crypto/x509: invalid authority key identifier parsing older root "Starfield Class 2 Certification Authority"

[joeshaw]

What version of Go are you using (go version)?

$ go version
go version devel go1.17-7a5e704 Mon Jun 21 14:58:02 2021 +0000 darwin/amd64

Does this issue reproduce with the latest release?

It does not reproduce with Go 1.16. It only occurs on tip on or after 51ff3a6, when the certificate parser was rewritten.

What operating system and processor architecture are you using (go env)?

darwin/amd64, macOS 11.4. It has also been reproduced on Linux.

What did you do?

Given the sample program:

package main

import (
	"crypto/x509"
	"encoding/pem"
	"io/ioutil"
	"log"
	"os"
)

func main() {
	d, err := ioutil.ReadFile(os.Args[1])
	if err != nil {
		log.Fatal(err)
	}

	block, _ := pem.Decode(d)
	_, err = x509.ParseCertificate(block.Bytes)
	if err != nil {
		log.Fatal(err)
	}
}

And the "Starfield Class 2 Certification Authority" certificate: https://crt.sh/?q=d73494e3446b02167573b3cde3ae1c8584ac26e15e45ac3ec0326708425d90fb

Running this program on Go 1.16.5 produces no error. On tip (7a5e704) following 1.17beta1, you get the error x509: invalid authority key identifier.

What did you expect to see?

The program should terminate without error.

What did you see instead?

x509: invalid authority key identifier

[joeshaw]

Old parser code:

type authKeyId struct {
        Id []byte `asn1:"optional,tag:0"`
}

// RFC 5280, 4.2.1.1
var a authKeyId
if rest, err := asn1.Unmarshal(e.Value, &a); err != nil {
	return nil, err
} else if len(rest) != 0 {
	return nil, errors.New("x509: trailing data after X.509 authority key-id")
}
out.AuthorityKeyId = a.Id

New parser code:

// RFC 5280, 4.2.1.1
val := cryptobyte.String(e.Value)
var akid cryptobyte.String
if !val.ReadASN1(&akid, cbasn1.SEQUENCE) {
	return errors.New("x509: invalid authority key identifier")
}
if !akid.ReadASN1(&akid, cbasn1.Tag(0).ContextSpecific()) {
	return errors.New("x509: invalid authority key identifier")
}
out.AuthorityKeyId = akid

This certificate does have a weird structure compared to modern certificates in the wild. OpenSSL renders it as:

X509v3 Authority Key Identifier:
    DirName:/L=ValiCert Validation Network/O=ValiCert, Inc./OU=ValiCert Class 2 Policy Validation Authority/CN=http://www.valicert.com//emailAddress=info@valicert.com
    serial:01

[toothrot]

/cc @rolandshoemaker @FiloSottile

[rolandshoemaker]

Blergh. This is caused by the new parser being somewhat stricter about what it accepts and sort of a bug. Whereas the old parser would allow SEQUENCEs to lack expected fields, and contain unexpected trailing fields (the latter of which is a perfectly reasonable thing to do in ASN.1).

We're expecting an optional field to be present (because it's the only actual field that is actually used) and ignoring other optional fields, really we shouldn't do the former but should continue to do the latter. I'll write up a fix for this.

[gopherbot]

Change https://golang.org/cl/331689 mentions this issue: crypto/x509: don't fail on optional auth key id fields