caeser encryption special case?!

[ivory osprey]

can someone explain why my caesar solving algorithm works for every letter except of 'Z' cause 'Z' should turn to 'O' but it turns to 'arrow down'

#include<iostream>
#include<string> 

void unsolve(std::string nachricht);
int offset = 0;
int main()
{
    std::string nachricht 
    unsolve(nachricht);
    return 0;

}
void unsolve(std::string nachricht){
int highest = 0;
int index = 0;
int letters[24] = {};
for (int i = 0; i < nachricht.size() ; i++)
{
    if (nachricht[i] >= 'A' && nachricht[i] <= 'Z')
    {
    nachricht[i] = nachricht[i] - 'A';
    letters[nachricht[i]]++;
    }
}
for (int i = 0; i < sizeof(letters)/sizeof(letters[1]); i++)
{
    if(letters[i] > highest){
        highest = letters[i];
        index = i;}
}
for (int i = 0; i < nachricht.size(); i++)
{
    if (nachricht[i] >= 0 && nachricht[i] <= 24)
    {
        nachricht[i] = nachricht[i] + 'A';
    }
    if(nachricht[i] >= 76 && nachricht[i] <= 90) {
    nachricht[i] = nachricht[i] - (index - 4);
    }
    else if(nachricht[i] >= 65 && nachricht[i] <= 75)
    {
        nachricht[i] = nachricht[i] + 15;
    }
}
}```

[forest kestrel]

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[ivory osprey]

std::string nachricht =    "OTP NLPDLC - GPCDNSWFPDDPWFYR(LFNS LWD NLPDLC - NSTQQCP, "
                        "NLPDLC - LWRZCTESXFD, NLPDLC - GPCDNSTPMFYR, GPCDNSTPMPNSTQQCP "
                        "ZOPC LWD PTYQLNSPC NLPDLC MPKPTNSYPE) TDE PTY PTYQLNSPD DJXXPECTDNSPD "
                        "GPCDNSWFPDDPWFYRDGPCQLSCPY, OLD LFQ OPC XZYZRCLASTDNSPY FYO "
                        "XZYZLWASLMPETDNSPY DFMDETEFETZY MLDTPCE.LWD PTYPD OPC PTYQLNSDEPY "
                        "FYO FYDTNSPCDEPY GPCQLSCPY OTPYE PD SPFEP SLFAEDLPNSWTNS OLKF, "
                        "RCFYOACTYKTATPY OPC VCJAEZWZRTP LYDNSLFWTNS OLCKFDEPWWPY.OPC "
                        "PTYQLNSSPTE SLWMPC HPCOPY ZQEXLWD YFC OTP 26 MFNSDELMPY OPD "
                        "WLEPTYTDNSPY LWASLMPED ZSYP FYEPCDNSPTOFYR GZY RCZDD - "
                        "FYO VWPTYMFNSDELMPY LWD LWASLMPE QFPC VWLCEPIE FYO RPSPTXEPIE GPCHPYOPE "
                        "FYO DZYOPCKPTNSPY, DLEKKPTNSPY FDH. YTNSE MPLNSEPE.";```

and why does it work when I do cpp nachricht[i] = nachricht[i] - (index - 4); and not cpp nachricht[i] = nachricht[i] - index;

[rugged sierra]

probably because you're doing a simple frequency attack and the most common letter is e

[ivory osprey]

yes the most common letter is E

[rugged sierra]

and since E is the 5th letter (index 4), that's what you want to offset by

[ivory osprey]

ah okay that makes sense

but why is Z not after this calculation O but arrowdown

because its going into this if statement cpp if(nachricht[i] >= 76 && nachricht[i] <= 90) { nachricht[i] = nachricht[i] - (index - 4); }

but idk why its not just doing -11 then

[rugged sierra]

index is 15 right?

[ivory osprey]

yes

its O

[rugged sierra]

idk, use you debugger

[ivory osprey]

is that then a coincidence?

[rugged sierra]

or maybe show a screenshot of the arrowdown

[ivory osprey]

I got it

 if (nachricht[i] >= 0 && nachricht[i] <= 24)
    {
        nachricht[i] = nachricht[i] + 'A';
    }``` this needs to be <= 25 not <=24

but thank you

for ur help

!solved