Password hashing

1. Password Hashing: A Beginner's Guide

Password hashing is a fundamental security practice used to protect user credentials in virtually every modern application and service. Instead of storing passwords in plain text – a disastrous security practice – systems store *hashes* of passwords. This article will provide a comprehensive introduction to password hashing for beginners, covering the core concepts, historical evolution, modern algorithms, best practices, and common pitfalls. Understanding these principles is crucial for anyone involved in web development, system administration, or cybersecurity.

1. 1. Why Not Store Passwords in Plain Text?

Storing passwords in plain text is a catastrophic security risk. If a database containing plain text passwords is compromised (through a data breach, SQL injection, or other attack), attackers have immediate access to all user accounts. They can log in as any user, steal data, and cause significant damage. The consequences can be devastating, leading to financial loss, reputational damage, and legal liabilities.

1. 1. What is Hashing?

Hashing is a one-way function. This means that it takes an input (in this case, a password) and produces a fixed-size string of characters (the hash). The crucial property of a good hashing function is that it’s computationally infeasible to reverse the process – to determine the original password from its hash.

Think of it like a blender. You can easily put fruit into a blender and get a smoothie. But trying to recover the original fruit from the smoothie is virtually impossible.

Key characteristics of a good hashing function include:

• **Deterministic:** The same input password *always* produces the same hash.
• **One-way:** It's computationally infeasible to derive the original password from the hash.
• **Collision Resistance:** It's difficult to find two different passwords that produce the same hash (a "collision"). While collisions are theoretically possible, a strong hashing algorithm minimizes the probability.
• **Avalanche Effect:** A small change in the input password should result in a significant and unpredictable change in the hash.

1. 1. Early Hashing Algorithms & Their Weaknesses

Early attempts at password hashing were simplistic and quickly proven vulnerable.

• **MD5:** Message Digest 5, a widely used hashing algorithm, was found to be susceptible to collision attacks. Attackers could create different passwords that produced the same MD5 hash, allowing them to bypass authentication. Cryptographic Hash Functions details the vulnerabilities of MD5 and similar algorithms.
• **SHA-1:** Secure Hash Algorithm 1, intended as a successor to MD5, also suffered from similar vulnerabilities, although it took longer to break. Its use is now strongly discouraged.
• **Plain Hashing (without salt):** Even using stronger hashing algorithms like SHA-256 *without* a salt (explained below) is insufficient. Attackers can use precomputed tables of hashes known as "rainbow tables" to quickly identify common passwords. Rainbow Table Attack explains this vulnerability in detail.

These early weaknesses highlighted the need for more sophisticated techniques.

1. 1. The Importance of Salting

Salting is a crucial technique that significantly enhances password security. A *salt* is a random string of characters that is added to each password *before* it is hashed. Each user has a unique salt.

Here's how it works:

1. **Generate a unique salt for each user.** This salt should be randomly generated and stored along with the hash. 2. **Concatenate the salt and the password.** For example, `salt + password`. 3. **Hash the combined string.** This produces the final hash value, which is stored in the database.

Why does salting help?

• **Defeats Rainbow Tables:** Rainbow tables are useless because the salt modifies the password before hashing, making each hash unique even for common passwords.
• **Prevents Precomputation Attacks:** Attackers cannot precompute hashes for common passwords with different salts.
• **Increases Computational Cost:** Salting adds an extra step, increasing the computational cost for attackers attempting to crack passwords.

1. 1. Modern Password Hashing Algorithms

Modern password hashing algorithms are designed to be computationally expensive and resistant to common attacks. These algorithms often incorporate key derivation functions.

