What a salt is and why it matters
A salt is a random value that gets mixed with a password or other secret before a cryptographic operation such as hashing or key derivation. Its main purpose is to make precomputed attacks infeasible: if every password is combined with a unique, unpredictable salt, an attacker cannot reuse a single set of precomputed hashes (rainbow tables) across many accounts. Salts also force attackers to compute cracking work per account, which raises the cost of offline attacks and buys time for defenders to detect and respond to breaches.
How salts affect password security
When a password is hashed with a salt, the resulting value differs even when two users choose the same password, because each operation uses a distinct salt. This defeats strategies that rely on identifying identical hashes to find reused passwords across accounts or systems. Salts do not make weak passwords strong, but they narrow the feasible attack methods: an attacker still needs to perform expensive hashing attempts individually for each user instead of looking up values in a single table.
Salts vs. secret keys (peppers)
A salt is usually non-secret and stored alongside the hash. A pepper is a secret value stored separately (for instance, in application configuration or an HSM) and combined with the password before hashing. Using a pepper can protect stored hashes from attackers who obtain only the database but not the application secrets. However, peppers add operational complexity: they become a single point of failure if mismanaged, and they require secure key handling and rotation policies.
Recommended salt properties
Pick salts that are long enough and unpredictable. Practically, use at least 16 random bytes (128 bits) generated by a cryptographically secure random number generator. Longer salts are fine and cheap; the goal is to avoid collisions and predictability. Ensure salts are unique per hashed item and never derived from predictable data such as timestamps, user IDs, or usernames alone. Uniqueness and randomness together are what stop precomputation.
How salts interact with hashing algorithms and KDFs
Salts are one piece of a broader password-hardening strategy. Use a slow, memory-hard key derivation function such as Argon2, scrypt, or bcrypt (with appropriate parameters) instead of a plain fast hash like SHA-256. These functions are purposely expensive to compute and can be tuned to balance defense and performance. Always feed a good random salt into the KDF; the salt multiplies the attacker’s cost, while the KDF increases the cost per guess.
Common mistakes to avoid
- Reusing the same salt across many accounts , this lets attackers amortize effort across targets.
- Using short or predictable salts such as user IDs or timestamps , these are vulnerable to targeted precomputation.
- Storing salts in non-immutable or unprotected places where they can be altered or removed without detection.
- Relying on salts alone , if passwords are weak, attackers will still succeed given enough time and compute.
Salt storage and format
Salts are commonly stored in plain text together with the hash in the authentication database, often concatenated or stored in separate columns. That is normal and safe because salt value does not need to be secret to be effective. What matters more is integrity: the application should treat the salt as authoritative for each stored hash and avoid accidental reuse or replacement. When migrating or upgrading hashing schemes, include the salt in any stored metadata so the system knows which algorithm and parameters were used.
Where salts do and do not help
Salts are specifically designed to protect stored password hashes and derived keys from precomputed and bulk attacks, but they do not stop online attacks like credential stuffing or password spraying. To reduce online risk, complement salted hashing with rate limiting, multi-factor authentication, and monitoring. Salts also cannot defend against an attacker who gains both the stored hashes and application secrets such as peppers, nor can they help if users choose extremely weak passwords that are easily guessed.
Practical implementation checklist
- Use a CSPRNG to generate unique salts, at least 16 bytes long.
- Choose a modern KDF: Argon2 (preferred), scrypt, or bcrypt; configure memory and time parameters appropriate for your environment.
- Store salt and algorithm parameters with the hash so verification knows how to recompute values.
- Consider a server-side pepper stored separately if you can secure and rotate it properly.
- Combine salted hashing with rate limiting, MFA, and breach detection for layered defense.
Recommended choices and parameter guidance
For new systems, Argon2id is a strong default because it offers memory hardness and tunable parallelism to resist GPU and ASIC attackers. If Argon2 is not available, scrypt is an acceptable alternative; bcrypt remains widely used and supported but has limitations on memory hardness and maximum input length. Tune parameters such that legitimate authentication remains fast enough for users while forcing attackers to spend meaningful time and resources on each guess. Test settings on representative hardware before deployment and revisit parameters as hardware improves.
Summary
Salts are a simple but powerful tool to stop large-scale precomputed attacks and force attackers to expend effort per target. They should be long, random, and unique for each stored secret, and used together with a slow, memory-aware key derivation function. Salts are normally stored with the hashes they protect, while optional secret peppers can add an additional layer when managed securely. In short, salts raise the attacker’s cost and buy defenders time, but they are one element of a broader security program that must include good password policies, MFA, and monitoring.
FAQs
Do salts need to be secret?
No. Salts are effective even when visible to attackers. Their job is to be unique and unpredictable for each item, not to be confidential. Keep salts accessible with the hash so verification can recreate the same inputs.
How long should a salt be?
Aim for at least 16 bytes (128 bits) generated by a cryptographically secure RNG. Longer salts further reduce the chance of collisions and are cheap to store and use.
Can I use the same salt for all users?
Avoid that. Reusing a salt across many accounts allows attackers to amortize cracking effort and defeats the purpose of salting. Use unique salts per account or per hashed value.
Is a pepper better than a salt?
A pepper complements a salt but does not replace it. A pepper is secret and must be managed securely; it can protect hashes if the database is leaked but the application secret remains safe. Peppers add complexity and operational requirements such as secure storage and rotation.
Which hashing function should I use with a salt?
Use a purpose-built key derivation function like Argon2id, scrypt, or bcrypt. Pair the chosen KDF with a proper salt and tune its parameters to make brute-force attempts expensive without harming user experience.
