Random data

Purpose

This class produces unpredictable, cryptographically secure random numbers. Using a predictable random number generator, such as System.Random, is insecure.

These functions should be used to randomly generate encryption keys, nonces, salts, seeds, integers, strings, and passphrases.

Usage

Fill

Fills a span with random bytes.

SecureRandom.Fill(Span<byte> buffer)

Exceptions

ArgumentOutOfRangeException

buffer has a length of 0.

GetInt32

Generates a random integer between 0 (inclusive) and the upper bound (exclusive).

SecureRandom.GetInt32(int upperBound)

Exceptions

ArgumentOutOfRangeException

upperBound is less than MinUpperBound.

GetString

Generates a random string of a given length. A custom character set can be provided, but several character sets are available via constants.

SecureRandom.GetString(int length, string characterSet = AlphanumericChars)

Exceptions

ArgumentOutOfRangeException

length is less than MinStringLength or greater than MaxStringLength.

ArgumentNullException

characterSet is null.

ArgumentOutOfRangeException

characterSet has a length of 0.

GetPassphrase

Generates a random passphrase using the EFF's long wordlist (minus hyphenated words).

SecureRandom.GetPassphrase(int wordCount, char separatorChar = '-', bool capitalize = false, bool includeNumber = false)

ArgumentOutOfRangeException

wordCount is less than MinWordCount or greater than MaxWordCount.

FillDeterministic

Fills a span with deterministic bytes indistinguishable from random without knowing the seed.

SecureRandom.FillDeterministic(Span<byte> buffer, ReadOnlySpan<byte> seed)

This should be reserved for tests and custom constructions (e.g. an XOF).

Exceptions

ArgumentOutOfRangeException

buffer has a length of 0.

ArgumentOutOfRangeException

seed has a length not equal to SeedSize.

Constants

These are used for validation and/or save you defining your own constants.

public const int SeedSize = 32;
public const string AlphabeticChars = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
public const string NumericChars = "0123456789";
public const string SymbolChars = "!#$%&'()*+,-./:;<=>?@[]^_`{}~";
public const string AlphanumericChars = AlphabeticChars + NumericChars;
public const string AlphanumericSymbolChars = AlphanumericChars + SymbolChars;
public const int MinUpperBound = 2;
public const int MinStringLength = 8;
public const int MaxStringLength = 128;
public const int MinWordCount = 4;
public const int MaxWordCount = 20;

Notes

If these functions are called inside a virtual machine (VM) which is snapshotted and restored, the same output may be produced.​

The libsodium library uses RtlGenRandom() on Windows and getrandom or /dev/urandom on Linux and macOS to generate cryptographically secure random numbers non-deterministically. Deterministic generation is done using the IETF version of ChaCha20.