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# Elfcryption
Encryption
## Symmetric Encryption
Symmetric encryption is where we use the same key to encrypt and decrypt data.
We can use `gpg` to encrypt a file, using the command `gpg -c file.txt` and typing the password when prompted, creating the file `file.txt.gpg`. This file is encrypted and the data is all scrambled. GPG uses the AES algorithm to encrypt the files.
Once encrypted, to decrypt the file we use the command `gpg -d file.txt.gpg` and type the password (key) used to encrypt it.
To check the integrity of a file, we can see it hash value with `md5sum file.txt`. If the hash value is different from the one when the file was first created, it means the file was modified, if not it means it is the original file.
## Asymmetric Encryption
Asymmetric Encryption uses a public and a private key. If we encrypt data with someone else's public key, it can only be decrypted with that persons private key.
SSH key uses public and private keys. We generate a private key, and with that we have a public key generated also. The we place our public key onto the server, then when we want to SSH into a machine, we use our private key to authenticate ourselves as if the server can successfully decrypt our message with the public key.
So, if we use a public key to encrypt a message, it can only be decrypted with our private key. If we use a private key to encrypt a message, it can only be decrypted with our public key.
To generate a private key we use the following command (8912 creates the key 8912 bits long):
`openssl genrsa -aes256 -out private.key 8912`
To generate a public key we use our previously generated private key:
`openssl rsa -in private.key -pubout -out public.key`
Lets now encrypt a file (plaintext.txt) using our public key:
`openssl rsautl -encrypt -pubin -inkey public.key -in plaintext.txt -out encrypted.txt`
Now, if we use our private key, we can decrypt the file and get the original message:
`openssl rsautl -decrypt -inkey private.key -in encrypted.txt -out plaintext.txt`
