Key stretching in cryptography is a technique used to increase the security of cryptographic keys, especially in password-based encryption systems. The purpose of key stretching is to slow down the key derivation process, making it computationally expensive and time-consuming for an attacker to try different possible keys.
In password-based encryption, users often choose weak passwords that can be easily guessed by attackers. To counter this vulnerability, key stretching algorithms apply additional computational iterations or operations to the original password to generate a longer and more secure cryptographic key.
One of the common key stretching algorithms is bcrypt. It repeatedly applies a computationally intensive hash function to the password along with a randomly generated salt. The number of iterations is configurable, and by increasing the number of iterations, the key stretching process becomes more time-consuming, thereby increasing the difficulty for attackers to brute-force the password.
Key stretching helps protect against dictionary attacks, where attackers try a large list of possible passwords, as well as against brute-force attacks, where attackers systematically try all possible combinations of characters to guess the password.