In this post, I will be going through cryptography basic concepts, how to setup gpg, and how to convert keys between different formats.

Terminology
Setup GPG
Symmetric Key Encryption
- Using GPG
- Using openssl
Asymmetric Key Encryption
- Using GPG
- Using OpenSSL
Hybrid Encryption
Conversions
Conclusion

Terminology

Privacy-Enhanced Mail (PEM) is a de facto file format for storing and sending cryptographic keys, certificates, and other data, based on a set of 1993 IETF
PGP - Pretty Good Privacy
OpenPGP - OpenPGP was initially derived from the PGP software, created by Phil Zimmermann
GnuPG - GNU Privacy Guard. GnuPG is a complete and free implementation of the OpenPGP standard as defined by RFC4880 (also known as PGP)¹

Setup GPG

Follow steps mentioned in gnupg.org to install gpg.
Use these settings to harden configuration gpg.conf, copy this file to ~/.gnupg/gpg.conf
Use these settings to configure gpg agent gpg-agent.conf, depending on operating system change the pinentry-program, for MacOS use /usr/local/bin/pinentry-mac and for linux use /usr/bin/pinentry-curses
Make sure you have in your shell rc file.
```
  GPG_TTY=`tty`
  export GPG_TTY
```
gpg --full-gen-key to generate keys. Mostly you will choose RSA with 4096 bits. If you want to generate ECC (Elliptic Curve Cryptography) keys, then you need to use expert option gpg --expert --full-gen-key
gpg --list-secret-keys to list secret/private keys
gpg --list-keys to list public keys
gpg --armor --export YOUR_KEY_ID to export public key in ASCII format
gpg --armor --export-secret-keys > secret_key.gpg to export secret/private key. Note this should NOT be shared and should be kept in a safe location.
If you are vim user, then refer this section to configure vim for gpg
By default, one encryption subkey is created for the private key; you can create as many subkeys as you want, one subkey for signing, one key for authentication, etc. Refer this guide
You can use the authentication subkey for SSH, follow below steps
- gpg --expert --edit-key <KEY ID> to edit your key
- Type addkey in gpg prompt
- Select RSA (set your own capabilities) option 8
- Select s to Toggle the sign capability
- Select e to Toggle the encrypt capability
- Select a to Toggle the authenticate capability
- In Current allowed actions, make sure only Authenticate is selected
- Select q to Finished
- Specify the key size and expiry
- save and quit
- Make sure you have enable-ssh-support in ~/.gnupg/gpg-agent.conf
- Execute gpg -K --with-keygrip and then look for subkey that has [A] and copy Keygrip and paste in ~/.gnupg/sshcontrol
- Tell SSH how to access the gpg-agent, export SSH_AUTH_SOCK=$(gpgconf --list-dirs agent-ssh-socket)
- And launch gpg agent gpgconf --launch gpg-agent
- Run ssh-add -L to list public keys which can be shared and put in authorized_keys of the server which you want to login.
If you want to use an already existing ssh RSA key to add to your GPG, then follow the below steps
- Make sure ssh RSA has PEM format - ssh-keygen -p -m PEM -f id_rsa
- Make temp home directory - mkdir temp_gpg
- Give right permissions - chmod go-rwx -R temp_gpg/
- Export private key gpg -a --export-secret-keys <KEY_ID> > my_gpg_key.asc
- With temp_gpg as home directory import above key - gpg --homedir temp_gpg --import my_gpg_key.asc
- Verify above import - gpg2 -K --homedir temp_gpg/
- Install Monkeysphere to convert from SSH PEM format to OpenPGP format.
```
wget https://download-ib01.fedoraproject.org/pub/epel/7/x86_64/Packages/m/monkeysphere-0.37-1.el7.noarch.rpm
rpm -Uvh monkeysphere-0.37-1.el7.noarch.rpm
# Make sure you have perl installed, prior to this step
yum install monkeysphere 
```
- Convert and import SSH RSA key pem2openpgp temporary_id < .ssh/id_rsa | gpg --import --homedir temp_gpg/
- Now when you do gpg -K --with-keygrip --homedir temp_gpg/ you should see secret key sec with uid as temporary_id, copy that id
- Add SSH key as the subkey
  - Edit your primary key - gpg --homedir temp_gpg --expert --edit-key <KEY ID>
  - Then type addkey and select Existing key (option 13)
  - For the prompt Enter the keygrip - paste the keygrip you copied in the above step.
  - Select s to Toggle the sign capability
  - Select e to Toggle the encrypt capability
  - Select a to Toggle the authenticate capability
  - In Current allowed actions, make sure only Authenticate is selected
  - Select q to Finished
  - Specify the key size and expiry
  - save and quit
- export the new key gpg --homedir temp_gpg -a --export-secret-keys <KEY ID> > my_new_gpg_key.asc
- import the new key gpg --import my_new_gpg_key.asc
- Verify import gpg -K --with-keygrip
- Make sure you have enable-ssh-support in ~/.gnupg/gpg-agent.conf
- Execute gpg -K --with-keygrip and then look for subkey that has [A] and copy Keygrip and paste in ~/.gnupg/sshcontrol
- Tell SSH how to access the gpg-agent, export SSH_AUTH_SOCK=$(gpgconf --list-dirs agent-ssh-socket)
- And launch gpg agent gpgconf --launch gpg-agent
- Run ssh-add -L to list public keys which can be shared and put in authorized_keys of the server which you want to login.
You can use Mailvelope browser plugin to encrypt and decrypt emails in Gmail/Yahoo mail.
If you are on Andriod, you can use OpenKeyChain to manage your keys.

