1. How will the encryption keys be generated?
A primary requirement when generating cryptographic keys is that they should be random or unpredictable. Unfortunately, the problem with generating random numbers is that you have to supply a sufficiently random number as seed in the first place - a classical chicken and the egg situation. Having said this, however several techniques (to be covered in a future post) exist to generate random numbers.
2. What encryption technique will your application use?
First things first. Hashing, message digesting and digital signatures DO NOT constitute encryption. Encryption means garbling of data with the help of a secret "encryption key" and ungarbling it using a secret (of course) "decryption key".
When encryption key == decryption key, this is called Symmetric Key encryption.
When encryption key != decryption key, this is called Asymmetric Key (or Public Key) encryption.
You need to decide first whether your application will use symmetric or asymmetric key encryption. Each of these have their +/- but you need to decide which one to use. [Note: When you use asymmetric key encryption, you often end up using symmetric key but that's another story]
3. What encryption algorithm will you use for encryption?
Algorithms are nothing but the sequence of steps to be carried out when encrypting data and decrypting (usually the reverse) previously encrypted data. Enough has been written about not using home-grown encryption algorithms, so lets assume that you are planning to use a standard and tested algorithm. Here's what I recommend -
Symmetric key encryption - Use AES (Rijndael - pronounced "Rhine doll")
Aysmmetric key encryption - Use ECC, else use the prolific RSA algorithm.
4. What are the recommended key sizes to be used?
This is a function of the application's security requirements. If you are protecting low-worth data you can can settle for smaller key sizes than when protecting high-worth data. But there are two important points to remember:
- Key sizes for symmetric and asymmetric algorithms vary greatly from each other.
- When using a both symmetric and asymmetric algorithms, ensure that the crack resistivity provided by the combination is equivalent or higher than that required by the application.