1 // Copyright 2012 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 /*
6 Package pbkdf2 implements the key derivation function PBKDF2 as defined in RFC
7 2898 / PKCS #5 v2.0.
8 9 A key derivation function is useful when encrypting data based on a password
10 or any other not-fully-random data. It uses a pseudorandom function to derive
11 a secure encryption key based on the password.
12 13 While v2.0 of the standard defines only one pseudorandom function to use,
14 HMAC-SHA1, the drafted v2.1 specification allows use of all five FIPS Approved
15 Hash Functions SHA-1, SHA-224, SHA-256, SHA-384 and SHA-512 for HMAC. To
16 choose, you can pass the `New` functions from the different SHA packages to
17 pbkdf2.Key.
18 */
19 package pbkdf2
20 21 import (
22 "crypto/hmac"
23 "hash"
24 )
25 26 // Key derives a key from the password, salt and iteration count, returning a
27 // []byte of length keylen that can be used as cryptographic key. The key is
28 // derived based on the method described as PBKDF2 with the HMAC variant using
29 // the supplied hash function.
30 //
31 // For example, to use a HMAC-SHA-1 based PBKDF2 key derivation function, you
32 // can get a derived key for e.g. AES-256 (which needs a 32-byte key) by
33 // doing:
34 //
35 // dk := pbkdf2.Key([]byte("some password"), salt, 4096, 32, sha1.New)
36 //
37 // Remember to get a good random salt. At least 8 bytes is recommended by the
38 // RFC.
39 //
40 // Using a higher iteration count will increase the cost of an exhaustive
41 // search but will also make derivation proportionally slower.
42 func Key(password, salt []byte, iter, keyLen int, h func() hash.Hash) []byte {
43 prf := hmac.New(h, password)
44 hashLen := prf.Size()
45 numBlocks := (keyLen + hashLen - 1) / hashLen
46 47 var buf [4]byte
48 dk := make([]byte, 0, numBlocks*hashLen)
49 U := make([]byte, hashLen)
50 for block := 1; block <= numBlocks; block++ {
51 // N.B.: || means concatenation, ^ means XOR
52 // for each block T_i = U_1 ^ U_2 ^ ... ^ U_iter
53 // U_1 = PRF(password, salt || uint(i))
54 prf.Reset()
55 prf.Write(salt)
56 buf[0] = byte(block >> 24)
57 buf[1] = byte(block >> 16)
58 buf[2] = byte(block >> 8)
59 buf[3] = byte(block)
60 prf.Write(buf[:4])
61 dk = prf.Sum(dk)
62 T := dk[len(dk)-hashLen:]
63 copy(U, T)
64 65 // U_n = PRF(password, U_(n-1))
66 for n := 2; n <= iter; n++ {
67 prf.Reset()
68 prf.Write(U)
69 U = U[:0]
70 U = prf.Sum(U)
71 for x := range U {
72 T[x] ^= U[x]
73 }
74 }
75 }
76 return dk[:keyLen]
77 }
78