• **bcrypt:** A popular and well-respected algorithm that uses a work factor (cost parameter) to control the computational effort required to hash a password. Increasing the work factor makes it harder for attackers to crack passwords, but also increases the time it takes to authenticate users. bcrypt Algorithm provides a detailed technical overview.
• **scrypt:** Similar to bcrypt, scrypt is designed to be memory-hard, meaning it requires a significant amount of memory to compute the hash. This makes it more resistant to attacks using specialized hardware like GPUs and ASICs. scrypt Algorithm offers in-depth technical information.
• **Argon2:** The winner of the Password Hashing Competition (PHC) in 2015, Argon2 is considered the state-of-the-art password hashing algorithm. It offers three variants: Argon2d (resistant to GPU cracking), Argon2i (resistant to side-channel attacks), and Argon2id (a hybrid approach). Argon2 Specification is the definitive technical document. It allows for configurable memory usage, iterations, and parallelism, offering flexibility and strong security.
• **PBKDF2 (Password-Based Key Derivation Function 2):** While older than Argon2, bcrypt and scrypt, PBKDF2 is still widely used. It iteratively applies a pseudorandom function (like HMAC-SHA256) to the password and salt. It requires a high iteration count to be secure. PBKDF2 RFC is the formal specification.

1. 1. Key Derivation Functions (KDFs)

Key derivation functions (KDFs) are algorithms that take a password and a salt as input and produce a cryptographic key. Password hashing algorithms like bcrypt, scrypt, and Argon2 are actually *types* of KDFs specifically designed for password storage. KDFs are also used in other cryptographic applications, such as generating encryption keys from passwords.

1. 1. Best Practices for Password Hashing

• **Always use a strong, modern algorithm:** Argon2, bcrypt, or scrypt are the recommended choices. Avoid MD5, SHA-1, and plain hashing.
• **Use a unique, randomly generated salt for each password:** Salts should be at least 16 bytes long.
• **Choose an appropriate work factor/iteration count:** Increase the work factor until it takes a reasonable amount of time (e.g., 0.5 - 1 second) to hash a password on your server. This makes brute-force attacks much more difficult.
• **Store the salt alongside the hash:** The salt is essential for verifying the password.
• **Regularly rehash passwords:** As computing power increases, algorithms that were once considered secure may become vulnerable. Periodically rehash passwords using a stronger algorithm or higher work factor. Password Re-hashing Strategies details specific approaches.
• **Implement rate limiting:** Limit the number of login attempts from a single IP address to prevent brute-force attacks. Rate Limiting Techniques explains various methods.
• **Consider Adaptive Hashing:** Algorithms like Argon2 allow for dynamic adjustment of the work factor based on attacker capabilities, providing an adaptive defense. Adaptive Password Hashing delves into this concept.
• **Implement Multi-Factor Authentication (MFA):** MFA adds an extra layer of security beyond just a password. Multi-Factor Authentication Overview provides a comprehensive explanation.
• **Proper Error Handling:** Avoid revealing whether a username exists or not during login attempts. Implement generic error messages. Secure Login Error Handling discusses best practices.
• **Regular Security Audits:** Conduct regular security audits to identify and address any vulnerabilities in your password storage system. Security Audit Checklist offers a starting point.

1. 1. Common Pitfalls to Avoid

• **Using weak or outdated algorithms:** MD5, SHA-1, and plain hashing are all unacceptable.
• **Not using salts:** Salting is essential for preventing rainbow table attacks.
• **Using a constant salt for all users:** This defeats the purpose of salting.
• **Using a short or predictable salt:** The salt must be sufficiently random and unique.
• **Using a low work factor:** A low work factor makes it easier for attackers to crack passwords.
• **Storing passwords in plain text:** This is the most fundamental security mistake.
• **Ignoring security updates:** Keep your libraries and software up to date to patch vulnerabilities.

