The few manuals and text-books3 of cryptology in existence devoted themselves mainly to such 'poly-alphabetic' ciphers.
当时的一些讲授密码学的资料或教材，主要都是讲这种字母替换法。
Slightly more complex was the use of a system which substituted not for single letters, but for the 676 possible letter pairs.
某些字母替换系统，使用了一些技巧，比如说轮流使用多套规则。
One British cipher system of this period was of this nature, combining the idea with the use of a code book. It was used by the Merchant Navy.4
当时英国有一种密码系统，就是这种类型的，与密码手册配套使用。英国商船队使用的就是这样的系统。
The cipher clerk would first have to render the message into Merchant Navy Code, thus:
密码操作员首先要把信息翻译成商船通信码，比如：
Text Coded, Expect to arrive at VQUW, 14, CFUD, 40 UQGL
信息通信码，预计到达时间VQUW，14 CFUD，40 UQGL
The next step required an even number of rows, so the clerk would have to add a nonsense word to make it up:Balloon ZJVY
下一步需要将行数变成偶数，因此操作员可能会添上一个没有意义的词，比如：气球ZJVY。
Then the encipherment would be done.
然后就可以进行编码。
The clerk would take the first vertical pair of letters, here VC, and look it up in a table of letter pairs.
操作员先取出纵向的第一对字母，在这里就是VC，然后在替换表中找到对应的替换。
The table would specify some other pair, say XX.
这张表包括了所有的XX这样的字母对。
The clerk would continue to go through the message, substituting for each letter pair in this way.
操作员重复这个过程，对每一对字母进行替换。
There was little more to it, except that as with the 'adding on' kind of cipher, the process was futile unless the legitimate receiver knew which substitution table was being used.
其它就没什么特别的，与叠加型的密码一样，接收者必须知道使用的是哪个替换表，否则就无法解密信息。
To preface the transmission with 'Table number 8', say, would allow the enemy analyst to collect and collate the transmissions enciphered with the same table, and make an attack.
然而，如果在通信中说出来"使用8号表"，那么敌方分析员就会将使用同一张表的信息搜集到一起，寻找规律进行破解。
There had to be some element of disguise involved.
因此，还要再做一些伪装工作。