math/rand/v2: add ChaCha8.Read

Fixes #67059
Closes #67452
Closes #67498

Change-Id: I84eba2ed787a17e9d6aaad2a8a78596e3944909a
Reviewed-on: https://go-review.googlesource.com/c/go/+/587280
Reviewed-by: Roland Shoemaker <roland@golang.org>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

api/next/67059.txt

@@ -0,0 +1 @@
+pkg math/rand/v2, method (*ChaCha8) Read([]uint8) (int, error) #67059

doc/next/6-stdlib/99-minor/math/rand/v2/67059.md

@@ -0,0 +1 @@
+The new [ChaCha8.Read] method implements the [io.Reader] interface.

src/math/rand/rand.go

@@ -474,6 +474,7 @@ func Shuffle(n int, swap func(i, j int)) { globalRand().Shuffle(n, swap) }
 // Read, unlike the [Rand.Read] method, is safe for concurrent use.
 //
 // Deprecated: For almost all use cases, [crypto/rand.Read] is more appropriate.
+// If a deterministic source is required, use [math/rand/v2.ChaCha8.Read].
 func Read(p []byte) (n int, err error) { return globalRand().Read(p) }
 
 // NormFloat64 returns a normally distributed float64 in the range

src/math/rand/v2/chacha8.go

@@ -4,12 +4,20 @@
 
 package rand
 
-import "internal/chacha8rand"
+import (
+	"errors"
+	"internal/byteorder"
+	"internal/chacha8rand"
+)
 
 // A ChaCha8 is a ChaCha8-based cryptographically strong
 // random number generator.
 type ChaCha8 struct {
 	state chacha8rand.State
+
+	// The last readLen bytes of readBuf are still to be consumed by Read.
+	readBuf [8]byte
+	readLen int // 0 <= readLen <= 8
 }
 
 // NewChaCha8 returns a new ChaCha8 seeded with the given seed.
@@ -22,6 +30,8 @@ func NewChaCha8(seed [32]byte) *ChaCha8 {
 // Seed resets the ChaCha8 to behave the same way as NewChaCha8(seed).
 func (c *ChaCha8) Seed(seed [32]byte) {
 	c.state.Init(seed)
+	c.readLen = 0
+	c.readBuf = [8]byte{}
 }
 
 // Uint64 returns a uniformly distributed random uint64 value.
@@ -35,12 +45,66 @@ func (c *ChaCha8) Uint64() uint64 {
 	}
 }
 
+// Read reads exactly len(p) bytes into p.
+// It always returns len(p) and a nil error.
+//
+// If calls to Read and Uint64 are interleaved, the order in which bits are
+// returned by the two is undefined, and Read may return bits generated before
+// the last call to Uint64.
+func (c *ChaCha8) Read(p []byte) (n int, err error) {
+	if c.readLen > 0 {
+		n = copy(p, c.readBuf[len(c.readBuf)-c.readLen:])
+		c.readLen -= n
+		p = p[n:]
+	}
+	for len(p) >= 8 {
+		byteorder.LePutUint64(p, c.Uint64())
+		p = p[8:]
+		n += 8
+	}
+	if len(p) > 0 {
+		byteorder.LePutUint64(c.readBuf[:], c.Uint64())
+		n += copy(p, c.readBuf[:])
+		c.readLen = 8 - len(p)
+	}
+	return
+}
+
 // UnmarshalBinary implements the encoding.BinaryUnmarshaler interface.
 func (c *ChaCha8) UnmarshalBinary(data []byte) error {
+	data, ok := cutPrefix(data, []byte("readbuf:"))
+	if ok {
+		var buf []byte
+		buf, data, ok = readUint8LengthPrefixed(data)
+		if !ok {
+			return errors.New("invalid ChaCha8 Read buffer encoding")
+		}
+		c.readLen = copy(c.readBuf[len(c.readBuf)-len(buf):], buf)
+	}
 	return chacha8rand.Unmarshal(&c.state, data)
 }
 
