There are various procedures for turning a passphrase into a arbitrary length key for use with a symmetric cipher. A general interface for such algorithms is presented in pbkdf.h. The main function is derive_key, which takes a passphrase, a salt, an iteration count, and the desired length of the output key, and returns a key of that length, deterministically produced from the passphrase and salt. If an algorithm can’t produce a key of that size, it will throw an exception (most notably, PKCS #5’s PBKDF1 can only produce strings between 1 and $n$ bytes, where $n$ is the output size of the underlying hash function).
The purpose of the iteration count is to make the algorithm take longer to compute the final key (reducing the speed of brute-force attacks of various kinds). Most standards recommend an iteration count of at least 10000. Currently defined PBKDF algorithms are “PBKDF1(digest)”, “PBKDF2(digest)”, and “OpenPGP-S2K(digest)”; you can retrieve any of these using the get_pbkdf, found in lookup.h. As of this writing, “PBKDF2(SHA-256)” with 10000 iterations and a 16 byte salt is recommend for new applications.
- OctetString PBKDF::derive_key(size_t output_len, const std::string& passphrase, const byte* salt, size_t salt_len, size_t iterations) const¶
Computes a key from passphrase and the salt (of length salt_len bytes) using an algorithm-specific interpretation of iterations, producing a key of length output_len.
Use an iteration count of at least 10000. The salt should be randomly chosen by a good random number generator (see Random Number Generators for how), or at the very least unique to this usage of the passphrase.
If you call this function again with the same parameters, you will get the same key.
PBKDF* pbkdf = get_pbkdf("PBKDF2(SHA-256)"); AutoSeeded_RNG rng; secure_vector<byte> salt = rng.random_vec(16); OctetString aes256_key = pbkdf->derive_key(32, "password", &salt, salt.size(), 10000);
There are some oddities about OpenPGP’s S2K algorithms that are documented here. For one thing, it uses the iteration count in a strange manner; instead of specifying how many times to iterate the hash, it tells how many bytes should be hashed in total (including the salt). So the exact iteration count will depend on the size of the salt (which is fixed at 8 bytes by the OpenPGP standard, though the implementation will allow any salt size) and the size of the passphrase.
To get what OpenPGP calls “Simple S2K”, set iterations to 0, and do not specify a salt. To get “Salted S2K”, again leave the iteration count at 0, but give an 8-byte salt. “Salted and Iterated S2K” requires an 8-byte salt and some iteration count (this should be significantly larger than the size of the longest passphrase that might reasonably be used; somewhere from 1024 to 65536 would probably be about right). Using both a reasonably sized salt and a large iteration count is highly recommended to prevent password guessing attempts.