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// Copyright 2021 by David A. Golden. All rights reserved.
//
// Licensed under the Apache License, Version 2.0 (the "License"); you may
// not use this file except in compliance with the License. You may obtain
// a copy of the License at http://www.apache.org/licenses/LICENSE-2.0
// Package pbkdf2 implements password-based key derivation using the PBKDF2
// algorithm described in RFC 2898 and RFC 8018.
//
// It provides a drop-in replacement for `golang.org/x/crypto/pbkdf2`, with
// the following benefits:
//
// - Released as a module with semantic versioning
//
// - Does not pull in dependencies for unrelated `x/crypto/*` packages
//
// - Supports Go 1.9+
//
// See https://tools.ietf.org/html/rfc8018#section-4 for security considerations
// in the selection of a salt and iteration count.
package pbkdf2
import (
"crypto/hmac"
"encoding/binary"
"hash"
)
// Key generates a derived key from a password using the PBKDF2 algorithm. The
// inputs include salt bytes, the iteration count, desired key length, and a
// constructor for a hashing function. For example, for a 32-byte key using
// SHA-256:
//
// key := Key([]byte("trustNo1"), salt, 10000, 32, sha256.New)
func Key(password, salt []byte, iterCount, keyLen int, h func() hash.Hash) []byte {
prf := hmac.New(h, password)
hLen := prf.Size()
numBlocks := keyLen / hLen
// Get an extra block if keyLen is not an even number of hLen blocks.
if keyLen%hLen > 0 {
numBlocks++
}
Ti := make([]byte, hLen)
Uj := make([]byte, hLen)
dk := make([]byte, 0, hLen*numBlocks)
buf := make([]byte, 4)
for i := uint32(1); i <= uint32(numBlocks); i++ {
// Initialize Uj for j == 1 from salt and block index.
// Initialize Ti = U1.
binary.BigEndian.PutUint32(buf, i)
prf.Reset()
prf.Write(salt)
prf.Write(buf)
Uj = Uj[:0]
Uj = prf.Sum(Uj)
// Ti = U1 ^ U2 ^ ... ^ Ux
copy(Ti, Uj)
for j := 2; j <= iterCount; j++ {
prf.Reset()
prf.Write(Uj)
Uj = Uj[:0]
Uj = prf.Sum(Uj)
for k := range Uj {
Ti[k] ^= Uj[k]
}
}
// DK = concat(T1, T2, ... Tn)
dk = append(dk, Ti...)
}
return dk[0:keyLen]
}
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