+func cutPrefix(s, prefix []byte) (after []byte, found bool) {
+	if len(s) < len(prefix) || string(s[:len(prefix)]) != string(prefix) {
+		return s, false
+	}
+	return s[len(prefix):], true
+}
+
+func readUint8LengthPrefixed(b []byte) (buf, rest []byte, ok bool) {
+	if len(b) == 0 || len(b) < int(1+b[0]) {
+		return nil, nil, false
+	}
+	return b[1 : 1+b[0]], b[1+b[0]:], true
+}
+
 // MarshalBinary implements the encoding.BinaryMarshaler interface.
 func (c *ChaCha8) MarshalBinary() ([]byte, error) {
+	if c.readLen > 0 {
+		out := []byte("readbuf:")
+		out = append(out, uint8(c.readLen))
+		out = append(out, c.readBuf[len(c.readBuf)-c.readLen:]...)
+		return append(out, chacha8rand.Marshal(&c.state)...), nil
+	}
 	return chacha8rand.Marshal(&c.state), nil
 }

src/math/rand/v2/chacha8_test.go

@@ -5,8 +5,13 @@
 package rand_test
 
 import (
+	"bytes"
+	"crypto/sha256"
+	"encoding/hex"
+	"io"
 	. "math/rand/v2"
 	"testing"
+	"testing/iotest"
 )
 
 func TestChaCha8(t *testing.T) {
@@ -25,6 +30,74 @@ func TestChaCha8(t *testing.T) {
 	}
 }
 
+func TestChaCha8Read(t *testing.T) {
+	p := NewChaCha8(chacha8seed)
+	h := sha256.New()
+
+	buf := make([]byte, chacha8outlen)
+	if nn, err := p.Read(buf); err != nil {
+		t.Fatal(err)
+	} else if nn != len(buf) {
+		t.Errorf("Read short: got %d, expected %d", nn, len(buf))
+	}
+	h.Write(buf)
+	if got := h.Sum(nil); !bytes.Equal(got, chacha8hash) {
+		t.Errorf("transcript incorrect: got %x, want %x", got, chacha8hash)
+	}
+
+	p.Seed(chacha8seed)
+	h.Reset()
+
+	buf = make([]byte, chacha8outlen)
+	if _, err := io.ReadFull(iotest.OneByteReader(p), buf); err != nil {
+		t.Errorf("one byte reads: %v", err)
+	}
+	h.Write(buf)
+	if got := h.Sum(nil); !bytes.Equal(got, chacha8hash) {
+		t.Errorf("transcript incorrect (one byte reads): got %x, want %x", got, chacha8hash)
+	}
+
+	p.Seed(chacha8seed)
+	h.Reset()
+
+	if n, err := p.Read(make([]byte, 0)); err != nil {
+		t.Errorf("zero length read: %v", err)
+	} else if n != 0 {
+		t.Errorf("Read zero length: got %d, expected %d", n, 0)
+	}
+
+	var n int
+	for n < chacha8outlen {
+		if IntN(2) == 0 {
+			out, err := p.MarshalBinary()
+			if err != nil {
+				t.Fatal(err)
+			}
+			if IntN(2) == 0 {
+				p = NewChaCha8([32]byte{})
+			}
+			if err := p.UnmarshalBinary(out); err != nil {
+				t.Fatal(err)
+			}
+		}
+		buf := make([]byte, IntN(100))
+		if n+len(buf) > chacha8outlen {
+			buf = buf[:chacha8outlen-n]
+		}
+		n += len(buf)
+		t.Logf("reading %d bytes", len(buf))
+		if nn, err := p.Read(buf); err != nil {
+			t.Fatal(err)
+		} else if nn != len(buf) {
+			t.Errorf("Read short: got %d, expected %d", nn, len(buf))
+		}
+		h.Write(buf)
+	}
+	if got := h.Sum(nil); !bytes.Equal(got, chacha8hash) {
+		t.Errorf("transcript incorrect: got %x, want %x", got, chacha8hash)
+	}
+}
+
 func TestChaCha8Marshal(t *testing.T) {
 	p := NewChaCha8(chacha8seed)
 	for i, x := range chacha8output {
@@ -33,7 +106,7 @@ func TestChaCha8Marshal(t *testing.T) {
 			t.Fatalf("#%d: MarshalBinary: %v", i, err)
 		}
 		if string(enc) != chacha8marshal[i] {
-			t.Fatalf("#%d: MarshalBinary=%q, want %q", i, enc, chacha8marshal[i])
+			t.Errorf("#%d: MarshalBinary=%q, want %q", i, enc, chacha8marshal[i])
 		}
 		*p = ChaCha8{}
 		if err := p.UnmarshalBinary(enc); err != nil {
@@ -45,6 +118,24 @@ func TestChaCha8Marshal(t *testing.T) {
 	}
 }
 
