Add whatsappmulti buildflag for whatsapp with multidevice support (whatsapp)

vendor/github.com/Rhymen/go-whatsapp/crypto/hkdf/hkdf.go

@@ -0,0 +1,47 @@
+/*
+HKDF is a simple key derivation function (KDF) based on
+a hash-based message authentication code (HMAC). It was initially proposed by its authors as a building block in
+various protocols and applications, as well as to discourage the proliferation of multiple KDF mechanisms.
+The main approach HKDF follows is the "extract-then-expand" paradigm, where the KDF logically consists of two modules:
+the first stage takes the input keying material and "extracts" from it a fixed-length pseudorandom key, and then the
+second stage "expands" this key into several additional pseudorandom keys (the output of the KDF).
+*/
+package hkdf
+
+import (
+	"crypto/sha256"
+	"fmt"
+	"golang.org/x/crypto/hkdf"
+	"io"
+)
+
+/*
+Expand expands a given key with the HKDF algorithm.
+*/
+func Expand(key []byte, length int, info string) ([]byte, error) {
+	var h io.Reader
+	if info == "" {
+		/*
+			Only used during initial login
+			Pseudorandom Key is provided by server and has not to be created
+		*/
+		h = hkdf.Expand(sha256.New, key, []byte(info))
+	} else {
+		/*
+			Used every other time
+			Pseudorandom Key is created during kdf.New
+			This is the normal that crypto/hkdf is used
+		*/
+		h = hkdf.New(sha256.New, key, nil, []byte(info))
+	}
+	out := make([]byte, length)
+	n, err := io.ReadAtLeast(h, out, length)
+	if err != nil {
+		return nil, err
+	}
+	if n != length {
+		return nil, fmt.Errorf("new key to short")
+	}
+
+	return out, nil
+}