Decrypting Caesar cipher fails when offset is 11 or more

Question

I am doing a decryption program. I've been looking and trying to figure this out, but I don't see anything. So when I decrypt such as 9adwrqxvni0348&4#9 it comes out fine, but when I have an offset of 11 or more it decrypts all but the last character. No matter what the offset past 11 is the last character is that same. I just now stuck all letters in, and they work. It is just the last number character that does not work past 11.

for (int count = 0; count < length; count++)
{
    if (msg[count] >= 'a' && msg[count] <= 'z')//Letter wraping
    {
        dmsg += ((msg[count] - 'a' - offset + 26) % 26) + 'a';
    }
    else if (msg[count] >= '0' && msg[count] <= '9')//Number wraping
    {
        dmsg += (abs(msg[count] - '0' - offset + 10) % 10) + '0';
    }       
}

Might be your problem: http://stackoverflow.com/questions/7594508/modulo-operator-with-negative-values — Silvio Mayolo, Mar 15 '17 at 19:56
If code is 10bexrrywoi1459&4#9 then it comes in as run home 1459. which is right. If the offset is 11 or more then it comes out run home 1451. No matter what the offset is above 11. Weird — Jake Purdom, Mar 15 '17 at 20:09
@JakePurdom Have you tried stepping through the program with a debugger? — Barmar, Mar 15 '17 at 20:28
Why does `1459` decode to `1459`? Shouldn't it decode to something else that depends on the offset? — Barmar, Mar 15 '17 at 20:32
To know the correct way to decode it, we need to see how the encoding algorithm works. — Barmar, Mar 15 '17 at 20:33

tinstaafl · Answer 1 · 2017-03-15T21:22:38.670

-1

It looks to me that when the offset becomes greater than 10, adding 10 doesn't create the appropriate positive value on any digit that is equal to or less than the difference between the offset and 10. One work around is to mod 10 the offset:

dmsg += ( abs( msg[count] - '0' - ( offset % 10 ) + 10 ) % 10 ) + '0';

edited Mar 15 '17 at 21:22

answered Mar 15 '17 at 20:29

tinstaafl

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@JakePurdom - You're very welcome. One thing to keep in mind. The same thing will probably occur with letters, if the offset is greater than 26. – tinstaafl Mar 15 '17 at 21:25
@JakePurdom - If you've found an answer that answers your question, please remember to mark it so. Thanks. – tinstaafl Mar 15 '17 at 23:01

Decrypting Caesar cipher fails when offset is 11 or more

1 Answers1