Ejemplo028.c

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#include <stdio.h>
 
#define N 50
#define NC 52
 
int main(int argc, char *argv[])
{
    char str[N], strC[N], strD[N], c;
    int i, a, b, ai, nc;
    ai = -1;
    do{
        printf("a = ");
        scanf("%d", &a);
        for(i=0; i<NC; i++)
            if(((a*i)%NC)==1)
            {
                ai = i;
                break;
            }
    }while(ai<0);
    printf("b = ");
    scanf("%d", &b);
    while ((c = getchar()) != '\n' && c != EOF) {}
    i = 0;
    do{
        c = getchar();
        if((c>='A'&&c<='Z')||((c>='a'&&c<='z')||c==' '))
        {
            str[i] = c;
            i++;
        }
    }while(c!=10&&i<(N-1));
    nc = i;
    str[nc] = '\0';
    printf("MSG: %s\n", str);
    for(i=0; i<nc; i++)
    {
        if(str[i]>='A'&&str[i]<='Z')
            strC[i] = str[i]-'A';
        else if(str[i]>='a'&&str[i]<='z')
            strC[i] = str[i]-'a'+26;
        else
            strC[i] = NC;
    }
    strC[nc] = '\0';
    for(i=0; i<nc; i++)
        if(strC[i]<NC)
            strC[i] = (a*strC[i]+b)%NC;
    for(i=0; i<nc; i++)
    {
        if(strC[i]<NC/2)
            strC[i] = strC[i]+'A';
        else if(strC[i]<NC)
            strC[i] = strC[i]+'a'-26;
        else
            strC[i] = ' ';
    }
    printf("MSG ENCRIPTADO: %s\n", strC);
    for(i=0; i<nc; i++)
    {
        if(strC[i]>='A'&&strC[i]<='Z')
            strD[i] = strC[i]-'A';
        else if(strC[i]>='a'&&strC[i]<='z')
            strD[i] = strC[i]-'a'+26;
        else
            strD[i] = NC;
    }
    strD[nc] = '\0';
    for(i=0; i<nc; i++)
        if(strD[i]<NC)
        {
            if(strD[i]>=b)
                strD[i] = (ai*(strD[i]-b))%NC;
            else
                strD[i] = (ai*(NC+strD[i]-b))%NC;
        }
    for(i=0; i<nc; i++)
    {
        if(strD[i]<NC/2)
            strD[i] = strD[i]+'A';
        else if(strD[i]<NC)
            strD[i] = strD[i]+'a'-26;
        else
            strD[i] = ' ';
    }
    printf("MSG DESENCRIPTADO: %s\n", strD);
    return 0;
}