Symmetric Key Encryption

One key, same key is used to encrypt and decrypt.
Example Algorithms - AES-128, AES-192 and AES-256

Using GPG

echo "hello world" | gpg -ca --cipher-algo AES256

Or to encrypt a file

gpg --symmetric --armor --cipher-algo AES256 file.txt

Usually you should be getting a pop-up asking for password/secret.

Note: By default encryption will happen in binary format. -a or --armor will output the encrypted text in ASCII format.

To decrypt

gpg -d cyphertext.txt

Using openssl

echo "hello world" | openssl enc -aes-128-cbc -salt -a -e

Or to encrypt a file

openssl enc -aes-128-cbc -salt -a -e -in /tmp/test1.txt -out /tmp/test1.txt.enc

To decrypt

openssl enc -aes-128-cbc -salt -a -d -in /tmp/test1.txt.enc -out /tmp/test1.txt

You can also generate a long password like below

openssl rand -base64 128 > /tmp/pw.txt

And use that to encrypt

openssl enc -aes-128-cbc -salt -a -e -pass file:/tmp/pw.txt -in /tmp/test1.txt -out /tmp/test1.txt.enc

And to decrypt

openssl enc -aes-128-cbc -salt -a -d -pass file:/tmp/pw.txt -in /tmp/test1.txt.enc

Note: cbc stands for Cipher Block Chaining

Asymmetric Key Encryption

Two keys, one key to encrypt and one key to decrypt. One key is kept secret, and another is shared to the public. The public key is used to encrypt, and the private key is used to decrypt.
Example Algorithms - EIGamal, RSA, DSA, Elliptic curve techniques, and PKCS.

Using GPG

echo "hello world" | gpg -ea --recipient recipient@mail.com

Or to encrypt a whole file

gpg --output doc.gpg -ea --recipient recipient@mail.com doc

To Decrypt

gpg --output doc --decrypt doc.gpg

Using OpenSSL

OpenSSL is used to generate keys for SSL communication. SSL uses Asymmetric Key Encryption for SSL Handshake and uses Symmetric Key Encryption for exchanging data between server and client.

