What Is "64 Bit Encryption"?by G.S. Jackson
Encryption strengths are often advertised by the length of the encryption key that's used. Encryption key length trends towards a "bigger is better" perspective: the longer the key, the better the encryption. So, by this logic, a 64-bit encryption string is always worse than one with a higher number. However, referring to something as "64-bit" encryption says nothing about the underlying encryption method. In order to understand this difference, you need to see how encryption algorithms and encryption keys work with one another.
There are two different ways to discuss encryption methods: the application and the algorithm. In application, the differences typically revolve around how encryption and decryption keys are shared between people encrypting and decrypting messages. In the algorithmic sense, the differences depend on the process of encryption employed to scramble the message or data encrypted. Both are somewhat dependent on each other, but when talking about complexity of encryption in terms of bits, you must take both into consideration.
Encryption keys are the devices that encryption algorithms use to encrypt and decrypt messages. Every encryption algorithm, as part of its encryption process, refers to a key to modify its encryption. This ensures that the decryption of the message requires the use of the corresponding decryption key to unlock the message's contents. In different applications and algorithms, different keys will have different uses: symmetric keys will encrypt and decrypt the same message, while asymmetric keys come in pairs, one for encryption and one for decryption. This is to say, not all keys are created equal.
Typically, key strength is measured in part through key length. Encryption keys are measured in bits, representing how many bits of computer memory a key takes up. So a 64-bit key takes 64 bits of memory, a 128-bit key takes 128 bits, and so on. When a particular algorithm for encryption is advertised, it will typically be advertised through the length of keys it allows for. The Advanced Encryption Standard, for example, uses either 128-, 192-, or 256-bit encryption keys.
Key Length and Strength
Key strength, as mentioned, is typically measured through key length. A longer key is exponentially harder to break; the time scale for breaking larger encryption like an AES 256-bit encryption is in the neighborhood of billions of years. However, this number only refers to the ability to brute force a symmetric key by guessing each possible combination of key digits. While 64-bit encryption would be much easier to break than 128-bit encryption -- by a factor of two at least -- this does not speak to the strength of the underlying encryption algorithm.