Symmetric Encryption & Decryption

What Is Symmetric Encryption?

Symmetric encryption locks data with a password. The same password unlocks it again. You do not need a key pair.

Normal OpenPGP encryption is different. It uses a public key to encrypt and a private key to decrypt. That is called asymmetric encryption. See Text Operations.

Symmetric encryption is:

• Fast.
• Good for large files, or for sharing something once.
• Only safe when both people can share the password safely.

Guard the password

Anyone who learns the password can read the data. So always use a strong password that you use nowhere else, and share it in a safe way.

How It Works

1. You choose a strong password.
2. GpgFrontend uses that password to encrypt the data.
3. Anyone with the same password can decrypt it.

The password is everything

The safety of the data depends fully on the password: how strong it is, and how well you keep it secret.

Encrypting Data Symmetrically

GpgFrontend can do this since v2.1.9. The action is called Sym. Encrypt.

Find Sym. Encrypt

The easiest way: open the Operations menu and choose Sym. Encrypt, or press Ctrl+Alt+E (Cmd+Option+E on macOS). You can only use it while no key is ticked in the Key ToolBox.

You can also put it on the toolbar:

1. Open Settings and choose Appearance.
2. Under Toolbar, in the Actions list, tick Sym. Encrypt.

Sym. Encrypt does not get a button of its own. It joins the Encrypt button. To use it, right-click the Encrypt button (or press and hold it), then choose Sym. Encrypt from the list. If no key is ticked, a plain click on Encrypt also opens this list. For more about such buttons, see Buttons With More Than One Action.

Changed in v2.2.3

Before v2.2.3, Sym. Encrypt had its own toolbar button. Now it is in the Encrypt button’s list.

Enter the Text or Select a File

• For text: type or paste the text into a Text Editor tab.
• For a file: select the file in the File Panel.

Symmetric Encryption

1. Make sure no key is ticked in the Key ToolBox.
2. Choose Sym. Encrypt from the Operations menu, or from the Encrypt button’s list.
3. When asked, type a strong password that you use nowhere else.
4. GpgFrontend encrypts the text or file with that password.
5. Save the result, or send it to the other person.

Same password

The other person needs exactly the same password to decrypt the message.

Decrypting Symmetric Encrypted Data

To decrypt text that was encrypted with a password:

1. Paste the encrypted message into a Text Editor tab. It starts with -----BEGIN PGP MESSAGE----- and ends with -----END PGP MESSAGE-----.
2. Click Decrypt.
3. When asked, type the same password that was used to encrypt.
4. If the password is right, the readable text appears in the tab.

To decrypt a file, select it in the File Panel and click Decrypt. See Decrypt a File.

Wrong password?

If the password is wrong or mistyped, decryption fails.

Technical Details: GnuPG vs rPGP

This part is for readers who want the technical details. You do not need it to use Sym. Encrypt.

How a password becomes a key

Symmetric encryption makes an OpenPGP message that is protected by a passphrase (another word for password) instead of a public key. The passphrase goes through an OpenPGP String-to-Key (S2K) function, which turns it into the real encryption key. The data is then encrypted with a symmetric cipher.

How this is done depends on the engine in use. It also decides which tools can decrypt the result.

Aspect	GnuPG engine	rPGP engine
Implementation	gpgme_op_encrypt_symmetric (GnuPG decides the details)	rPGP, fixed cipher, adjustable Argon2 memory
Symmetric cipher	GnuPG default, typically AES-256 (configurable)	AES-256
Data packet	GnuPG default (SEIPD; may use AEAD on newer GnuPG)	SEIPD v2 (AES-256 with OCB)
Key derivation	Iterated-and-salted S2K by default (configurable)	Argon2id S2K (memory-hard), 2 GiB or 64 MiB
Interoperability	Broad; readable by most OpenPGP tools	Requires an RFC 9580 tool; GnuPG cannot read it

GnuPG Engine

With the GnuPG engine, GpgFrontend asks GnuPG to do the symmetric encryption. So the cipher, the S2K function, and the packet format follow your GnuPG version and settings (for example, s2k-cipher-algo, s2k-digest-algo, and s2k-count in gpg.conf). Current GnuPG defaults to AES-256 with an iterated-and-salted S2K. Most other OpenPGP tools can read that. Newer GnuPG versions may use AEAD (OCB) when set up to do so.

rPGP Engine

With the rPGP engine, the message is an SEIPD v2 packet encrypted with AES-256 in OCB mode. SEIPD v2 is the newer message format from the current OpenPGP standard, and it checks the data for changes as it reads it. The passphrase is turned into the symmetric key with an Argon2id S2K. Argon2 needs a lot of memory on purpose. This makes guessing a weak passphrase much more costly than with the classic iterated-and-salted S2K.

Changed in v2.2.3

rPGP now writes passphrase-encrypted messages as SEIPD v2. Up to v2.2.2 it used SEIPD v1. Both kinds still decrypt.

Choose How Much Memory Argon2 Uses

Argon2 needs memory to turn your password into a key. The same amount is needed again to decrypt, on every computer that opens the message. You can choose the amount:

1. Open Settings and choose rPGP (under Keys & Engines).
2. Find the Passphrase Encryption box.
3. In Argon2 Parameters:, pick one option:
   • High memory, 2 GiB: the default and the strongest choice.
   • Low memory, 64 MiB: pick this if your computer, or the computer of the person who will open the message, cannot spare 2 GiB of memory.

The choice applies only to messages you encrypt from now on.

New in v2.2.3

The Argon2 Parameters: setting is new. Before, rPGP always used 2 GiB.

rPGP symmetric output needs a modern tool

Argon2 S2K and SEIPD v2 are part of the current OpenPGP standard (RFC 9580). A message that rPGP encrypts with a passphrase can only be decrypted by a tool that follows RFC 9580, such as GpgFrontend with rPGP. GnuPG does not read SEIPD v2, so it cannot decrypt the message. If the message must open in as many tools as possible, encrypt it with the GnuPG engine instead.

When you encrypt symmetrically with rPGP, GpgFrontend always asks for the passphrase again. It does not use a remembered one. It also asks you to type the passphrase a second time, so you do not encrypt with a mistyped one.

When to Use Symmetric Encryption?

Symmetric encryption is a good choice when:

• You encrypt something for yourself, such as private notes or backup files.
• You can give the password to the other person safely, for example in person.
• You do not want to make and manage key pairs.

It is not a good choice when you cannot share the password safely. Then use normal encryption with keys instead.

Tips for Better Security

• Always use a strong password. Longer is better.
• Never send a password over an unsafe channel, such as an email that is not encrypted.
• You can combine both kinds of encryption. For example, encrypt a file with a password, then encrypt the password with the other person’s public key.