To generate a private key of length 2048 bits

openssl genrsa -aes256 -out /tmp/priv.key 2048

To generate public key for above key

openssl rsa -in /tmp/priv.key -pubout -out /tmp/pub.key

To get the public key in ssh format

chmod 600 /tmp/priv.key
ssh-keygen -y -f /tmp/priv.key

Encrypt using public key

openssl rsautl -encrypt -pubin -inkey /tmp/pub.key -in /tmp/test1.txt -out /tmp/test1.txt.enc

Decrypt using private key

openssl rsautl -decrypt -inkey /tmp/priv.key -in /tmp/test1.txt.enc -out /tmp/test1.txt

Using above the encrypted file will be in binary. You can also below method, to get in ASCII format

#Encrypt
openssl rsautl -encrypt -pubin -inkey /tmp/pub.key -in /tmp/test1.txt | 
   openssl base64 > /tmp/test1.txt.enc.asc

#Decrypt
openssl base64 -d < /tmp/test1.txt.enc.asc | 
   openssl rsautl -decrypt -inkey /tmp/priv.key

Note: Asymmetric Key Encryption is mostly used to exchange short encrypted messages (for example symmetric key secrets), they are not intended to encrypt large files. For example Asymmetric key of length 2048 bits can encrypt only 245 bytes - refer this. Use symmetric key encryption for large files.

Hybrid Encryption

In general, Hybrid Encryption is used where both Asymmetric and symmetric key is used. That’s how usually SSL and SSH works.

For example, if you have someone ssh public key, you can follow the below steps. To send the encrypted message

Generate a 256 bit (32 byte) random key - openssl rand -out /tmp/secret.key 32

Using the above key encrypt the data using symmetric key algorithm AES

openssl enc -aes-128-cbc -salt -a -e -pass file:/tmp/secret.key -in /tmp/test1.txt -out /tmp/test1.txt.enc

Using asymmetric key algorithm RSA, encrypt above secret key using the public key of the recipient

openssl rsautl -encrypt -oaep -pubin -inkey <(ssh-keygen -e -f /tmp/ssh_pub.key -m PKCS8) -in /tmp/secret.key | openssl base64 > /tmp/secret.key.enc.asc

Delete the symmetric key - rm -rf /tmp/secret.key
Now both, encrypted symmetric secret /tmp/secret.key.enc.asc and encrypted text /tmp/test1.txt.enc can be shared to the recipient.

The recipient can use below steps to decrypt the message

Decrypt the secret.key.enc.asc to get secret.key using private key (using RSA Asymmetric algorithm)

openssl rsautl -decrypt -oaep -inkey /tmp/priv.key -in <(openssl base64 -d < /tmp/secret.key.enc.asc) -out /tmp/secret.key

Now, decrypt the data using the symmetric key (secret.key).

openssl enc -aes-128-cbc -salt -a -d -pass file:/tmp/secret.key -in /tmp/test1.txt.enc

Conversions

You can convert keys from one format to another by below steps.

OpenSSH to OpenSSL

To convert from OpenSSH private key to OpenSSL private key
- Make sure the OpenSSH private key is in PEM format. If the first line (head -1 id_rsa) of id_rsa is -----BEGIN OPENSSH PRIVATE KEY-----, then it is not in PEM format.
- You can convert to PEM format using ssh-keygen -p -m PKCS8 -f id_rsa
- After conversion, you should see the first line of id_rsa changed to -----BEGIN ENCRYPTED PRIVATE KEY-----
- To convert to OpenSSL key - openssl rsa -in ~/.ssh/id_rsa -out key_rsa.pem
To create a certification request - openssl req -new -key ~/.ssh/id_rsa -out myid.csr
To create OpenSSL self-signed certificate (public key) openssl x509 -req -days 3650 -in myid.csr -signkey ~/.ssh/id_rsa -out myid.crt
OpenSSL to OpenSSH
Private keys format is the same between OpenSSL and OpenSSH. So you just have to rename your OpenSSL key - cp myid.key id_rsa
In OpenSSL, there is no specific file for public keys (public keys are generally embedded in certificates). However, you extract the public key from the private key file - ssh-keygen -y -f myid.key > id_rsa.pub
GnuPG to OpenSSH
First, you need to know the fingerprint of your RSA key. You can use - gpg --list-secret-keys --keyid-format short
Next, you can use openpgp2ssh tool distributed in with monkeyshpere project - gpg --export-secret-keys 01234567 | openpgp2ssh 01234567 > id_rsa
You can now extract ssh public key using - ssh-keygen -y -f id_rsa > id_rsa.pub