jim Rogers

First_Half_Index : Positive := 1;
Second_Half_Index : Positive := 2;


>for Code_Index in Codetext'Length loop
if Code_Index <= Codetext'Length / 2 then
Plaintext(First_Half_index) := Codetext(Code_Index);
First_Half_Index := First_Half_Index + 2;
else
Plaintext(Second_Half_Index) := Codetext(Code_Index);
Second_Half_Index := Second_Half_Index + 2;
end if;
>end loop;

this code confuses me
what is Code_Index and Codetext'Length

>Your two examples above do not agree.
>Following the encrypted example, Mother would encrypt to
>Mteohr. I suspect that may have been a slip of the fingers during
>typing, but it does add confusion to your requirements.

that is my mistake it is really Mteohr

>Now Length is the # of characters read + 1.
the reason for doing that is to loop through the string and reading all
char to length.

thanks for the help guys

isaac2004 wrote:

an example is this
Mother would be Mtoehr encrypted

How about a little rearrangement and some functional
decomposition (if you can afford to be a little wasteful
in the first draft):

procedure encrypt (s: Message) is

type Confusion is array(Positive range s'Range) of Positive;

function permute(s: Message) return Confusion;

new_idx: constant Confusion := permute(s);

begin
for k in s'Range loop
<copy characters to their new positions via `new_idx`>
end loop;
end encrypt;

>You could put in some debugging code:
the thing is that i know what is wrong with the code but i cant figure
out the algorithm

>Code_Index is a variable declared as part of for loop. C99 can do that as
well - finally ;-)

so Code_index would be like this

for Code_Index in Count loop

bla bla ;
end loop;

Codetext'Length is just a variable for length and codetext'range is one
for max index
im kinda confused

isaac2004 wrote:
>You could put in some debugging code:
the thing is that i know what is wrong with the code but i cant figure
out the algorithm
>
>Code_Index is a variable declared as part of for loop. C99 can do that as
well - finally ;-)

so Code_index would be like this

for Code_Index in Count loop

bla bla ;
end loop;

Codetext'Length is just a variable for length and codetext'range is one
for max index
im kinda confused

Codetext'Length is the length of the Codetext array. It is the number
of elements
in the array.

Codetext'range is the range of index values for the array Codetext.

Jim Rogers

Well I quickly put a description together for you:
#Array_Attributes
do follow some of the links - especialy the one about loops on arrays.
Martin