标准字头密码

#include
#include
#include
typedef struct tagKeywordTable
{
    char p;
    char c;
}KeywordTable;
void cst_keyword_table (char *key);
void prnt_keyword_table ();
int keyword_encrypt (char *plain_txt, char *cipher_txt);
int keyword_decrypt (char *cipher_txt, char *plain_txt);
KeywordTable keyword_table[26];
void cst_keyword_table (char *in_key)
{
    int i = 0, j, k;
    int char_count = 0, left_ch_index = 0;
    char ch, left_chs[26] = {0}, key[26] = {0};
    int key_len;
    strcpy (key, in_key);
    key_len = strlen (key);
    while (1)
    {
        if (key[i] == ‘ ‘)
        {
            for (j = i; j < key_len - 1; ++j)
                key[j] = key[j + 1];
            key_len--;
        }
        if (i == key_len)
            break;
        i++;
    }
    for (i = 0; i < 26; ++i)
    {
        ch = ‘a‘ + i;
        j = 0;
        char_count = 0;
        while (1)
        {
            if (key[j] == ch)
            {
                char_count++;
                if (char_count > 1)
                {
                    for (k = j; k < key_len -1; ++k)
                        key[k] = key[k + 1];
                    key_len--;
                }
                else
                    j++;
            }
            else
                j++;
            if (j == key_len)
                break;
        }
        if (char_count == 0)
            left_chs[left_ch_index++] = ch;
    }

    for (i = 0; i < left_ch_index; ++i)
        key[key_len++] = left_chs[i];
    for (i = 0; i < 26; ++i)
    {
        keyword_table[i].p = ‘a‘ + i;
        keyword_table[i].c = key[i];
    }
}

void prnt_keyword_table ()
{
    int i;
    printf ("Plain:\t");
    for (i = 0; i < 26; ++i)
        printf ("%c ", keyword_table[i].p);
    printf ("\nCipher:\t");
    for (i = 0; i < 26; ++i)
        printf ("%c ", keyword_table[i].c);
    puts("\n");
}

int keyword_encrypt (char *plain_txt, char *cipher_txt)
{
    int length = strlen (plain_txt);
    int i;
    for (i = 0; i < length; ++i)
    {
        if (plain_txt[i] >= ‘a‘ && plain_txt[i] <= ‘z‘)
        {
            cipher_txt[i] = keyword_table[plain_txt[i] - ‘a‘].c;
        }
        else if (plain_txt[i] >= ‘A‘ && plain_txt[i] <= ‘Z‘)
        {
            cipher_txt[i] = keyword_table[plain_txt[i] - ‘A‘].c - 32;
        }
        else
            cipher_txt[i] = plain_txt[i];
    }
    cipher_txt[length] = ‘\0‘;

    return length;
}
int main ()
{
    char plain_txt[1000], cipher_txt[1000];
    char key[26] = {0};
    printf ("请输入你的密钥: ");
    scanf ("%s", key);
    cst_keyword_table (key);
    printf ("标准字头密码体制的密表为:\n");
    prnt_keyword_table ();
    printf ("现在输入明文: ");
    scanf ("%s", plain_txt);
    keyword_encrypt (plain_txt, cipher_txt);
    printf ("加密后的密文为 : %s\n", cipher_txt);
    return 0;
}
****************************************************************************************
解密
#include
#include
#include
typedef struct tagKeywordTable
{
        char p;
        char c;
}KeywordTable;
void cst_keyword_table (char *key);
void prnt_keyword_table ();
int keyword_encrypt (char *plain_txt, char *cipher_txt);
int keyword_decrypt (char *cipher_txt, char *plain_txt);
KeywordTable keyword_table[26];
void cst_keyword_table (char *in_key)
{
        int i = 0, j, k;
        int char_count = 0, left_ch_index = 0;
        char ch, left_chs[26] = {0}, key[26] = {0};
        int key_len;
        strcpy (key, in_key);
        key_len = strlen (key);
        while (1)
        {
                if (key[i] == ‘ ‘)
                {
                        for (j = i; j < key_len - 1; ++j)
                                key[j] = key[j + 1];
                        key_len--;
                }
                if (i == key_len)
                        break;
                i++;
        }
        for (i = 0; i < 26; ++i)
        {
                ch = ‘a‘ + i;
                j = 0;
                char_count = 0;
                while (1)
                {
                        if (key[j] == ch)
                        {
                                char_count++;
                                if (char_count > 1)
                                {
                                        for (k = j; k < key_len -1; ++k)
                                                key[k] = key[k + 1];
                                        key_len--;
                                }
                                else
                                        j++;
                        }
                        else
                                j++;
                        if (j == key_len)
                                break;
                }
                if (char_count == 0)
                        left_chs[left_ch_index++] = ch;
        }
        for (i = 0; i < left_ch_index; ++i)
                key[key_len++] = left_chs[i];
        for (i = 0; i < 26; ++i)
        {
                keyword_table[i].p = ‘a‘ + i;
                keyword_table[i].c = key[i];
        }
}
void prnt_keyword_table ()
{
        int i;
        printf ("Plain:\t");
        for (i = 0; i < 26; ++i)
                printf ("%c ", keyword_table[i].p);
        printf ("\nCipher:\t");
        for (i = 0; i < 26; ++i)
                printf ("%c ", keyword_table[i].c);
        puts("\n");
}

int keyword_decrypt (char *cipher_txt, char *plain_txt)
{
        int length = strlen (cipher_txt);
        int i, j;
        for (i = 0; i < length; ++i)
        {
                if (cipher_txt[i] >= ‘a‘ && cipher_txt[i] <= ‘z‘)
                {
                        for (j = 0; j < 26; ++j)
                        {
                                if (keyword_table[j].c == cipher_txt[i])
                                {
                                        plain_txt[i] = keyword_table[j].p;
                                        break;
                                }
                        }
                }
                else if (cipher_txt[i] >= ‘A‘ && cipher_txt[i] <= ‘Z‘)
                {
                        for (j = 0; j < 26; ++j)
                        {
                                if (keyword_table[j].c == cipher_txt[i] + 32)
                                {
                                        plain_txt[i] = keyword_table[j].p - 32;
                                        break;
                                }
                        }

                }
                else
                        plain_txt[i] = cipher_txt[i];
        }
        plain_txt[length] = ‘\0‘;
        return length;
}
int main ()
{
        char plain_txt[1000], cipher_txt[1000];
        char key[26] = {0};
        printf ("请输入你的密钥: ");
        scanf ("%s", key);
        cst_keyword_table (key);
        printf ("标准字头密码体制的密表为:\n");
        prnt_keyword_table ();
        printf ("输入原密文: ");
        scanf ("%s", cipher_txt);
        keyword_decrypt (cipher_txt, plain_txt);
        printf ("则原明文为:%s\n", plain_txt);

        return 0;
}
****************************************************************************************