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/bmp-string.go

@@ -0,0 +1,50 @@
+// 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 (
+	"errors"
+	"unicode/utf16"
+)
+
+// bmpString returns s encoded in UCS-2 with a zero terminator.
+func bmpString(s string) ([]byte, error) {
+	// References:
+	// https://tools.ietf.org/html/rfc7292#appendix-B.1
+	// http://en.wikipedia.org/wiki/Plane_(Unicode)#Basic_Multilingual_Plane
+	//  - non-BMP characters are encoded in UTF 16 by using a surrogate pair of 16-bit codes
+	//	  EncodeRune returns 0xfffd if the rune does not need special encoding
+	//  - the above RFC provides the info that BMPStrings are NULL terminated.
+
+	ret := make([]byte, 0, 2*len(s)+2)
+
+	for _, r := range s {
+		if t, _ := utf16.EncodeRune(r); t != 0xfffd {
+			return nil, errors.New("pkcs12: string contains characters that cannot be encoded in UCS-2")
+		}
+		ret = append(ret, byte(r/256), byte(r%256))
+	}
+
+	return append(ret, 0, 0), nil
+}
+
+func decodeBMPString(bmpString []byte) (string, error) {
+	if len(bmpString)%2 != 0 {
+		return "", errors.New("pkcs12: odd-length BMP string")
+	}
+
+	// strip terminator if present
+	if l := len(bmpString); l >= 2 && bmpString[l-1] == 0 && bmpString[l-2] == 0 {
+		bmpString = bmpString[:l-2]
+	}
+
+	s := make([]uint16, 0, len(bmpString)/2)
+	for len(bmpString) > 0 {
+		s = append(s, uint16(bmpString[0])<<8+uint16(bmpString[1]))
+		bmpString = bmpString[2:]
+	}
+
+	return string(utf16.Decode(s)), nil
+}