There are three modern algorithms that utilize hash stretching: Password-Based Key Derivation Function 2 (PBKDF2), bcrypt, and scrypt. The difference between PBKDF2 and bcrypt is that bcrypt costs more to run on GPU than PBKDF2, and is therefore harder for an attacker to parallelize the operations using many GPUs.

Both PBKDF2 and bcrypt use a small and constant amount of memory, which makes them vulnerable to brute-force attacks using application-specific integrated circuit chips (ASICs) and/or field-programmable gate arrays (FPGA). scrypt was invented to tackle this issue, and allows you to adjust the amount of RAM required to compute the hash. However, scrypt was only published in 2009, and has not been battle-tested as much as the other two algorithms.

Therefore, in this book, we will use the bcrypt algorithm, since it's been around since 1999 and no vulnerabilities have yet been found.