asn1: add big support.

Initially I wanted to minimise dependencies but it's become clear that
big int support in ASN.1 is a common need and that it should be part
of the core.

R=bradfitz
CC=golang-dev
https://golang.org/cl/4550063

src/pkg/asn1/asn1.go

@@ -20,6 +20,7 @@ package asn1
 // everything by any means.
 
 import (
+	"big"
 	"fmt"
 	"os"
 	"reflect"
@@ -88,6 +89,27 @@ func parseInt(bytes []byte) (int, os.Error) {
 	return int(ret64), nil
 }
 
+var bigOne = big.NewInt(1)
+
+// parseBigInt treats the given bytes as a big-endian, signed integer and returns
+// the result.
+func parseBigInt(bytes []byte) *big.Int {
+	ret := new(big.Int)
+	if len(bytes) > 0 && bytes[0]&0x80 == 0x80 {
+		// This is a negative number.
+		notBytes := make([]byte, len(bytes))
+		for i := range notBytes {
+			notBytes[i] = ^bytes[i]
+		}
+		ret.SetBytes(notBytes)
+		ret.Add(ret, bigOne)
+		ret.Neg(ret)
+		return ret
+	}
+	ret.SetBytes(bytes)
+	return ret
+}
+
 // BIT STRING
 
 // BitString is the structure to use when you want an ASN.1 BIT STRING type. A
@@ -425,6 +447,7 @@ var (
 	timeType             = reflect.TypeOf(&time.Time{})
 	rawValueType         = reflect.TypeOf(RawValue{})
 	rawContentsType      = reflect.TypeOf(RawContent(nil))
+	bigIntType           = reflect.TypeOf(new(big.Int))
 )
 
 // invalidLength returns true iff offset + length > sliceLength, or if the
@@ -639,6 +662,10 @@ func parseField(v reflect.Value, bytes []byte, initOffset int, params fieldParam
 	case flagType:
 		v.SetBool(true)
 		return
+	case bigIntType:
+		parsedInt := parseBigInt(innerBytes)
+		v.Set(reflect.ValueOf(parsedInt))
+		return
 	}
 	switch val := v; val.Kind() {
 	case reflect.Bool:

src/pkg/asn1/asn1_test.go

@@ -42,6 +42,33 @@ func TestParseInt64(t *testing.T) {
 	}
 }
 
+var bigIntTests = []struct {
+	in     []byte
+	base10 string
+}{
+	{[]byte{0xff}, "-1"},
+	{[]byte{0x00}, "0"},
+	{[]byte{0x01}, "1"},
+	{[]byte{0x00, 0xff}, "255"},
+	{[]byte{0xff, 0x00}, "-256"},
+	{[]byte{0x01, 0x00}, "256"},
+}
+
+func TestParseBigInt(t *testing.T) {
+	for i, test := range bigIntTests {
+		ret := parseBigInt(test.in)
+		if ret.String() != test.base10 {
+			t.Errorf("#%d: bad result from %x, got %s want %s", i, test.in, ret.String(), test.base10)
+		}
+		fw := newForkableWriter()
+		marshalBigInt(fw, ret)
+		result := fw.Bytes()
+		if !bytes.Equal(result, test.in) {
+			t.Errorf("#%d: got %x from marshaling %s, want %x", i, result, ret, test.in)
+		}
+	}
+}
+
 type bitStringTest struct {
 	in        []byte
 	ok        bool

src/pkg/asn1/common.go

@@ -132,6 +132,8 @@ func getUniversalType(t reflect.Type) (tagNumber int, isCompound, ok bool) {
 		return tagUTCTime, false, true
 	case enumeratedType:
 		return tagEnum, false, true
+	case bigIntType:
+		return tagInteger, false, true
 	}
 	switch t.Kind() {
 	case reflect.Bool:

src/pkg/asn1/marshal.go

@@ -5,6 +5,7 @@
 package asn1
 
 import (
+	"big"
 	"bytes"
 	"fmt"
 	"io"
@@ -125,6 +126,43 @@ func int64Length(i int64) (numBytes int) {
 	return
 }
 
+func marshalBigInt(out *forkableWriter, n *big.Int) (err os.Error) {
+	if n.Sign() < 0 {
+		// A negative number has to be converted to two's-complement
+		// form. So we'll subtract 1 and invert. If the
+		// most-significant-bit isn't set then we'll need to pad the
+		// beginning with 0xff in order to keep the number negative.
+		nMinus1 := new(big.Int).Neg(n)
+		nMinus1.Sub(nMinus1, bigOne)
+		bytes := nMinus1.Bytes()
+		for i := range bytes {
+			bytes[i] ^= 0xff
+		}
+		if len(bytes) == 0 || bytes[0]&0x80 == 0 {
+			err = out.WriteByte(0xff)
+			if err != nil {
+				return
+			}
+		}
+		_, err = out.Write(bytes)
+	} else if n.Sign() == 0 {
+		// Zero is written as a single 0 zero rather than no bytes.
+		err = out.WriteByte(0x00)
+	} else {
+		bytes := n.Bytes()
+		if len(bytes) > 0 && bytes[0]&0x80 != 0 {
+			// We'll have to pad this with 0x00 in order to stop it
+			// looking like a negative number.
+			err = out.WriteByte(0)
+			if err != nil {
+				return
+			}
+		}
+		_, err = out.Write(bytes)
+	}
+	return
+}
+
 func marshalLength(out *forkableWriter, i int) (err os.Error) {
 	n := lengthLength(i)
 
@@ -334,6 +372,8 @@ func marshalBody(out *forkableWriter, value reflect.Value, params fieldParameter
 		return marshalBitString(out, value.Interface().(BitString))
 	case objectIdentifierType:
 		return marshalObjectIdentifier(out, value.Interface().(ObjectIdentifier))
+	case bigIntType:
+		return marshalBigInt(out, value.Interface().(*big.Int))
 	}
 
 	switch v := value; v.Kind() {
@@ -420,6 +460,9 @@ func marshalField(out *forkableWriter, v reflect.Value, params fieldParameters)
 
 	if v.Type() == rawValueType {
 		rv := v.Interface().(RawValue)
+		if rv.Class == 0 && rv.Tag == 0 && len(rv.Bytes) == 0 && params.optional {
+			return
+		}
 		err = marshalTagAndLength(out, tagAndLength{rv.Class, rv.Tag, len(rv.Bytes), rv.IsCompound})
 		if err != nil {
 			return