text: // Copyright 2012 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 pbkdf2 implements the key derivation function PBKDF2 as defined in RFC
  2898 / PKCS #5 v2.0.
  
  A key derivation function is useful when encrypting data based on a password
  or any other not-fully-random data. It uses a pseudorandom function to derive
  a secure encryption key based on the password.
  
  While v2.0 of the standard defines only one pseudorandom function to use,
  HMAC-SHA1, the drafted v2.1 specification allows use of all five FIPS Approved
  Hash Functions SHA-1, SHA-224, SHA-256, SHA-384 and SHA-512 for HMAC. To
  choose, you can pass the `New` functions from the different SHA packages to
  pbkdf2.Key.
  */
  package pbkdf2 // import "golang.org/x/crypto/pbkdf2"
  
  import (
  	"crypto/hmac"
  	"hash"
  )
  
  // Key derives a key from the password, salt and iteration count, returning a
  // []byte of length keylen that can be used as cryptographic key. The key is
  // derived based on the method described as PBKDF2 with the HMAC variant using
  // the supplied hash function.
  //
  // For example, to use a HMAC-SHA-1 based PBKDF2 key derivation function, you
  // can get a derived key for e.g. AES-256 (which needs a 32-byte key) by
  // doing:
  //
  // 	dk := pbkdf2.Key([]byte("some password"), salt, 4096, 32, sha1.New)
  //
  // Remember to get a good random salt. At least 8 bytes is recommended by the
  // RFC.
  //
  // Using a higher iteration count will increase the cost of an exhaustive
  // search but will also make derivation proportionally slower.
  func Key(password, salt []byte, iter, keyLen int, h func() hash.Hash) []byte {
  	prf := hmac.New(h, password)
  	hashLen := prf.Size()
  	numBlocks := (keyLen + hashLen - 1) / hashLen
  
  	var buf [4]byte
  	dk := make([]byte, 0, numBlocks*hashLen)
  	U := make([]byte, hashLen)
  	for block := 1; block <= numBlocks; block++ {
  		// N.B.: || means concatenation, ^ means XOR
  		// for each block T_i = U_1 ^ U_2 ^ ... ^ U_iter
  		// U_1 = PRF(password, salt || uint(i))
  		prf.Reset()
  		prf.Write(salt)
  		buf[0] = byte(block >> 24)
  		buf[1] = byte(block >> 16)
  		buf[2] = byte(block >> 8)
  		buf[3] = byte(block)
  		prf.Write(buf[:4])
  		dk = prf.Sum(dk)
  		T := dk[len(dk)-hashLen:]
  		copy(U, T)
  
  		// U_n = PRF(password, U_(n-1))
  		for n := 2; n <= iter; n++ {
  			prf.Reset()
  			prf.Write(U)
  			U = U[:0]
  			U = prf.Sum(U)
  			for x := range U {
  				T[x] ^= U[x]
  			}
  		}
  	}
  	return dk[:keyLen]
  }
  