有必要那么麻烦吗？
$ C_i=M_i+k_{i\mod \mathtt{len(k)}}-1 $
$ M_i=C_i-k_{i\mod \mathtt{len(k)}}+1 $
前提：从0开始
by hdkghc @ 2022-06-18 06:23:59
$\because C_i =M_i+k_{i\mod\mathtt{len(k)}}-1 $
$\therefore M_i=C_i-k_{i\mod \mathtt{len(k)}}+1 $
by hdkghc @ 2022-06-18 06:27:00
$\because C_i =M_i+k_{i\mod\mathtt{len(k)}}-1 $
$ \therefore M_i=C_i-k_{i\mod \mathtt{len(k)}}+1 $
by hdkghc @ 2022-06-18 06:27:38