+func TestChaCha8MarshalRead(t *testing.T) {
+	p := NewChaCha8(chacha8seed)
+	for i := range 50 {
+		enc, err := p.MarshalBinary()
+		if err != nil {
+			t.Fatalf("#%d: MarshalBinary: %v", i, err)
+		}
+		if string(enc) != chacha8marshalread[i] {
+			t.Errorf("#%d: MarshalBinary=%q, want %q", i, enc, chacha8marshalread[i])
+		}
+		*p = ChaCha8{}
+		if err := p.UnmarshalBinary(enc); err != nil {
+			t.Fatalf("#%d: UnmarshalBinary: %v", i, err)
+		}
+		p.Read(make([]byte, 1))
+	}
+}
+
 func BenchmarkChaCha8(b *testing.B) {
 	p := NewChaCha8([32]byte{1, 2, 3, 4, 5})
 	var t uint64
@@ -54,11 +145,26 @@ func BenchmarkChaCha8(b *testing.B) {
 	Sink = t
 }
 
+func BenchmarkChaCha8Read(b *testing.B) {
+	p := NewChaCha8([32]byte{1, 2, 3, 4, 5})
+	buf := make([]byte, 32)
+	b.SetBytes(32)
+	var t uint8
+	for n := b.N; n > 0; n-- {
+		p.Read(buf)
+		t += buf[0]
+	}
+	Sink = uint64(t)
+}
+
 // Golden output test to make sure algorithm never changes,
 // so that its use in math/rand/v2 stays stable.
 
 var chacha8seed = [32]byte([]byte("ABCDEFGHIJKLMNOPQRSTUVWXYZ123456"))
 
