You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: pbkdf2.go
    path of file: ./repos/sealer/vendor/golang.org/x/crypto/pbkdf2
    
    the code of the file until where you have to start completion: // 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 o
    
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]
}