crypto/x509: Add certificate signature request (CSR) support.

This change adds support for parsing and serialisation of PKCS #10,
certificate signature requests.

LGTM=agl
R=golang-codereviews, agl
CC=agl, golang-codereviews, nick
https://golang.org/cl/49830048

src/pkg/crypto/x509/pkix/pkix.go

@@ -30,6 +30,13 @@ type AttributeTypeAndValue struct {
 	Value interface{}
 }
 
+// AttributeTypeAndValueSET represents a set of ASN.1 sequences of
+// AttributeTypeAndValue sequences from RFC 2986 (PKCS #10).
+type AttributeTypeAndValueSET struct {
+	Type  asn1.ObjectIdentifier
+	Value [][]AttributeTypeAndValue `asn1:"set"`
+}
+
 // Extension represents the ASN.1 structure of the same name. See RFC
 // 5280, section 4.2.
 type Extension struct {

src/pkg/encoding/asn1/asn1.go

@@ -23,6 +23,7 @@ import (
 	"fmt"
 	"math/big"
 	"reflect"
+	"strconv"
 	"time"
 )
 
@@ -197,6 +198,19 @@ func (oi ObjectIdentifier) Equal(other ObjectIdentifier) bool {
 	return true
 }
 
+func (oi ObjectIdentifier) String() string {
+	var s string
+
+	for i, v := range oi {
+		if i > 0 {
+			s += "."
+		}
+		s += strconv.Itoa(v)
+	}
+
+	return s
+}
+
 // parseObjectIdentifier parses an OBJECT IDENTIFIER from the given bytes and
 // returns it. An object identifier is a sequence of variable length integers
 // that are assigned in a hierarchy.

src/pkg/encoding/asn1/asn1_test.go

@@ -232,6 +232,10 @@ func TestObjectIdentifier(t *testing.T) {
 			}
 		}
 	}
+
+	if s := ObjectIdentifier([]int{1, 2, 3, 4}).String(); s != "1.2.3.4" {
+		t.Errorf("bad ObjectIdentifier.String(). Got %s, want 1.2.3.4", s)
+	}
 }
 
 type timeTest struct {