+var chacha8outlen = 2976
+var chacha8hash, _ = hex.DecodeString("bfec3d418b829afe5df2d8887d1508348409c293b73758d7efd841dd995fe021")
+
 var chacha8output = []uint64{
 	0xb773b6063d4616a5, 0x1160af22a66abc3c, 0x8c2599d9418d287c, 0x7ee07e037edc5cd6,
 	0xcfaa9ee02d1c16ad, 0x0e090eef8febea79, 0x3c82d271128b5b3e, 0x9c5addc11252a34f,
@@ -529,3 +635,56 @@ var chacha8marshal = []string{
 	"chacha8:\x00\x00\x00\x00\x00\x00\x00zK3\x9bB!,\x94\x9d\x975\xce'O_t\xee|\xb21\x87\xbb\xbb\xfd)\x8f\xe52\x01\vP\fk",
 	"chacha8:\x00\x00\x00\x00\x00\x00\x00{K3\x9bB!,\x94\x9d\x975\xce'O_t\xee|\xb21\x87\xbb\xbb\xfd)\x8f\xe52\x01\vP\fk",
 }
+
+var chacha8marshalread = []string{
+	"chacha8:\x00\x00\x00\x00\x00\x00\x00\x00ABCDEFGHIJKLMNOPQRSTUVWXYZ123456",
+	"readbuf:\a\x16F=\x06\xb6s\xb7chacha8:\x00\x00\x00\x00\x00\x00\x00\x01ABCDEFGHIJKLMNOPQRSTUVWXYZ123456",
+	"readbuf:\x06F=\x06\xb6s\xb7chacha8:\x00\x00\x00\x00\x00\x00\x00\x01ABCDEFGHIJKLMNOPQRSTUVWXYZ123456",
+	"readbuf:\x05=\x06\xb6s\xb7chacha8:\x00\x00\x00\x00\x00\x00\x00\x01ABCDEFGHIJKLMNOPQRSTUVWXYZ123456",
+	"readbuf:\x04\x06\xb6s\xb7chacha8:\x00\x00\x00\x00\x00\x00\x00\x01ABCDEFGHIJKLMNOPQRSTUVWXYZ123456",
+	"readbuf:\x03\xb6s\xb7chacha8:\x00\x00\x00\x00\x00\x00\x00\x01ABCDEFGHIJKLMNOPQRSTUVWXYZ123456",
+	"readbuf:\x02s\xb7chacha8:\x00\x00\x00\x00\x00\x00\x00\x01ABCDEFGHIJKLMNOPQRSTUVWXYZ123456",
+	"readbuf:\x01\xb7chacha8:\x00\x00\x00\x00\x00\x00\x00\x01ABCDEFGHIJKLMNOPQRSTUVWXYZ123456",
+	"chacha8:\x00\x00\x00\x00\x00\x00\x00\x01ABCDEFGHIJKLMNOPQRSTUVWXYZ123456",
+	"readbuf:\a\xbcj\xa6\"\xaf`\x11chacha8:\x00\x00\x00\x00\x00\x00\x00\x02ABCDEFGHIJKLMNOPQRSTUVWXYZ123456",
+	"readbuf:\x06j\xa6\"\xaf`\x11chacha8:\x00\x00\x00\x00\x00\x00\x00\x02ABCDEFGHIJKLMNOPQRSTUVWXYZ123456",
+	"readbuf:\x05\xa6\"\xaf`\x11chacha8:\x00\x00\x00\x00\x00\x00\x00\x02ABCDEFGHIJKLMNOPQRSTUVWXYZ123456",
+	"readbuf:\x04\"\xaf`\x11chacha8:\x00\x00\x00\x00\x00\x00\x00\x02ABCDEFGHIJKLMNOPQRSTUVWXYZ123456",
+	"readbuf:\x03\xaf`\x11chacha8:\x00\x00\x00\x00\x00\x00\x00\x02ABCDEFGHIJKLMNOPQRSTUVWXYZ123456",
+	"readbuf:\x02`\x11chacha8:\x00\x00\x00\x00\x00\x00\x00\x02ABCDEFGHIJKLMNOPQRSTUVWXYZ123456",
+	"readbuf:\x01\x11chacha8:\x00\x00\x00\x00\x00\x00\x00\x02ABCDEFGHIJKLMNOPQRSTUVWXYZ123456",
+	"chacha8:\x00\x00\x00\x00\x00\x00\x00\x02ABCDEFGHIJKLMNOPQRSTUVWXYZ123456",
+	"readbuf:\a(\x8dAٙ%\x8cchacha8:\x00\x00\x00\x00\x00\x00\x00\x03ABCDEFGHIJKLMNOPQRSTUVWXYZ123456",
+	"readbuf:\x06\x8dAٙ%\x8cchacha8:\x00\x00\x00\x00\x00\x00\x00\x03ABCDEFGHIJKLMNOPQRSTUVWXYZ123456",
+	"readbuf:\x05Aٙ%\x8cchacha8:\x00\x00\x00\x00\x00\x00\x00\x03ABCDEFGHIJKLMNOPQRSTUVWXYZ123456",
+	"readbuf:\x04ٙ%\x8cchacha8:\x00\x00\x00\x00\x00\x00\x00\x03ABCDEFGHIJKLMNOPQRSTUVWXYZ123456",
+	"readbuf:\x03\x99%\x8cchacha8:\x00\x00\x00\x00\x00\x00\x00\x03ABCDEFGHIJKLMNOPQRSTUVWXYZ123456",
