Encryption is the process of scrambling data using a mathematical algorithm so that it becomes unreadable to anyone without the correct key to reverse the process. VPNs rely on encryption as their core protective mechanism, but understanding how they actually use it requires distinguishing between two fundamentally different types of encryption working together.
Symmetric versus asymmetric encryption
Symmetric encryption uses a single, shared key for both encrypting and decrypting data. It is fast and efficient, which makes it well suited to encrypting large volumes of ongoing traffic, but it has an obvious practical problem: both sides need the same secret key, and safely sharing that key over an untrusted network is not trivial. Asymmetric encryption solves that specific problem using a mathematically linked pair of keys, a public key that can be shared openly and a private key that must never be shared, where data encrypted with one key can only be decrypted with the other. Asymmetric encryption is computationally slower, so it is generally used only to securely establish a shared secret, rather than to encrypt an entire ongoing data stream.
The standard behind common encryption algorithms
The specific algorithm most VPNs rely on for the fast, ongoing symmetric encryption of traffic is AES, the Advanced Encryption Standard, formally specified by the U.S. National Institute of Standards and Technology in FIPS 197. AES was selected in 2000 after an open, multi-year international competition evaluating competing cipher designs, and it remains, decades later, the accepted global standard for symmetric encryption across government, financial, and consumer applications, including the overwhelming majority of VPN implementations.
How VPNs establish and exchange keys
When you connect to a VPN, the client and server first perform a key exchange using asymmetric encryption techniques, essentially agreeing on a shared secret without ever transmitting that secret directly across the network in a way an eavesdropper could capture. Once this handshake completes, both sides switch to fast symmetric encryption, using AES or a similar cipher, to protect the actual ongoing flow of traffic through the tunnel. This two-step process gets the security benefits of asymmetric key exchange and the speed benefits of symmetric encryption without forcing a tradeoff between the two.
128-bit versus 256-bit, in practical terms
The number attached to an AES cipher, 128-bit or 256-bit, refers to the length of the encryption key, and longer keys mean exponentially more possible combinations an attacker would need to try to break the encryption by brute force. AES-128 is already considered computationally infeasible to brute-force with current or foreseeable technology, and AES-256 adds a substantial further margin, primarily valued as a hedge against future advances in computing power rather than because AES-128 is currently considered weak in practice. For most VPN use cases, the difference between the two is not the deciding factor in how secure the connection actually is.
The limits of encryption at a compromised endpoint
Encryption protects data in transit between your device and the VPN server, but it cannot protect anything once it reaches an endpoint that is itself compromised. If your device is infected with malware, or if you are logged into an account that already identifies you, encryption of the network traffic does nothing to change that exposure. Encryption secures the pipe the data travels through; it says nothing about the safety of what is sitting at either end of that pipe.
Why the algorithm matters more than the marketing
Providers sometimes market encryption in vague terms, such as “military-grade encryption,” without specifying the actual algorithm and key length in use. A concrete, verifiable claim, such as confirming AES-256 is actually used for the data channel and checking which key exchange method secures the initial handshake, tells you far more than a generic marketing phrase ever could. Reputable providers are generally transparent about these technical specifics, publishing them directly in their documentation, and a provider that is vague about exactly what encryption it uses is itself a reasonable warning sign worth taking seriously. For more detail, see the IETF (RFC 4301), which covers how VPN security architecture combines key exchange with ongoing encrypted tunnel protection.
