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hkdf.go
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hkdf.go
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// Copyright 2014 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 qtls
// Mostly derived from golang.org/x/crypto/hkdf, but with an exposed
// Extract API.
//
// HKDF is a cryptographic key derivation function (KDF) with the goal of
// expanding limited input keying material into one or more cryptographically
// strong secret keys.
//
// RFC 5869: https://tools.ietf.org/html/rfc5869
import (
"crypto"
"crypto/hmac"
)
func hkdfExpand(hash crypto.Hash, prk, info []byte, l int) []byte {
var (
expander = hmac.New(hash.New, prk)
res = make([]byte, l)
counter = byte(1)
prev []byte
)
if l > 255*expander.Size() {
panic("hkdf: requested too much output")
}
p := res
for len(p) > 0 {
expander.Reset()
expander.Write(prev)
expander.Write(info)
expander.Write([]byte{counter})
prev = expander.Sum(prev[:0])
counter++
n := copy(p, prev)
p = p[n:]
}
return res
}
// HkdfExtract generates a pseudorandom key for use with Expand from an input secret and an optional independent salt.
func HkdfExtract(hash crypto.Hash, secret, salt []byte) []byte {
return hkdfExtract(hash, secret, salt)
}
func hkdfExtract(hash crypto.Hash, secret, salt []byte) []byte {
if salt == nil {
salt = make([]byte, hash.Size())
}
if secret == nil {
secret = make([]byte, hash.Size())
}
extractor := hmac.New(hash.New, salt)
extractor.Write(secret)
return extractor.Sum(nil)
}