What Is Encryption? Keys and End-to-End Explained

TL;DR

• Encryption scrambles readable data (plaintext) into unreadable data (ciphertext) using an algorithm and a key, so only someone with the right key can read it [1].
• Symmetric encryption uses one shared key. Asymmetric encryption uses a linked pair: a public key and a private key [3][4].
• End-to-end encryption means only the sender's and recipient's devices can decrypt a message, not the service in the middle [5].
• Encryption protects content, but not everything: details such as who talks to whom can still be visible.

What encryption is #

NIST describes encryption as turning plaintext into ciphertext with an algorithm and a key, so that nobody except the intended recipient can read it [1]. Decryption is the reverse step.

Picture a locked box. The message sits inside, the box can travel through many hands, and only a person with the key can open it. The strength comes from the key, not from keeping the method secret. Well-known algorithms are public, and security rests on keeping keys private.

Encryption is used in two main situations [5]:

• Data in transit, moving across a network, such as a web page over HTTPS.
• Data at rest, stored on a device or server, such as a laptop with full-disk encryption.

Symmetric vs asymmetric keys #

There are two families of encryption, and they solve different problems.

Symmetric encryption is simple: one key locks and unlocks. Its weak point is sharing that key. If you send it over an open channel, someone might copy it.

Asymmetric encryption solves this. Anyone can use your public key to lock a message for you, but only your private key can open it. NIST notes that working out the private key from the public key is computationally infeasible [4]. The same key pair idea is also used for digital signatures, which let others check that something really came from you [4].

In real systems the two are combined. Asymmetric methods safely set up a shared secret, and then fast symmetric encryption protects the bulk of the data. The HTTPS handshake works this way.

End-to-end encryption #

With ordinary encryption in transit, your message may be protected on the way to a company's server, then decrypted there, and re-encrypted for the next hop. The company could read it in the middle.

End-to-end encryption (E2EE) keeps the content encrypted all the way from the sender's device to the recipient's device. Only those two endpoints can decrypt it, so the app's servers and internet providers cannot read the content [5]. NIST's glossary describes it as encryption applied while data passes through a network [2].

It is the idea behind many private messaging apps. The key pairs live on the devices, so even the company running the service cannot read the messages.

What encryption does not protect #

The EFF's guide is clear on the limits [5]:

• Metadata. Encryption of content does not hide that you are communicating, or details such as when and how long.
• Compromised devices. If malware controls your phone or laptop, it can read messages before they are encrypted or after they are decrypted.
• The other person. A recipient can screenshot or share what you send.
• Backups. Encrypted chats may be backed up to the cloud without encryption, so check your backup settings.

Encryption also does not replace strong passwords or two-factor authentication.

FAQ #

What is the difference between encryption and hashing? #

Encryption is designed to be reversed by someone with the key. Hashing is a one-way process that turns data into a fixed fingerprint and is not meant to be undone. Sites often hash passwords for that reason.

Can encryption be broken? #

Modern, well-chosen algorithms are not practically breakable by guessing keys. Attacks usually target weak passwords, stolen keys, or flaws in how software is used instead.

Is encrypted data always safe? #

Safe from being read without the key, yes. But if someone gets the key, or if your device is compromised, protection is lost [5].

Do I use encryption already? #

Almost certainly. HTTPS websites, phone lock screens, and many messaging apps rely on it every day.

Sources #