Why is System.Random unsafe for security purposes, and what is the cryptographic alternative?

Short answer

System.Random is a deterministic pseudorandom algorithm whose next values can be predicted; use RandomNumberGenerator from System.Security.Cryptography instead.

Explanation

Random uses a simple time-based seed (Clock Seed), allowing attackers to guess security tokens, account activation codes, and session keys after observing a few prior values. RandomNumberGenerator provides a cryptographically secure CSPRNG seeded from hardware and operating-system events, with convenient methods such as RandomNumberGenerator.GetInt32() and GetBytes().

Code example

// Generate a cryptographically secure six-digit OTP
int otp = RandomNumberGenerator.GetInt32(100000, 1000000);

// Generate an encrypted random key
byte[] salt = RandomNumberGenerator.GetBytes(32);

Common mistakes

• Using new Random().Next() to create password reset tokens or security signatures.

Follow-up questions

• What does Always Encrypted with Secure Enclaves in SQL Server and .NET provide?

Sources

• Microsoft Learn — RandomNumberGenerator Class