How to Encrypt a Message, 1641

Giambattista della Porta, De furtivis
litterarum notis (1591), Folger

"The second way of secrecy in speech, is by an alteration of any knowne language... by augmenting words with the addition of other letters. Of which kind, is that secret way of discoursing in ordinary use, by doubling the vowels that make the syllables, and interposing G. or any other consonant... Thus, if I would say, Our plot is discovered, it must be pronounced thus, Ougur plogot igis digiscogovegereged. Which doe's not seeme so obscure in writing, as it will in speech and pronuntiation. And it is so easie to be learnt, that I have knowne little children, almost as soone as they could speake, discourse to one another as fast this way, as they could in their plainest English. But all these later kinds of secrecy in speech, have this grand inconvenience in them, that they are not without suspition...
There are likewise some other inventions to expresse any inward sence by barbarous words, wherein onely the first, and middle, and last letters shall be significant. As in this example. Fildy, fagodur wyndeeldrare discogure rantibrad. Which in its resolution is no more than this. Fly for we are discovered." 

John Wilkins, Mercury, or, The Secret and Swift Messenger

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