In cryptography, PBKDF1 and PBKDF2 (Password-Based Key Derivation Function 1 and 2) are key derivation functions with a sliding computational cost, used to reduce vulnerability to brute-force attacks. PBKDF2 is part of RSA Laboratories' Public-Key Cryptography Standards (PKCS) series, specifically PKCS#5 v2.0, also published as Internet Engineering Task Force's RFC2898. It supersedes PBKDF1, which could only produce derived keys up to 160 bits long. RFC8018 (PKCS#5 v2.1), published in 2017, recommends PBKDF2 for password hashing. Purpose and operation PBKDF2 applies a pseudorandom function, such as hash-based message authentication code (HMAC), to the input password or passphrase along with a salt value and repeats the process many times to produce a derived key, which can then be used as a cryptographic key in subsequent operations. The added computational work makes password cracking much more difficult, and is known as key stretching. When the standard was written in the year 2000 the recommended minimum number of iterations was 1,000, but the parameter is intended to be increased over time as CPU speeds increase. A Kerberos standard in 2005 recommended 4,096 iterations; Apple reportedly used 2,000 for iOS 3, and 10,000 for iOS 4; while LastPass in 2011 used 5,000 iterations for JavaScript clients and 100,000 iterations for server-side hashing. In 2023, OWASP recommended to use 600,000 iterations for PBKDF2-HMAC-SHA256 and 210,000 for PBKDF2-HMAC-SHA512. Having a salt added to the password reduces the ability to use precomputed hashes (rainbow tables) for attacks, and means that multiple passwords have to be tested individually, not all at once. The standard recommends a salt length of at least 64 bits. The US National Institute of Standards and Technology recommends a salt length of 128 bits. Key derivation process The PBKDF2 key derivation function has five input parameters: where: is a pseudorandom function of two parameters with output length (e.g., a keyed HMAC) is the master password from which a derived key is generated is a sequence of bits, known as a cryptographic salt is the number of iterations desired is the desired bit-length of the derived key is the generated derived key Each -bit block of derived key , is computed as follows (with marking string concatenation): The function is the xor () of c iterations of chained PRFs. The first iteration of PRF uses Password as the PRF key and Salt concatenated with encoded as a big-endian 32-bit integer as the input. (Note that i