First this, from 1641: a bit of early encryption advice - check out these togop segecreget techniques:
"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 (web version)
Now that you've encrypted that important information, here's how you can pass it on (from 1677):
"Now we will teach the techniques for writing on various objects in such a way that, even though the marks may be seen, nevertheless they will deceive spies and interceptors, through artful tricks...
Giambattista Della Porta, Della magia naturale (Italian edition, 1677)One can write messages quite effectively on playing cards. It is first necessary to lay out the cards in a certain order, each one beside the next, either face up or face down. Once you have arranged them in this way, you can write whatever message you want along the borders between cards. Then you flip the cards and shuffle them well. The message will no longer appear, and if anyone is curious enough to examine the cards closely, he will see only some disorderly markings. But when the intended recipient wants to read the message, he will lay out the cards in the predetermined order, so that the corners and edges join and line up with each other, and it will be possible to read the message perfectly."
via Ask The Past (blog)