reader.go

package gopherng

import (
	"crypto/cipher"
	"fmt"

	"golang.org/x/crypto/chacha20"
	"golang.org/x/crypto/sha3"
)

// PseudorandomReader is a Reader providing seeded pseudorandom output from the
// keystream of an internal stream cipher.
//
// It is not safe for concurrent use.
type PseudorandomReader struct {
	// cipher is the stream cipher used to generate output.
	cipher cipher.Stream
}

// NewReader initializes a PseudorandomReader by initializing its internal
// stream cipher.
func NewReader(seed []byte) *PseudorandomReader {
	var p PseudorandomReader
	keyNonce := sha3.Sum512(seed)
	var err error
	p.cipher, err = chacha20.NewUnauthenticatedCipher(keyNonce[0:32],
		keyNonce[32:56])
	if err != nil {
		panic(fmt.Errorf(
			"failed to construct XChaCha20 unauthenticated cipher: %w", err))
	}
	return &p
}

// Read reads pseudorandom bytes by calling XORKeyStream with 0s in order to
// produce the keystream output.
func (p *PseudorandomReader) Read(b []byte) (int, error) {
	l := len(b)
	srcZero := make([]byte, l)
	p.cipher.XORKeyStream(b, srcZero)
	return l, nil
}