encoding/asn1: handle ASN1's string type BMPString

This code enables handling of ASN1's string type BMPString, used in some digital signatures.
Parsing code taken from golang.org/x/crypto/pkcs12.

Change-Id: Ibeae9cf4d8ae7c18f8b5420ad9244a16e117ff6b
GitHub-Last-Rev: 6945253514
GitHub-Pull-Request: golang/go#26690
Reviewed-on: https://go-review.googlesource.com/c/go/+/126624
Run-TryBot: Andrew Bonventre <andybons@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Bonventre <andybons@golang.org>

src/encoding/asn1/asn1.go

@@ -27,6 +27,7 @@ import (
 	"reflect"
 	"strconv"
 	"time"
+	"unicode/utf16"
 	"unicode/utf8"
 )
 
@@ -475,6 +476,29 @@ func parseUTF8String(bytes []byte) (ret string, err error) {
 	return string(bytes), nil
 }
 
+// BMPString
+
+// parseBMPString parses an ASN.1 BMPString (Basic Multilingual Plane of
+// ISO/IEC/ITU 10646-1) from the given byte slice and returns it.
+func parseBMPString(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
+}
+
 // A RawValue represents an undecoded ASN.1 object.
 type RawValue struct {
 	Class, Tag int
@@ -589,7 +613,7 @@ func parseSequenceOf(bytes []byte, sliceType reflect.Type, elemType reflect.Type
 			return
 		}
 		switch t.tag {
-		case TagIA5String, TagGeneralString, TagT61String, TagUTF8String, TagNumericString:
+		case TagIA5String, TagGeneralString, TagT61String, TagUTF8String, TagNumericString, TagBMPString:
 			// We pretend that various other string types are
 			// PRINTABLE STRINGs so that a sequence of them can be
 			// parsed into a []string.
@@ -691,6 +715,8 @@ func parseField(v reflect.Value, bytes []byte, initOffset int, params fieldParam
 				result, err = parseGeneralizedTime(innerBytes)
 			case TagOctetString:
 				result = innerBytes
+			case TagBMPString:
+				result, err = parseBMPString(innerBytes)
 			default:
 				// If we don't know how to handle the type, we just leave Value as nil.
 			}
@@ -759,7 +785,7 @@ func parseField(v reflect.Value, bytes []byte, initOffset int, params fieldParam
 	if universalTag == TagPrintableString {
 		if t.class == ClassUniversal {
 			switch t.tag {
-			case TagIA5String, TagGeneralString, TagT61String, TagUTF8String, TagNumericString:
+			case TagIA5String, TagGeneralString, TagT61String, TagUTF8String, TagNumericString, TagBMPString:
 				universalTag = t.tag
 			}
 		} else if params.stringType != 0 {
@@ -957,6 +983,9 @@ func parseField(v reflect.Value, bytes []byte, initOffset int, params fieldParam
 			// that allow the encoding to change midstring and
 			// such. We give up and pass it as an 8-bit string.
 			v, err = parseT61String(innerBytes)
+		case TagBMPString:
+			v, err = parseBMPString(innerBytes)
+
 		default:
 			err = SyntaxError{fmt.Sprintf("internal error: unknown string type %d", universalTag)}
 		}

src/encoding/asn1/asn1_test.go

@@ -6,6 +6,7 @@ package asn1
 
 import (
 	"bytes"
+	"encoding/hex"
 	"fmt"
 	"math"
 	"math/big"
@@ -1096,3 +1097,35 @@ func TestTaggedRawValue(t *testing.T) {
 		}
 	}
 }
+
+var bmpStringTests = []struct {
+	decoded    string
+	encodedHex string
+}{
+	{"", "0000"},
+	// Example from https://tools.ietf.org/html/rfc7292#appendix-B.
+	{"Beavis", "0042006500610076006900730000"},
+	// Some characters from the "Letterlike Symbols Unicode block".
+	{"\u2115 - Double-struck N", "21150020002d00200044006f00750062006c0065002d00730074007200750063006b0020004e0000"},
+}
+
+func TestBMPString(t *testing.T) {
+	for i, test := range bmpStringTests {
+		encoded, err := hex.DecodeString(test.encodedHex)
+		if err != nil {
+			t.Fatalf("#%d: failed to decode from hex string", i)
+		}
+
+		decoded, err := parseBMPString(encoded)
+
+		if err != nil {
+			t.Errorf("#%d: decoding output gave an error: %s", i, err)
+			continue
+		}
+
+		if decoded != test.decoded {
+			t.Errorf("#%d: decoding output resulted in %q, but it should have been %q", i, decoded, test.decoded)
+			continue
+		}
+	}
+}

src/encoding/asn1/common.go

@@ -37,6 +37,7 @@ const (
 	TagUTCTime         = 23
 	TagGeneralizedTime = 24
 	TagGeneralString   = 27
+	TagBMPString       = 30
 )
 
 // ASN.1 class types represent the namespace of the tag.