Derive a 32-byte key from a password deterministically in PHP

Question

Today I learned that \"password\" tends to mean a memorizable string of an arbitrary number of characters, while \"key\" means a highly random string of bi

Answer 1

For hash-pbkdf2 you say:

"The following approach works but ignores the instruction of "[The salt] should be generated randomly"

Well, the fix to that is to do generate the salt randomly, and store it with the ciphertext. See this question for methods on how to generate secure random bytes within PHP. The output can then be used as key to encrypt; of course the key will always be regenerated using the stored salt and memorized password, and doesn't need to be stored. Note that keys consist of raw bytes; it's probably best to retrieve a raw key from hash-pbkdf2 (the last parameter).

Note that the iteration count should be as high as possible. Normally 100,000 or so is considered optimal nowadays, but the higher the more secure. It takes about as much time for an attacker to calculate the resulting key for each password, and as passwords only contain 30 to 60 bits (for good passwords) it really helps against dictionary attacks.