Symmetric Encryption: The Easiest Way to Encrypt an Email

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Symmetric encryption is a core concept in cryptography.

In a nutshell, symmetric encryption uses a single key for both encrypting and decrypting data.

While that may sound complex, it’s actually really simple! You don’t need any knowledge of encryption algorithms or advanced computer science. This method is valued for its straightforwardness and speed, making it widely used for securing information.

In this guide, we will cover:

  • what symmetric encryption actually is;
  • the pros and cons of using symmetric encryption;
  • how to send a password-encrypted email with Mailfence;

… and a lot more! So let’s get started.

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Mailfence - Get your free, secure email today.

4.1 based on 177 user reviews

What is Symmetric Encryption?

First things first, let’s define what symmetric encryption actually is.

Symmetric encryption is an encryption method that uses the same key for both encrypting and decrypting data between parties.

Sending a PDF that is password-protected? That’s symmetric encryption. Sending an email where the receiver needs a password to read it? That’s also symmetric encryption.

Practically speaking, this means both parties use the same “key” (in most cases a password) to decrypt the email or file. This is also why this encryption method is called “symmetric”. Check out this guide to learn what sets it apart from asymmetric encryption.

Believe it or not, symmetric encryption is actually one of the oldest forms of encryption.

The Scytale cipher, one of the oldest forms of symmetric encryption
The Scytale cipher, one of the oldest forms of symmetric encryption

For example, the Spartans used the Scytale cipher around 500 BCE. This cipher involved wrapping a strip of parchment around a rod to encode messages.

In the 20th century, symmetric encryption advanced. The Enigma machine is a notable example. Used by the Germans in World War II, it created complex ciphers with rotors and plugboards.

These methods might seem completely different from protecting a file with a password. However, the core mechanism is the same. Both sender and receiver agree on an encryption method and use the same “key” to encrypt and decrypt the information.

Today, symmetric encryption is a cornerstone of cryptographic practices. It supports many security protocols and systems.

How Does Symmetric Encryption Actually Work?

The keys, in practice, represent a shared secret between two or more parties.

As long as both the sender and the recipient know the shared secret, they can encrypt and decrypt all messages that use this shared secret.

In practice, this is how a symmetric encryption might look like:

  1. The message is encrypted using a symmetric key derived from the user password (via a S2K function) on the front-end. The back-end stores the encrypted message.
  2. The recipient receives the access URL of the encrypted message.
  3. The recipient clicks on the access URL and provides the right password (the password chosen by the sender). It will be used to derive the symmetric key, which in turn decrypts the message. This step also includes several performance optimization measures.

This means symmetric encryption is mostly abstracted from the user experience. The only requirement is to use a program that supports it.

The other requirement is that both the sender and receiver know the secret key. This means you need to share your password by some other means (using an end-to-end encrypted messaging service like Signal for example).

Using Signal to share a secret key
Using Signal to share a secret key

Pros and Cons of Using Symmetric Encryption

Advantages of symmetric encryption

Ease of use

The main advantage of symmetric key encrypted emails is the ease of use. You can send encrypted emails to any person without having to make sure that this person has a (asymmetrical) public key or a special program/add-on/plugin to decrypt the message.

You can send secure password-encrypted messages to anyone, without any need for technical know-how about encryption. The only thing you will need to do is communicate a shared secret/password to the recipient of your message.

Symmetric encryption is as simple as typing in a password
Symmetric encryption is as simple as typing in a password

Sender control

The messages remain hosted on servers of the sender service. It does not transit via the open internet. They are stored encrypted on our servers and are kept there until they are retrieved by the recipient or deleted by the sender.

Accessibility

After receiving the secure link, the recipient can access the message from any device (with an updated browser). The sender can set an expiration date for the secure link, which means the recipient can only use that link to decrypt and read the secure message before it has expired.

Disadvantages of symmetric encryption

Losing the shared secret/passphrase

The main issue with shared secret/passphrase is that you need to exchange it with the receiver of the encrypted message. You can do this over the internet or via an alternative channel such as SMS, phone, …

The danger is that if the shared passphrase falls into the wrong hands, anyone who knows it will be able to decrypt the message.

One answer to this problem is asymmetric encryption (OpenPGP-based), in which there is a key pair. With this encryption method, a public key is made freely available to anyone who wants to send you a message. A second private key is kept secret by its owner.

Platform-specific

Password-encrypted emails require trust in the service which manages and provides this feature. The recipient must view the message on the sender’s portal.

In case of an unreliable or breached sender service, there could be a significant security issue.

Also, there are no digital signatures (except the traditional DKIM header for anti-spoofing reasons) on these encrypted messages. For that, you should use an asymmetric signature (OpenPGP-based).

Restricted capabilities

Using symmetric might also include some restrictions in functionalities, such as:

  • Attachments can only be sent with the original (or first) email in a conversation. Recipient(s) cannot add attachments in response email.
  • You cannot forward password-encrypted emails.
  • Neither party (sender or recipient) can change the sender address and the recipient address for a given email conversation thread.

Does Mailfence Support Symmetric Encryption?

Of course! Following our design philosophy of providing absolute control and freedom, usage of this feature is not restricted. Therefore:

  • All Mailfence users can use password-encrypted messages with other Mailfence users.
  • All Mailfence users can use password-encrypted messages with non-Mailfence users.

To send a password-encrypted email, follow these simple steps:

  • simply select “Encryption” next to the “Send” button;
  • type in your password and confirm it;
  • include a hint that the receiver will be able to read (optional)
  • set an expiration date for your email (optional)

Best of all, you can also use symmetric encryption on our mobile app!

Using symmetric encryption in the Mailfence mobile app
Using symmetric encryption in the Mailfence mobile app

If you haven’t yet, make sure to download our free mobile app for iOS and Android!

All of the encryption/decryption process happens on the front-end (in your browser) using an open-source and security-audited library. The derived symmetric key is never shared with the back-end, thereby maintaining a zero-knowledge framework.

Check out this guide for more info on how to send a password-encrypted message.

Note: please do not use your account password, or your OpenPGP keypair passphrase, as the password for symmetrically encrypted messages.

Symmetric Encryption Best Practices

Feeling ready to send your first password-encrypted messages? Then here are some best practices you should always follow.

Key management

Symmetric encryption is only as safe as the password you select. That is why you should always generate strong keys (i.e. strong passwords) and avoid predictable patterns or weak keys.

It is also vital to store your keys securely. Keep keys in a protected environment, such as hardware security modules or secure key management services. Never store keys in plain text files.

Finally, regularly change encryption keys. This reduces the risk of key compromise. Implement a rotation policy that minimizes disruption.

Sharing key

Make sure to always passwords through secure channels. Use encrypted messaging services or secure key exchange protocols. Avoid sharing keys over insecure methods. Also, as much as possible, limit the access. Only authorized individuals should have access to encryption keys. Use role-based access control to enforce this in your organization.

User training

Educate users on the importance of encryption. Provide training on secure key management. Teach them how to handle encrypted data responsibly. For more information, check out our full email security and privacy course.

Symmetric Encryption: Final Words

That’s it for this guide on symmetric encryption. Hopefully, you found it useful, and now feel confident to start sending your first password-encrypted emails!

Got any questions? Anything unclear? Feel free to drop us a line at support@mailfence.com

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