About Encryption Key Length
In crude terms, the length of a key used when making a cypher determines how long it will take to crack using a brute force attack, with longer keys requiring exponentially more time than shorter ones (a brute force attack is also known an exhaustive key search and involves trying every possible combination until the correct one is found).
It is almost impossible to find VPN of less than 128-bit key length, and it is increasingly common to find 256-bit encryption on offer, sometimes going up to 2048-bit. SSTP uses 2048-bit encryption as standard for example. But what do these numbers mean in practice, and is 256-bit encryption really more secure than 128-bit encryption?
The short answer is that to all practical intents and purposes, no. While it is true that a 256-bit key would require 2128 times more computational power to break than a 128-bit key, that still means 3.4 x1038 operations would be required (the number of combinations in a 128-bit key) – a feat beyond conventional computing techniques for the foreseeable future. It would currently take the fastest supercomputer (figures from 2011, capable of 10.51 pentaflops peak speed) 1.02 x 1018 (around 1 billion) years to crack a 128-bit AES key by force.
As a 128-bit cypher cannot in any practical terms be cracked (through brute strength), it is fair to say that this more than strong enough for most purposes. Only those truly paranoid about security (such as governments when handling ultra-sensitive classified data that needs to remain secret for the next 100 years or so) may have a practical use for 256-bit encryption (the United States government for example uses NIST certified 256-bit AES encryption).
So why is it increasingly common to see VPN providers offering 256-bit encryption (let alone 2048-bit encryption)? Particularly when you consider that it takes computers considerably longer to encrypt information with 256-bit or greater keys? The simple answer is marketing. It sounds more impressive when trying to sell a product.
Large corporations and governments may feel the need for the added security margin afforded by longer key lengths, but for the average home VPN user 128-bits is more than sufficient.
Different cyphers do have vulnerabilities which may allow for faster key deduction, and sideways attacks using software such as key loggers can be used to get around encryption. However the point stands that when it comes to key length, sizes over 128-bits really are unlikely to matter for most users.