ハッシュ・暗号化

SHA-256 (Secure Hash Algorithm 256-bit) is the most widely used hash algorithm today. Standardized by the U.S. National Institute of Standards and Technology (NIST), it is adopted in countless systems including Bitcoin, TLS/SSL communications, and database security.

Basic Characteristics of SHA-256

• Output Size: Always outputs 256 bits (64 hexadecimal characters)
• Deterministic: Always generates the same hash value for the same input
• One-Way: Computationally impossible to restore original data from hash value
• Avalanche Effect: Even a 1-bit change in input drastically changes the output

Why SHA-256 is Secure

• Collision Resistance: The probability of generating the same hash value from different inputs theoretically requires 2^128 attempts, making it practically impossible
• High Security Margin: While vulnerabilities have been found in SHA-1, SHA-256 remains secure
• International Trust: Recommended by many government agencies and security experts including NICT

Comparison with Other Hash Algorithms

Passwords stored in databases must never be stored in plaintext. Proper hashing and salting are required by OWASP (Open Web Application Security Project) and NIST security guidelines.

Password Storage Methods to Absolutely Avoid

• ❌ Plaintext Storage: If the database is compromised, all user passwords are exposed
• ❌ Hashing with MD5 or SHA-1: Vulnerabilities have already been reported, weak against rainbow table attacks
• ❌ Hashing without Salt: The same password generates the same hash value, making it vulnerable to dictionary attacks

Recommended Password Storage Methods

• ✅ bcrypt: Password-specific hash function. Automatically generates salt and adjustable computation cost (PHP: password_hash())
• ✅ Argon2: Latest hash algorithm. Strong against both memory and time, resistant to GPU attacks (PHP 7.2+)
• ✅ PBKDF2: NIST recommended. Setting high iteration counts provides resistance to brute-force attacks

What is Salt?

Salt is a random string added to the password before hashing. The same password with different salts generates completely different hash values. This prevents the following attacks:

• Rainbow Table Attacks: Pre-computed hash value matching is nullified
• Dictionary Attacks: Common patterns among identical passwords are eliminated

Implementation Example (PHP)

// Hash password (registration)
$hashed_password = password_hash($user_password, PASSWORD_ARGON2ID);

// Verify password (login)
if (password_verify($input_password, $hashed_password)) {
    // Password is correct
} else {
    // Password is incorrect
}

Password Input Best Practices

• Require HTTPS: Login pages must be served over HTTPS to prevent eavesdropping
• Set Password Strength Requirements: Recommend minimum 8 characters with uppercase, lowercase, numbers, and symbols
• Implement Multi-Factor Authentication (MFA): Effective for mitigating password breach risks
• Limit Login Attempts: To prevent brute-force attacks, gradually increase wait time after failures