Source: hkdf.go in package vendor/golang.org/x/crypto/hkdf

Package hkdf implements the HMAC-based Extract-and-Expand Key Derivation Function (HKDF) as defined in RFC 5869. HKDF is a cryptographic key derivation function (KDF) with the goal of expanding limited input keying material into one or more cryptographically strong secret keys.

package hkdf // import "golang.org/x/crypto/hkdf"

import (
	"crypto/hmac"
	"errors"
	"hash"
	"io"
)

Extract generates a pseudorandom key for use with Expand from an input secret and an optional independent salt. Only use this function if you need to reuse the extracted key with multiple Expand invocations and different context values. Most common scenarios, including the generation of multiple keys, should use New instead.

func Extract(hash func() hash.Hash, secret, salt []byte) []byte {
	if salt == nil {
		salt = make([]byte, hash().Size())
	}
	extractor := hmac.New(hash, salt)
	extractor.Write(secret)
	return extractor.Sum(nil)
}

type hkdf struct {
	expander hash.Hash
	size     int

	info    []byte
	counter byte

	prev []byte
	buf  []byte
}

Check whether enough data can be generated

	need := len(p)
	remains := len(f.buf) + int(255-f.counter+1)*f.size
	if remains < need {
		return 0, errors.New("hkdf: entropy limit reached")

Read any leftover from the buffer

	n := copy(p, f.buf)
	p = p[n:]

Fill the rest of the buffer

	for len(p) > 0 {
		f.expander.Reset()
		f.expander.Write(f.prev)
		f.expander.Write(f.info)
		f.expander.Write([]byte{f.counter})
		f.prev = f.expander.Sum(f.prev[:0])
		f.counter++

Copy the new batch into p

		f.buf = f.prev
		n = copy(p, f.buf)
		p = p[n:]

Save leftovers for next run

	f.buf = f.buf[n:]

	return need, nil
}

Expand returns a Reader, from which keys can be read, using the given pseudorandom key and optional context info, skipping the extraction step. The pseudorandomKey should have been generated by Extract, or be a uniformly random or pseudorandom cryptographically strong key. See RFC 5869, Section 3.3. Most common scenarios will want to use New instead.

func Expand(hash func() hash.Hash, pseudorandomKey, info []byte) io.Reader {
	expander := hmac.New(hash, pseudorandomKey)
	return &hkdf{expander, expander.Size(), info, 1, nil, nil}
}

New returns a Reader, from which keys can be read, using the given hash, secret, salt and context info. Salt and info can be nil.

func New(hash func() hash.Hash, secret, salt, info []byte) io.Reader {
	prk := Extract(hash, secret, salt)
	return Expand(hash, prk, info)