counterecryptor.go

package goproxy

import (
	"crypto/aes"
	"crypto/cipher"
	"crypto/rsa"
	"crypto/sha256"
	"crypto/x509"
	"errors"
)

type CounterEncryptorRand struct {
	cipher  cipher.Block
	counter []byte
	rand    []byte
	ix      int
}

func NewCounterEncryptorRandFromKey(key interface{}, seed []byte) (r CounterEncryptorRand, err error) {
	var keyBytes []byte
	switch key := key.(type) {
	case *rsa.PrivateKey:
		keyBytes = x509.MarshalPKCS1PrivateKey(key)
	default:
		err = errors.New("only RSA keys supported")
		return
	}
	h := sha256.New()
	if r.cipher, err = aes.NewCipher(h.Sum(keyBytes)[:aes.BlockSize]); err != nil {
		return
	}
	r.counter = make([]byte, r.cipher.BlockSize())
	if seed != nil {
		copy(r.counter, h.Sum(seed)[:r.cipher.BlockSize()])
	}
	r.rand = make([]byte, r.cipher.BlockSize())
	r.ix = len(r.rand)
	return
}

func (c *CounterEncryptorRand) Seed(b []byte) {
	if len(b) != len(c.counter) {
		panic("SetCounter: wrong counter size")
	}
	copy(c.counter, b)
}

func (c *CounterEncryptorRand) refill() {
	c.cipher.Encrypt(c.rand, c.counter)
	for i := 0; i < len(c.counter); i++ {
		if c.counter[i]++; c.counter[i] != 0 {
			break
		}
	}
	c.ix = 0
}

func (c *CounterEncryptorRand) Read(b []byte) (n int, err error) {
	if c.ix == len(c.rand) {
		c.refill()
	}
	if n = len(c.rand) - c.ix; n > len(b) {
		n = len(b)
	}
	copy(b, c.rand[c.ix:c.ix+n])
	c.ix += n
	return
}