hkdf.go

     1  // Copyright 2014 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  // Package hkdf implements the HMAC-based Extract-and-Expand Key Derivation
     6  // Function (HKDF) as defined in RFC 5869.
     7  //
     8  // HKDF is a cryptographic key derivation function (KDF) with the goal of
     9  // expanding limited input keying material into one or more cryptographically
    10  // strong secret keys.
    11  package hkdf
    12  
    13  import (
    14  	"crypto/hmac"
    15  	"errors"
    16  	"hash"
    17  	"io"
    18  )
    19  
    20  // Extract generates a pseudorandom key for use with Expand from an input secret
    21  // and an optional independent salt.
    22  //
    23  // Only use this function if you need to reuse the extracted key with multiple
    24  // Expand invocations and different context values. Most common scenarios,
    25  // including the generation of multiple keys, should use New instead.
    26  func Extract(hash func() hash.Hash, secret, salt []byte) []byte {
    27  	if salt == nil {
    28  		salt = make([]byte, hash().Size())
    29  	}
    30  	extractor := hmac.New(hash, salt)
    31  	extractor.Write(secret)
    32  	return extractor.Sum(nil)
    33  }
    34  
    35  type hkdf struct {
    36  	expander hash.Hash
    37  	size     int
    38  
    39  	info    []byte
    40  	counter byte
    41  
    42  	prev []byte
    43  	buf  []byte
    44  }
    45  
    46  func (f *hkdf) Read(p []byte) (int, error) {
    47  	// Check whether enough data can be generated
    48  	need := len(p)
    49  	remains := len(f.buf) + int(255-f.counter+1)*f.size
    50  	if remains < need {
    51  		return 0, errors.New("hkdf: entropy limit reached")
    52  	}
    53  	// Read any leftover from the buffer
    54  	n := copy(p, f.buf)
    55  	p = p[n:]
    56  
    57  	// Fill the rest of the buffer
    58  	for len(p) > 0 {
    59  		if f.counter > 1 {
    60  			f.expander.Reset()
    61  		}
    62  		f.expander.Write(f.prev)
    63  		f.expander.Write(f.info)
    64  		f.expander.Write([]byte{f.counter})
    65  		f.prev = f.expander.Sum(f.prev[:0])
    66  		f.counter++
    67  
    68  		// Copy the new batch into p
    69  		f.buf = f.prev
    70  		n = copy(p, f.buf)
    71  		p = p[n:]
    72  	}
    73  	// Save leftovers for next run
    74  	f.buf = f.buf[n:]
    75  
    76  	return need, nil
    77  }
    78  
    79  // Expand returns a Reader, from which keys can be read, using the given
    80  // pseudorandom key and optional context info, skipping the extraction step.
    81  //
    82  // The pseudorandomKey should have been generated by Extract, or be a uniformly
    83  // random or pseudorandom cryptographically strong key. See RFC 5869, Section
    84  // 3.3. Most common scenarios will want to use New instead.
    85  func Expand(hash func() hash.Hash, pseudorandomKey, info []byte) io.Reader {
    86  	expander := hmac.New(hash, pseudorandomKey)
    87  	return &hkdf{expander, expander.Size(), info, 1, nil, nil}
    88  }
    89  
    90  // New returns a Reader, from which keys can be read, using the given hash,
    91  // secret, salt and context info. Salt and info can be nil.
    92  func New(hash func() hash.Hash, secret, salt, info []byte) io.Reader {
    93  	prk := Extract(hash, secret, salt)
    94  	return Expand(hash, prk, info)
    95  }
    96
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