A rainbow table is a precomputed table of hash values that are used to quickly reverse cryptographic hash functions. It is a form of a lookup table that allows for the rapid recovery of plaintext passwords from their hashed counterparts.
In cryptography, a hash function takes an input (such as a password) and produces a fixed-size string of characters, which is the hash value. Hash functions are designed to be one-way, meaning that it should be computationally difficult to derive the original input from the hash value. This property makes hash functions useful for password storage, as the actual passwords are not stored, only their hash values.
However, rainbow tables exploit the fact that hash functions are deterministic. A rainbow table contains precomputed hash values for a large number of possible inputs, typically covering a wide range of possible passwords. These tables are constructed in such a way that they can efficiently reverse the hash function and find the original password corresponding to a given hash value.
To use a rainbow table, one simply looks up the given hash value in the table to find the corresponding plaintext password. This process is much faster than traditional brute-force methods of trying every possible password until a match is found.
To protect yourself against rainbow table attacks, here are some recommended measures:
- Use Strong Passwords: Choose passwords that are long, complex, and unique for each account. Avoid using common words or easily guessable patterns. Strong passwords make it harder for attackers to find matches in a rainbow table.
- Implement Salted Hashing: Hash functions should be combined with a technique called salting. A salt is a random value unique to each user or password. It is concatenated with the password before hashing. The salt value is then stored alongside the hashed password. Salting makes the use of precomputed rainbow tables ineffective because each password requires a separate rainbow table. It significantly increases the resources needed to crack passwords.
- Use Cryptographically Secure Hash Functions: Ensure that the hash function you use is strong and resistant to various cryptographic attacks. Commonly recommended hash functions for password storage include bcrypt, scrypt, and Argon2. These functions are designed to be slow and computationally expensive, making rainbow table attacks more difficult.
- Use Key Stretching and Iterations: Key stretching is a technique that involves repeatedly hashing the password or using multiple iterations of a hash function. This increases the time required to compute the hash, slowing down an attacker’s attempts to crack passwords using rainbow tables. Functions like bcrypt and scrypt incorporate key stretching and allow you to adjust the number of iterations.
- Regularly Update Passwords: It’s important to change your passwords periodically, especially for critical accounts. If a password hash is compromised, changing the password renders the corresponding rainbow table entry useless.
- Implement Multi-Factor Authentication (MFA): MFA adds an extra layer of security by requiring additional verification factors, such as a code from a mobile app or a physical token, in addition to the password. Even if an attacker manages to obtain a password, they would still need the additional factor to gain access.
By following these best practices, you can significantly reduce the risk of rainbow table attacks and enhance the security of your password storage system.