1. 1. Related Concepts

• **Cryptographic Hash Functions:** The underlying mathematical functions used in password hashing.
• **Key Stretching:** The process of repeatedly applying a hashing function to increase the computational cost.
• **Brute-Force Attack:** An attempt to guess a password by trying all possible combinations.
• **Dictionary Attack:** An attempt to guess a password by trying words from a dictionary.
• **Credential Stuffing:** An attack that uses stolen credentials from one service to attempt to log in to other services. Credential Stuffing Mitigation provides strategies.
• **Side-Channel Attacks:** Attacks that exploit information leaked from the physical implementation of a cryptographic system. Side-Channel Attack Analysis explains different types of attacks.
• **Security Engineering Principles:** The broader principles of designing secure systems. Security Engineering Best Practices offers comprehensive guidance.
• **OWASP (Open Web Application Security Project):** A community focused on improving the security of software. OWASP Top Ten lists the most critical web application security risks.
• **NIST (National Institute of Standards and Technology):** Provides guidelines and standards for cryptography and security. NIST Cryptographic Standards offers detailed information.
• **Zero-Knowledge Proofs:** A cryptographic method that allows one party to prove to another that they know a secret without revealing the secret itself. Zero-Knowledge Proof Applications explores potential uses.
• **Homomorphic Encryption:** A form of encryption that allows computations to be performed on ciphertext without decrypting it first. Homomorphic Encryption Techniques discusses its capabilities.
• **Post-Quantum Cryptography:** Cryptographic algorithms that are believed to be secure against attacks by quantum computers. Post-Quantum Cryptography Algorithms outlines emerging standards.
• **Differential Cryptanalysis:** A technique used to analyze the security of cryptographic algorithms by studying how differences in the input affect the output. Differential Cryptanalysis Methods provides detailed explanations.
• **Linear Cryptanalysis:** Another technique for analyzing cryptographic algorithms that focuses on finding linear approximations to the underlying functions. Linear Cryptanalysis Techniques outlines the approach.
• **Block Cipher Modes of Operation:** Different ways to use block ciphers to encrypt data. Block Cipher Modes Comparison discusses the pros and cons of each mode.
• **Authenticated Encryption:** Encryption schemes that provide both confidentiality and integrity protection. Authenticated Encryption Schemes details various methods.
• **Secure Communication Protocols:** Protocols like TLS/SSL that provide secure communication over a network. TLS/SSL Protocol Analysis examines the security features.
• **Digital Signatures:** A cryptographic technique used to verify the authenticity and integrity of digital documents. Digital Signature Verification explains the process.
• **Hardware Security Modules (HSMs):** Dedicated hardware devices used to store and manage cryptographic keys. HSM Security Features outlines their capabilities.
• **Threat Modeling:** A process for identifying and prioritizing potential security threats. Threat Modeling Methodologies details various approaches.
• **Penetration Testing:** A simulated attack on a system to identify vulnerabilities. Penetration Testing Techniques provides a comprehensive overview.
• **Vulnerability Scanning:** Automated tools used to identify known vulnerabilities in a system. Vulnerability Scanning Tools lists popular options.
• **Incident Response:** The process of handling security incidents. Incident Response Plan provides a template.
• **Data Loss Prevention (DLP):** Technologies and practices used to prevent sensitive data from leaving an organization. DLP Implementation Strategies details best practices.
• **Security Information and Event Management (SIEM):** Systems that collect and analyze security logs to detect and respond to threats. SIEM Configuration Guide offers guidance.

Cryptographic Hash Functions bcrypt Algorithm scrypt Algorithm Argon2 Specification PBKDF2 RFC Rainbow Table Attack Password Re-hashing Strategies Rate Limiting Techniques Adaptive Password Hashing Multi-Factor Authentication Overview Secure Login Error Handling Security Audit Checklist Credential Stuffing Mitigation Side-Channel Attack Analysis Security Engineering Best Practices OWASP Top Ten NIST Cryptographic Standards Zero-Knowledge Proof Applications Homomorphic Encryption Techniques Post-Quantum Cryptography Algorithms Differential Cryptanalysis Methods Linear Cryptanalysis Techniques Block Cipher Modes Comparison Authenticated Encryption Schemes TLS/SSL Protocol Analysis Digital Signature Verification

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