昨晚在知乎看到一个问题，（ 可以帮忙破译一个密码吗？VRPHWLPHV L ZDQW WR FKDW ZLWK BRX）
看了一下各位答案，是凯撒密码，我百科了一下，发现蛮简单的，今天早上来实现了。
凯撒加密方法就是将字符串的每个字母按字母表顺序进行偏移。
知道原理后，就可以很容易写出来了。
/*************************************************************************
> File Name: caesar.c
> Author: Netcan
> Mail: 1469709759@qq.com
> Created Time: 2015/3/8 10:28:20
************************************************************************/
#include<stdio.h>
#include<string.h>
int main(int argc, char **argv)
{
char code[1024];
if(argc >= 3) {
strcpy(code, argv[2]);
for(char *c=code; *c!='\0'; ++c) {
if(strcmp(argv[1], "-decode") == 0)
if(*c>='A' && *c<='Z')
*c = *c - 'A';
if(strcmp(argv[1], "-code") == 0)
if(*c>='a' && *c<='z')
*c = *c - 'a';
}
for(int offset=1; offset<=26; ++offset) {
for(char *c=code; *c!='\0'; ++c)
if(*c >= 0 && *c <= 25) {
if(strcmp(argv[1], "-decode") == 0)
printf("%c", (*c + offset)%26 + 'a');
if(strcmp(argv[1], "-code") == 0)
printf("%c", (*c + offset)%26 + 'A');
}
else
printf("%c", *c);
if(strcmp(argv[1], "-decode") == 0)
printf("offset=%d\n",offset);
if(strcmp(argv[1], "-code") == 0)
printf("offset=%d\n",27-offset);
}
}
return 0;
}程序运行也很简单，
caesar.exe -code "string" #加密
caesar.exe -decode "STRING" #解密
所以这个答案也很容易得出：
caesar.exe -decode "VRPHWLPHV L ZDQW WR FKDW ZLWK BRX, EXW L KDYH QR UHDVRQ WRFKDW ZLWK BRX."
解密的结果：
wsqixmqiw m aerx xs glex amxl csy, fyx m lezi rs viewsr xs glex amxl csy. offset=1
xtrjynrjx n bfsy yt hmfy bnym dtz, gzy n mfaj st wjfxts yt hmfy bnym dtz. offset=2
yuskzosky o cgtz zu ingz cozn eua, haz o ngbk tu xkgyut zu ingz cozn eua. offset=3
zvtlaptlz p dhua av joha dpao fvb, iba p ohcl uv ylhzvu av joha dpao fvb. offset=4
awumbquma q eivb bw kpib eqbp gwc, jcb q pidm vw zmiawv bw kpib eqbp gwc. offset=5
bxvncrvnb r fjwc cx lqjc frcq hxd, kdc r qjen wx anjbxw cx lqjc frcq hxd. offset=6
cywodswoc s gkxd dy mrkd gsdr iye, led s rkfo xy bokcyx dy mrkd gsdr iye. offset=7
dzxpetxpd t hlye ez nsle htes jzf, mfe t slgp yz cpldzy ez nsle htes jzf. offset=8
eayqfuyqe u imzf fa otmf iuft kag, ngf u tmhq za dqmeaz fa otmf iuft kag. offset=9
fbzrgvzrf v jnag gb pung jvgu lbh, ohg v unir ab ernfba gb pung jvgu lbh. offset=10
gcashwasg w kobh hc qvoh kwhv mci, pih w vojs bc fsogcb hc qvoh kwhv mci. offset=11
hdbtixbth x lpci id rwpi lxiw ndj, qji x wpkt cd gtphdc id rwpi lxiw ndj. offset=12
iecujycui y mqdj je sxqj myjx oek, rkj y xqlu de huqied je sxqj myjx oek. offset=13
jfdvkzdvj z nrek kf tyrk nzky pfl, slk z yrmv ef ivrjfe kf tyrk nzky pfl. offset=14
kgewlaewk a osfl lg uzsl oalz qgm, tml a zsnw fg jwskgf lg uzsl oalz qgm. offset=15
lhfxmbfxl b ptgm mh vatm pbma rhn, unm b atox gh kxtlhg mh vatm pbma rhn. offset=16
migyncgym c quhn ni wbun qcnb sio, von c bupy hi lyumih ni wbun qcnb sio. offset=17
njhzodhzn d rvio oj xcvo rdoc tjp, wpo d cvqz ij mzvnji oj xcvo rdoc tjp. offset=18
okiapeiao e swjp pk ydwp sepd ukq, xqp e dwra jk nawokj pk ydwp sepd ukq. offset=19
pljbqfjbp f txkq ql zexq tfqe vlr, yrq f exsb kl obxplk ql zexq tfqe vlr. offset=20
qmkcrgkcq g uylr rm afyr ugrf wms, zsr g fytc lm pcyqml rm afyr ugrf wms. offset=21
rnldshldr h vzms sn bgzs vhsg xnt, ats h gzud mn qdzrnm sn bgzs vhsg xnt. offset=22
sometimes i want to chat with you, but i have no reason to chat with you. offset=23
tpnfujnft j xbou up dibu xjui zpv, cvu j ibwf op sfbtpo up dibu xjui zpv. offset=24
uqogvkogu k ycpv vq ejcv ykvj aqw, dwv k jcxg pq tgcuqp vq ejcv ykvj aqw. offset=25
vrphwlphv l zdqw wr fkdw zlwk brx, exw l kdyh qr uhdvrq wr fkdw zlwk brx. offset=26只有第 23 行的 "sometimes i want to chat with you, but i have no reason to chat with you.“有意义，所以，明文出来了，是 " 有时我想和你聊天，但是我没有理由和你聊天。”
