凱撒加密，以26英文字字母來說只25種加密方法（加法）。
如果改以乘法，就可以有無限的可能性。
例如乘加質數 7
| 明文 | 數字 | 使用金鑰7加密 | 密文 |
| A | 0 | （0 * 7）% 26 = 0 | A |
| B | 1 | （1 * 7）% 26 = 7 | H |
| C | 2 | （2 * 7）% 26 = 14 | O |
| D | 3 | （3 * 7）% 26 = 21 | V |
| E | 4 | （4 * 7）% 26 = 2 | C |
| F | 5 | （5 * 7）% 26 = 9 | J |
| G | 6 | （6 * 7）% 26 = 16 | Q |
| H | 7 | （7 * 7）% 26 = 23 | X |
| I | 8 | （8 * 7）% 26 = 4 | E |
| J | 9 | （9 * 7）% 26 = 11 | L |
| K | 10 | （10 * 7）% 26 = 18 | S |
| L | 11 | （11 * 7）% 26 = 25 | Y |
| M | 12 | （12 * 7）% 26 = 6 | G |
| N | 13 | （13 * 7）% 26 = 13 | N |
| O | 14 | （14 * 7）% 26 = 20 | U |
| P | 15 | （15 * 7）% 26 = 1 | B |
| Q | 16 | （16 * 7）% 26 = 8 | I |
| R | 17 | （17 * 7）% 26 = 15 | P |
| S | 18 | （18 * 7）% 26 = 22 | W |
| T | 19 | （19 * 7）% 26 = 3 | D |
| U | 20 | （20 * 7）% 26 = 10 | K |
| V | 21 | （21 * 7）% 26 = 17 | R |
| W | 22 | （22 * 7）% 26 = 24 | Y |
| X | 23 | （23 * 7）% 26 = 5 | F |
| Y | 24 | （24 * 7）% 26 = 12 | M |
| Z | 25 | （25 * 7）% 26 = 19 | T |
但是否使用了乘數就可以有無限種的可能性了嗎？
我們把它列出來計算
| A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z | |
| 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 | |
| 1 | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 |
| 2 | 0 | 2 | 4 | 6 | 8 | 10 | 12 | 14 | 16 | 18 | 20 | 22 | 24 | 0 | 2 | 4 | 6 | 8 | 10 | 12 | 14 | 16 | 18 | 20 | 22 | 24 |
| 3 | 0 | 3 | 6 | 9 | 12 | 15 | 18 | 21 | 24 | 1 | 4 | 7 | 10 | 13 | 16 | 19 | 22 | 25 | 2 | 5 | 8 | 11 | 14 | 17 | 20 | 23 |
| 4 | 0 | 4 | 8 | 12 | 16 | 20 | 24 | 2 | 6 | 10 | 14 | 18 | 22 | 0 | 4 | 8 | 12 | 16 | 20 | 24 | 2 | 6 | 10 | 14 | 18 | 22 |
| 5 | 0 | 5 | 10 | 15 | 20 | 25 | 4 | 9 | 14 | 19 | 24 | 3 | 8 | 13 | 18 | 23 | 2 | 7 | 12 | 17 | 22 | 1 | 6 | 11 | 16 | 21 |
| 6 | 0 | 6 | 12 | 18 | 24 | 4 | 10 | 16 | 22 | 2 | 8 | 14 | 20 | 0 | 6 | 12 | 18 | 24 | 4 | 10 | 16 | 22 | 2 | 8 | 14 | 20 |
偶數乘數不可以使用，因為會有不同字母對映到同一個值，就無法被反解譯。
| 13 | 0 | 13 | 0 | 13 | 0 | 13 | 0 | 13 | 0 | 13 | 0 | 13 | 0 | 13 | 0 | 13 | 0 | 13 | 0 | 13 | 0 | 13 | 0 | 13 | 0 | 13 |
在 13 時，和英文字數 26 有因數關係，也不行。
如果乘數大於 26 ，開始出現重覆了。
| A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z | |
| 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 | |
| 1 | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 |
| 3 | 0 | 3 | 6 | 9 | 12 | 15 | 18 | 21 | 24 | 1 | 4 | 7 | 10 | 13 | 16 | 19 | 22 | 25 | 2 | 5 | 8 | 11 | 14 | 17 | 20 | 23 |
| 5 | 0 | 5 | 10 | 15 | 20 | 25 | 4 | 9 | 14 | 19 | 24 | 3 | 8 | 13 | 18 | 23 | 2 | 7 | 12 | 17 | 22 | 1 | 6 | 11 | 16 | 21 |
| 7 | 0 | 7 | 14 | 21 | 2 | 9 | 16 | 23 | 4 | 11 | 18 | 25 | 6 | 13 | 20 | 1 | 8 | 15 | 22 | 3 | 10 | 17 | 24 | 5 | 12 | 19 |
| 23 | 0 | 23 | 20 | 17 | 14 | 11 | 8 | 5 | 2 | 25 | 22 | 19 | 16 | 13 | 10 | 7 | 4 | 1 | 24 | 21 | 18 | 15 | 12 | 9 | 6 | 3 |
| 25 | 0 | 25 | 24 | 23 | 22 | 21 | 20 | 19 | 18 | 17 | 16 | 15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 |
| 27 | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 |
| 29 | 0 | 3 | 6 | 9 | 12 | 15 | 18 | 21 | 24 | 1 | 4 | 7 | 10 | 13 | 16 | 19 | 22 | 25 | 2 | 5 | 8 | 11 | 14 | 17 | 20 | 23 |
| 31 | 0 | 5 | 10 | 15 | 20 | 25 | 4 | 9 | 14 | 19 | 24 | 3 | 8 | 13 | 18 | 23 | 2 | 7 | 12 | 17 | 22 | 1 | 6 | 11 | 16 | 21 |
4. 換位加密