chacha8rand

Documentation

Overview

Package chacha8rand implements a pseudorandom generator based on ChaCha8. It is used by both runtime and math/rand/v2 and must have no dependencies.

ChaCha8 is ChaCha with 8 rounds. See https://cr.yp.to/chacha/chacha-20080128.pdf.

ChaCha8 operates on a 4x4 matrix of uint32 values, initially set to:

const1 const2 const3 const4
seed   seed   seed   seed
seed   seed   seed   seed
counter64     0      0

We use the same constants as ChaCha20 does, a random seed, and a counter. Running ChaCha8 on this input produces a 4x4 matrix of pseudo-random values with as much entropy as the seed.

Given SIMD registers that can hold N uint32s, it is possible to run N ChaCha8 block transformations in parallel by filling the first register with the N copies of const1, the second with N copies of const2, and so on, and then running the operations.

Each iteration of ChaCha8Rand operates over 32 bytes of input and produces 992 bytes of RNG output, plus 32 bytes of input for the next iteration.

The 32 bytes of input are used as a ChaCha8 key, with a zero nonce, to produce 1024 bytes of output (16 blocks, with counters 0 to 15). First, for each block, the values 0x61707865, 0x3320646e, 0x79622d32, 0x6b206574 are subtracted from the 32-bit little-endian words at position 0, 1, 2, and 3 respectively, and an increasing counter starting at zero is subtracted from each word at position 12. Then, this stream is permuted such that for each sequence of four blocks, first we output the first four bytes of each block, then the next four bytes of each block, and so on. Finally, the last 32 bytes of output are used as the input of the next iteration, and the remaining 992 bytes are the RNG output.

See https://c2sp.org/chacha8rand for additional details.

Normal ChaCha20 implementations for encryption use this same parallelism but then have to deinterlace the results so that it appears the blocks were generated separately. For the purposes of generating random numbers, the interlacing is fine. We are simply locked in to preserving the 4-way interlacing in any future optimizations.

Index

• func Marshal(s *State) []byte
• func Unmarshal(s *State, data []byte) error
• type State
  • func (s *State) Init(seed [32]byte)
  • func (s *State) Init64(seed [4]uint64)
  • func (s *State) Next() (uint64, bool)
  • func (s *State) Refill()
  • func (s *State) Reseed()

Functions

func Marshal

func Marshal(s *State) []byte

Marshal marshals the state into a byte slice. Marshal and Unmarshal are functions, not methods, so that they will not be linked into the runtime when it uses the State struct, since the runtime does not need these.

func Unmarshal

func Unmarshal(s *State, data []byte) error

Unmarshal unmarshals the state from a byte slice.

Types

type State

type State struct {
	// contains filtered or unexported fields
}

A State holds the state for a single random generator. It must be used from one goroutine at a time. If used by multiple goroutines at a time, the goroutines may see the same random values, but the code will not crash or cause out-of-bounds memory accesses.

func (*State) Init

func (s *State) Init(seed [32]byte)

Init seeds the State with the given seed value.

func (*State) Init64

func (s *State) Init64(seed [4]uint64)

Init64 seeds the state with the given seed value.

func (*State) Next

func (s *State) Next() (uint64, bool)

Next returns the next random value, along with a boolean indicating whether one was available. If one is not available, the caller should call Refill and then repeat the call to Next.

Next is //go:nosplit to allow its use in the runtime with per-m data without holding the per-m lock.

func (*State) Refill

func (s *State) Refill()

Refill refills the state with more random values. After a call to Refill, an immediate call to Next will succeed (unless multiple goroutines are incorrectly sharing a state).

func (*State) Reseed

func (s *State) Reseed()

Reseed reseeds the state with new random values. After a call to Reseed, any previously returned random values have been erased from the memory of the state and cannot be recovered.