+	"readbuf:\x02%\x8cchacha8:\x00\x00\x00\x00\x00\x00\x00\x03ABCDEFGHIJKLMNOPQRSTUVWXYZ123456",
+	"readbuf:\x01\x8cchacha8:\x00\x00\x00\x00\x00\x00\x00\x03ABCDEFGHIJKLMNOPQRSTUVWXYZ123456",
+	"chacha8:\x00\x00\x00\x00\x00\x00\x00\x03ABCDEFGHIJKLMNOPQRSTUVWXYZ123456",
+	"readbuf:\a\\\xdc~\x03~\xe0~chacha8:\x00\x00\x00\x00\x00\x00\x00\x04ABCDEFGHIJKLMNOPQRSTUVWXYZ123456",
+	"readbuf:\x06\xdc~\x03~\xe0~chacha8:\x00\x00\x00\x00\x00\x00\x00\x04ABCDEFGHIJKLMNOPQRSTUVWXYZ123456",
+	"readbuf:\x05~\x03~\xe0~chacha8:\x00\x00\x00\x00\x00\x00\x00\x04ABCDEFGHIJKLMNOPQRSTUVWXYZ123456",
+	"readbuf:\x04\x03~\xe0~chacha8:\x00\x00\x00\x00\x00\x00\x00\x04ABCDEFGHIJKLMNOPQRSTUVWXYZ123456",
+	"readbuf:\x03~\xe0~chacha8:\x00\x00\x00\x00\x00\x00\x00\x04ABCDEFGHIJKLMNOPQRSTUVWXYZ123456",
+	"readbuf:\x02\xe0~chacha8:\x00\x00\x00\x00\x00\x00\x00\x04ABCDEFGHIJKLMNOPQRSTUVWXYZ123456",
+	"readbuf:\x01~chacha8:\x00\x00\x00\x00\x00\x00\x00\x04ABCDEFGHIJKLMNOPQRSTUVWXYZ123456",
+	"chacha8:\x00\x00\x00\x00\x00\x00\x00\x04ABCDEFGHIJKLMNOPQRSTUVWXYZ123456",
+	"readbuf:\a\x16\x1c-\xe0\x9e\xaa\xcfchacha8:\x00\x00\x00\x00\x00\x00\x00\x05ABCDEFGHIJKLMNOPQRSTUVWXYZ123456",
+	"readbuf:\x06\x1c-\xe0\x9e\xaa\xcfchacha8:\x00\x00\x00\x00\x00\x00\x00\x05ABCDEFGHIJKLMNOPQRSTUVWXYZ123456",
+	"readbuf:\x05-\xe0\x9e\xaa\xcfchacha8:\x00\x00\x00\x00\x00\x00\x00\x05ABCDEFGHIJKLMNOPQRSTUVWXYZ123456",
+	"readbuf:\x04\xe0\x9e\xaa\xcfchacha8:\x00\x00\x00\x00\x00\x00\x00\x05ABCDEFGHIJKLMNOPQRSTUVWXYZ123456",
+	"readbuf:\x03\x9e\xaa\xcfchacha8:\x00\x00\x00\x00\x00\x00\x00\x05ABCDEFGHIJKLMNOPQRSTUVWXYZ123456",
+	"readbuf:\x02\xaa\xcfchacha8:\x00\x00\x00\x00\x00\x00\x00\x05ABCDEFGHIJKLMNOPQRSTUVWXYZ123456",
+	"readbuf:\x01\xcfchacha8:\x00\x00\x00\x00\x00\x00\x00\x05ABCDEFGHIJKLMNOPQRSTUVWXYZ123456",
+	"chacha8:\x00\x00\x00\x00\x00\x00\x00\x05ABCDEFGHIJKLMNOPQRSTUVWXYZ123456",
+	"readbuf:\a\xea\xeb\x8f\xef\x0e\t\x0echacha8:\x00\x00\x00\x00\x00\x00\x00\x06ABCDEFGHIJKLMNOPQRSTUVWXYZ123456",
+	"readbuf:\x06\xeb\x8f\xef\x0e\t\x0echacha8:\x00\x00\x00\x00\x00\x00\x00\x06ABCDEFGHIJKLMNOPQRSTUVWXYZ123456",
+	"readbuf:\x05\x8f\xef\x0e\t\x0echacha8:\x00\x00\x00\x00\x00\x00\x00\x06ABCDEFGHIJKLMNOPQRSTUVWXYZ123456",
+	"readbuf:\x04\xef\x0e\t\x0echacha8:\x00\x00\x00\x00\x00\x00\x00\x06ABCDEFGHIJKLMNOPQRSTUVWXYZ123456",
+	"readbuf:\x03\x0e\t\x0echacha8:\x00\x00\x00\x00\x00\x00\x00\x06ABCDEFGHIJKLMNOPQRSTUVWXYZ123456",
+	"readbuf:\x02\t\x0echacha8:\x00\x00\x00\x00\x00\x00\x00\x06ABCDEFGHIJKLMNOPQRSTUVWXYZ123456",
+	"readbuf:\x01\x0echacha8:\x00\x00\x00\x00\x00\x00\x00\x06ABCDEFGHIJKLMNOPQRSTUVWXYZ123456",
+	"chacha8:\x00\x00\x00\x00\x00\x00\x00\x06ABCDEFGHIJKLMNOPQRSTUVWXYZ123456",
+	"readbuf:\a[\x8b\x12q҂<chacha8:\x00\x00\x00\x00\x00\x00\x00\aABCDEFGHIJKLMNOPQRSTUVWXYZ123456",
+}