openssl: added a guide on how to en-/decrypt a file and create keys

wiki/cryptography.md

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+# Cryptography
+
+The practice of encryption and decryption is named cryptography.
+There are various ways to permit the readability of a message.
+
+## Linux
+
+- [openssl](linux/openssl.md) is a big software library for secure communication
+over computer networks. It can be used to encrypt and decrypt files using RSA.

wiki/linux/openssl.md

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+# OpenSSL
+
+[OpenSSL](https://www.openssl.org) is a software library for secure
+communication over computer networks.
+
+## Creating a key pair
+
+As described by [dreikanter](https://gist.github.com/dreikanter/c7e85598664901afae03fedff308736b)
+you can create and save a pretty secure private key to `private.key` with
+`openssl genrsa -aes256 -out private.key 8912`.
+After that run `openssl rsa -in private.key -pubout -out public.key` to save the
+according public key to `public.key`.
+
+## Encrypting and decrypting files
+
+[Dreikanter](https://gist.github.com/dreikanter/c7e85598664901afae03fedff308736b)
+describes to encrypt larger files with symmetric encryption and encrypt the key
+of that using asymmetric encryption.
+If however the file is small enough to be encrypted with the public key
+`public.key` directly run
+`openssl rsautl -encrypt -pubin -inkey public.key -in plaintext.txt -out encrypted.txt`
+where `plaintext.txt` is the message to encrypt and `encrypted.txt` is the
+encrypted message.
+You can decrypt the message using the private key `private.key` as follows
+`openssl rsautl -decrypt -inkey private.key -in encrypted.txt -out plaintext.txt`.