Cryptographic salt

As explained in Hacking Cryptography

A salt is a publicly known value combined with a password before hashing to make precomputed password-hash tables less reusable.

A cryptographic salt is a public value combined with a password before hashing. Store the salt alongside the digest; secrecy is not its purpose. Its value is to make a password hash depend on more than the password alone. section 6.4.2 section 6.5

Without a salt, an attacker who obtains a database of usernames and password digests can precompute hashes for candidate passwords, then perform fast reverse lookups. Rainbow tables reduce the storage cost of this precomputation with hash chains and column-specific reduction functions. The hash function is not reversed: many possible inputs were hashed and remembered in advance. section 6.4.2 section 6.5

Figure 6.9 Salts are used to make hash function output more unpredictable; the salts are stored in plaintext along with the digest values.

The stored record includes both the digest and its plaintext salt. The salt changes the hash input, so a table prepared without that salt does not directly match the stored digest. section 6.4.2

Salting invalidates tables computed without the salt and forces attackers to build new rainbow tables for the salted inputs. It does not make a fast general-purpose hash function suitable for password storage by itself. Use a dedicated password-hashing function such as PBKDF2 or Argon2, which also require salting and are deliberately slow enough to make table construction and sharing harder. section 6.5 section 10.1.4

A leaked unsalted password database

For a site limited to six-character alphanumerical passwords, a rainbow-table implementation cracked more than 20% of password hashes. Adding a salt value before hashing forces the attacker to construct a new table rather than reuse one built for unsalted hashes. section 6.4.2 section 10.1.4

How Hacking Cryptography covers it

How deep it goes

Explains precomputation attacks through rainbow-table code and identifies salts as a countermeasure, while deferring password hashing to dedicated functions.

Examples

Uses a leaked password-digest database, six-character passwords, plaintext salt storage, and a rainbow table exceeding a 20% cracking success rate.

What it assumes

Assumes familiarity with password hashing while explaining dictionary attacks, rainbow tables, and salting in context.

What you can do after

Apply it via a dedicated password-hashing function by adding a salt before password hashing.

How it connects

Connects salts with hash functions, dictionary attacks, rainbow tables, hash chains, PBKDF2, and Argon2.

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Also explained in

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  Grokking Relational Database DesignA cryptographic salt is a random value combined with password input before hashing to make precomputed hash attacks less effective.
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  Spring Security in Action, Second EditionA salt is additional key-like data combined with an input before a hashing or cryptographic operation to make repeated inputs produce distinct results.
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  Grokking Web Application SecurityA salt is a random value added to input data before hashing so that identical inputs produce different hash values.
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  Just Use Postgres!A cryptographic salt is a random value combined with a password before hashing to make identical passwords produce different hashes and to limit compromise scope.

Compare how 5 books explain cryptographic salt →

Related concepts

• saltingused by · Salting is the practice of combining data, especially a password, with an additional value before hashing to produce distinct hashes and hinder precomputed attacks.
• gen_saltproduced by · gen_salt is a pgcrypto function that generates a random salt for password hashing using a specified algorithm.
• password hashinguses · Password hashing is the use of a one-way, deliberately costly function to derive values for securely storing and verifying passwords without retaining the original passwords.
• dictionary attackcontrasts with · A dictionary attack is a credential-guessing technique that tests candidate passwords or values drawn from a prepared list of common or likely choices.
• rainbow tableaddresses · A rainbow table is a precomputed structure of chained password-hash values that accelerates recovery of plaintext passwords from unsalted hashes by trading storage for lookup time.

This explainer was generated by Manning's knowledge graph from the text of Hacking Cryptography. The section links point to the parts of the book it draws on.