/
salt.go
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/
salt.go
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// Copyright 2020 Jigsaw Operations LLC
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
package shadowsocks
import (
"crypto/rand"
"errors"
)
// SaltGenerator generates unique salts to use in Shadowsocks connections.
type SaltGenerator interface {
// Returns a new salt
GetSalt(salt []byte) error
}
// randomSaltGenerator generates a new random salt.
type randomSaltGenerator struct{}
// GetSalt outputs a random salt.
func (randomSaltGenerator) GetSalt(salt []byte) error {
_, err := rand.Read(salt)
return err
}
// RandomSaltGenerator is a basic SaltGenerator.
var RandomSaltGenerator SaltGenerator = randomSaltGenerator{}
type prefixSaltGenerator struct {
prefix []byte
}
func (g prefixSaltGenerator) GetSalt(salt []byte) error {
n := copy(salt, g.prefix)
if n != len(g.prefix) {
return errors.New("prefix is too long")
}
_, err := rand.Read(salt[n:])
return err
}
// NewPrefixSaltGenerator returns a SaltGenerator with output including
// the provided prefix, followed by random bytes. This is useful to change
// how shadowsocks traffic is classified by middleboxes.
//
// Note: Prefixes steal entropy from the initialization vector. This weakens
// security by increasing the likelihood that the same IV is used in two
// different connections (which becomes likely once 2^(N/2) connections are
// made, due to the birthday attack). If an IV is reused, the attacker can
// not only decrypt the ciphertext of those two connections; they can also
// easily recover the shadowsocks key and decrypt all other connections to
// this server. Use with care!
func NewPrefixSaltGenerator(prefix []byte) SaltGenerator {
return prefixSaltGenerator{prefix}
}