In German law, is it legal to send an encrypted message via a post card? E.g. encrypt a message text on a computer with RSA and print the Base64 encoding on the post card and send it. Or is it a requirement that the message content is readable (hand written) plain text?
The privacy of correspondence, posts, and telecommunications shall be inviolable.
You can send whatever you want via postcard (as long as it doesn't constitute a different criminal offense).
Technically, whoever were to decrypt your message, besides the addressee, would probably commit Data Espionage according to §202a (2) Criminal Code / § 202a Abs. 2 Strafgesetzbuch. The second paragraph states that this only applies if the data is transmitted electronically, magnetically, or otherwise in a manner which is not immediately perceptible. I would argue that a non-sealed postcard with an encrypted message would be "not immediately perceptible".