x/crypto: Add pkcs12 package for reading pkcs12 data

Package pkcs12 provides some Go implementations of PKCS#12.
This implementation is distilled from https://tools.ietf.org/html/rfc7292 and
referenced documents. It is intented for decoding P12/PFX-stored certificate+key
for use with the crypto/tls package.

Package includes @dgryski's RC2 implementation as a sub package as requested in
https://github.com/golang/go/issues/10621.

Change-Id: I78401241e39cd0099e9082a3a227cf0a3a36e6d1
Reviewed-on: https://go-review.googlesource.com/11986
Reviewed-by: Adam Langley <agl@golang.org>
Run-TryBot: Adam Langley <agl@golang.org>

pkcs12/mac.go

@@ -0,0 +1,45 @@
+// Copyright 2015 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 pkcs12
+
+import (
+	"crypto/hmac"
+	"crypto/sha1"
+	"crypto/x509/pkix"
+	"encoding/asn1"
+)
+
+type macData struct {
+	Mac        digestInfo
+	MacSalt    []byte
+	Iterations int `asn1:"optional,default:1"`
+}
+
+// from PKCS#7:
+type digestInfo struct {
+	Algorithm pkix.AlgorithmIdentifier
+	Digest    []byte
+}
+
+var (
+	oidSHA1 = asn1.ObjectIdentifier([]int{1, 3, 14, 3, 2, 26})
+)
+
+func verifyMac(macData *macData, message, password []byte) error {
+	if !macData.Mac.Algorithm.Algorithm.Equal(oidSHA1) {
+		return NotImplementedError("unknown digest algorithm: " + macData.Mac.Algorithm.Algorithm.String())
+	}
+
+	key := pbkdf(sha1Sum, 20, 64, macData.MacSalt, password, macData.Iterations, 3, 20)
+
+	mac := hmac.New(sha1.New, key)
+	mac.Write(message)
+	expectedMAC := mac.Sum(nil)
+
+	if !hmac.Equal(macData.Mac.Digest, expectedMAC) {
+		return ErrIncorrectPassword
+	}
+	return nil
+}