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chacha8.go
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chacha8.go
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// Copyright 2023 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Package chacha8rand implements a pseudorandom generator
// based on ChaCha8. It is used by both runtime and math/rand/v2
// and must have minimal dependencies.
package chacha8rand
import "internal/byteorder"
const (
ctrInc = 4 // increment counter by 4 between block calls
ctrMax = 16 // reseed when counter reaches 16
chunk = 32 // each chunk produced by block is 32 uint64s
reseed = 4 // reseed with 4 words
)
// block is the chacha8rand block function.
func block(seed *[4]uint64, blocks *[32]uint64, counter uint32)
// 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.
type State struct {
buf [32]uint64
seed [4]uint64
i uint32
n uint32
c uint32
}
// 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.
//
//go:nosplit
func (s *State) Next() (uint64, bool) {
i := s.i
if i >= s.n {
return 0, false
}
s.i = i + 1
return s.buf[i&31], true // i&31 eliminates bounds check
}
// Init seeds the State with the given seed value.
func (s *State) Init(seed [32]byte) {
s.Init64([4]uint64{
byteorder.LeUint64(seed[0*8:]),
byteorder.LeUint64(seed[1*8:]),
byteorder.LeUint64(seed[2*8:]),
byteorder.LeUint64(seed[3*8:]),
})
}
// Init64 seeds the state with the given seed value.
func (s *State) Init64(seed [4]uint64) {
s.seed = seed
block(&s.seed, &s.buf, 0)
s.c = 0
s.i = 0
s.n = chunk
}
// 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 (s *State) Refill() {
s.c += ctrInc
if s.c == ctrMax {
// Reseed with generated uint64s for forward secrecy.
// Normally this is done immediately after computing a block,
// but we do it immediately before computing the next block,
// to allow a much smaller serialized state (just the seed plus offset).
// This gives a delayed benefit for the forward secrecy
// (you can reconstruct the recent past given a memory dump),
// which we deem acceptable in exchange for the reduced size.
s.seed[0] = s.buf[len(s.buf)-reseed+0]
s.seed[1] = s.buf[len(s.buf)-reseed+1]
s.seed[2] = s.buf[len(s.buf)-reseed+2]
s.seed[3] = s.buf[len(s.buf)-reseed+3]
s.c = 0
}
block(&s.seed, &s.buf, s.c)
s.i = 0
s.n = uint32(len(s.buf))
if s.c == ctrMax-ctrInc {
s.n = uint32(len(s.buf)) - 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.
func (s *State) Reseed() {
var seed [4]uint64
for i := range seed {
for {
x, ok := s.Next()
if ok {
seed[i] = x
break
}
s.Refill()
}
}
s.Init64(seed)
}
// 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 Marshal(s *State) []byte {
data := make([]byte, 6*8)
copy(data, "chacha8:")
used := (s.c/ctrInc)*chunk + s.i
byteorder.BePutUint64(data[1*8:], uint64(used))
for i, seed := range s.seed {
byteorder.LePutUint64(data[(2+i)*8:], seed)
}
return data
}
type errUnmarshalChaCha8 struct{}
func (*errUnmarshalChaCha8) Error() string {
return "invalid ChaCha8 encoding"
}
// Unmarshal unmarshals the state from a byte slice.
func Unmarshal(s *State, data []byte) error {
if len(data) != 6*8 || string(data[:8]) != "chacha8:" {
return new(errUnmarshalChaCha8)
}
used := byteorder.BeUint64(data[1*8:])
if used > (ctrMax/ctrInc)*chunk-reseed {
return new(errUnmarshalChaCha8)
}
for i := range s.seed {
s.seed[i] = byteorder.LeUint64(data[(2+i)*8:])
}
s.c = ctrInc * (uint32(used) / chunk)
block(&s.seed, &s.buf, s.c)
s.i = uint32(used) % chunk
s.n = chunk
if s.c == ctrMax-ctrInc {
s.n = chunk - reseed
}
return nil
}