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A common approach (brute-force attack) is to repeatedly try guesses for the password and to check them against an available cryptographic hash of the password. [2] Another type of approach is password spraying, which is often automated and occurs slowly over time in order to remain undetected, using a list of common passwords. [3]
In a reverse brute-force attack, a single (usually common) password is tested against multiple usernames or encrypted files. [23] The process may be repeated for a select few passwords. In such a strategy, the attacker is not targeting a specific user.
The first public release of Crack was version 2.7a, which was posted to the Usenet newsgroups alt.sources and alt.security on 15 July 1991. Crack v3.2a+fcrypt, posted to comp.sources.misc on 23 August 1991, introduced an optimised version of the Unix crypt() function but was still only really a faster version of what was already available in other packages.
Though brute-force attacks (e.g. dictionary attacks) may be used to try to invert a hash function, they can become infeasible when the set of possible passwords is large enough. An alternative to brute-force is to use precomputed hash chain tables. Rainbow tables are a special kind of such table that overcome certain technical difficulties.
Despite their original use for key derivation, KDFs are possibly better known for their use in password hashing (password verification by hash comparison), as used by the passwd file or shadow password file. Password hash functions should be relatively expensive to calculate in case of brute-force attacks, and the key stretching of KDFs happen ...
One of the modes John can use is the dictionary attack. [6] It takes text string samples (usually from a file, called a wordlist, containing words found in a dictionary or real passwords cracked before), encrypting it in the same format as the password being examined (including both the encryption algorithm and key), and comparing the output to the encrypted string.
If the attacker finds a match, they know that their guess is the actual password for the associated user. Password cracking tools can operate by brute force (i.e. trying every possible combination of characters) or by hashing every word from a list; large lists of possible passwords in many languages are widely available on the Internet. [14]
For example, bcrypt cannot be used to derive a 512-bit key from a password. At the same time, algorithms like pbkdf2, scrypt, and argon2 are password-based key derivation functions - where the output is then used for the purpose of password hashing rather than just key derivation. Password hashing generally needs to complete < 1000 ms.