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94 lines
2.7 KiB
Go
94 lines
2.7 KiB
Go
// Copyright 2014 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// Package hkdf implements the HMAC-based Extract-and-Expand Key Derivation
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// Function (HKDF) as defined in RFC 5869.
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//
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// HKDF is a cryptographic key derivation function (KDF) with the goal of
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// expanding limited input keying material into one or more cryptographically
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// strong secret keys.
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package hkdf // import "golang.org/x/crypto/hkdf"
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import (
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"crypto/hmac"
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"errors"
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"hash"
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"io"
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)
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// Extract generates a pseudorandom key for use with Expand from an input secret
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// and an optional independent salt.
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//
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// Only use this function if you need to reuse the extracted key with multiple
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// Expand invocations and different context values. Most common scenarios,
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// including the generation of multiple keys, should use New instead.
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func Extract(hash func() hash.Hash, secret, salt []byte) []byte {
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if salt == nil {
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salt = make([]byte, hash().Size())
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}
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extractor := hmac.New(hash, salt)
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extractor.Write(secret)
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return extractor.Sum(nil)
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}
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type hkdf struct {
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expander hash.Hash
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size int
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info []byte
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counter byte
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prev []byte
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buf []byte
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}
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func (f *hkdf) Read(p []byte) (int, error) {
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// Check whether enough data can be generated
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need := len(p)
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remains := len(f.buf) + int(255-f.counter+1)*f.size
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if remains < need {
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return 0, errors.New("hkdf: entropy limit reached")
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}
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// Read any leftover from the buffer
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n := copy(p, f.buf)
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p = p[n:]
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// Fill the rest of the buffer
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for len(p) > 0 {
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f.expander.Reset()
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f.expander.Write(f.prev)
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f.expander.Write(f.info)
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f.expander.Write([]byte{f.counter})
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f.prev = f.expander.Sum(f.prev[:0])
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f.counter++
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// Copy the new batch into p
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f.buf = f.prev
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n = copy(p, f.buf)
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p = p[n:]
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}
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// Save leftovers for next run
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f.buf = f.buf[n:]
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return need, nil
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}
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// Expand returns a Reader, from which keys can be read, using the given
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// pseudorandom key and optional context info, skipping the extraction step.
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//
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// The pseudorandomKey should have been generated by Extract, or be a uniformly
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// random or pseudorandom cryptographically strong key. See RFC 5869, Section
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// 3.3. Most common scenarios will want to use New instead.
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func Expand(hash func() hash.Hash, pseudorandomKey, info []byte) io.Reader {
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expander := hmac.New(hash, pseudorandomKey)
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return &hkdf{expander, expander.Size(), info, 1, nil, nil}
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}
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// New returns a Reader, from which keys can be read, using the given hash,
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// secret, salt and context info. Salt and info can be nil.
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func New(hash func() hash.Hash, secret, salt, info []byte) io.Reader {
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prk := Extract(hash, secret, salt)
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return Expand(hash, prk, info)
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}
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