Tag Archives: pkijs

Escaping the Same-origin Policy for PKIjs

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PKIjs enables you to build rich PKI-aware applications inside of the browser, that said the browser implements a security policy called the Same-origin Policy that restricts the code running on the page from accessing resources served from other locations.

There is also a related capability that allows a remote server to state what origins can interact with it, this capability is called Cross-Origin Resource Sharing (CORS).

This background is important since the associated standards for PKI presume the client will be able to reach the Certificate Authority that issued the certificate to check revocation status (OCSP and CRL) as well as potentially fetch issuer certificates.

Unfortunately, the WebPKI CRL and OCSP responders do not set these headers, as such for a web page to fetch these resources an intermediate proxy is needed to enable your web application to access these servers.

There are lots of great resources on how to configure Nginx as forward-proxy. The problem is you do not want to be used as an open forward proxy. In this case, what we want to do is have a virtual host that will proxy request on behalf of the client to the appropriate origin server. For example sake, let’s say we have a host proxy.example.com. It is straightforward to configure Nginx as an open forward proxy and there are a lot of examples on the internet showing how to do this. That said we do not want to create an open proxy to be abused so we want some constraints:

  1. Only forward proxy to a whitelisted set of hosts,
  2. Only proxy specific methods (POST and GET),
  3. Rate limited the client to make it more difficult to abuse.

To enable this we want to be able to pass a query string to Nginx that contains the URL we want the request forwarded to, for example, https://proxy.example.com?url={urlencoded url}

A configuration that does this might look something like the following:

map "$request_method:$uri" $whitelist {
default "deny";
~^POST:/&url=http:/timestamp.globalsign.com "allow";
~^GET:/&url=http:/ocsp2.globalsign.com "allow";
}

limit_req_zone $binary_remote_addr zone=proxy:10m rate=5r/m;

server {
 resolver 8.8.8.8;
 listen 80 default_server;
 location / {
 limit_req zone=proxy burst=5;
 if ($whitelist = deny) { return 403; break; }
 if ($uri ~ \&url\=(https?):/([^\/]*)(.+)) { set $myproto $1; set $myhost $2; set $myuri $3; }
 if ($myproto = http) { proxy_pass http://$myhost$myuri; break; }
 if ($myproto = https) { proxy_pass https://$myhost$myuri; break; }
 }
}

NOTE: It seems that Nginx has a bug where when tokenizing it messes with the URL-encoded value stripping out some characters (including the /). To work around this in the above configuration we match on a single slash, this still works because this stripping appears consistent but does not effect the $uri value which is passed to the origin server.

NOTE: Unfortunately to accomplish the above we needed to use the if statement which has some downsides. If you know of how to accomplish the above without the use of them, or how to use less of them please let me know.

With this, the browser is now able to make requests to a limited set of hosts once every 12 seconds.

But what if you want this forward proxy(proxy.example.com) to exist in a different domain  from the application (pkijsapp.example.com)?  By default, the browser will not allow this but thankfully we can use  Cross-Origin Resource Sharing (CORS) to tell the clients they can send requests to our proxy, you do this by returning two additional headers to the client:

Access-Control-Allow-Origin: http://pkijsapp.example.com
Access-Control-Allow-Methods: POST, GET

With these two headers set, the browser will allow the pkijsapp.example.com application to send both POST and GETs to our proxy.

But what if we want to communicate with multiple servers? Unfortunately, the CORS specification only allows the server to set a single origin in this header. It recommends that cases that require access to multiple origins set the header dynamically. This, of course, requires the server to have knowledge of what the client needs and if you are building a Single Page Application it’s quite likely the server doesn’t have that context.

Thankfully it does support a wildcard so if you have multiple application domains you want your proxy to be accessible from you can specify *:

Access-Control-Allow-Origin: *

This approach works around the multiple origin limitations of CORS but has its own issues, for example:

  1. Your server is now an unauthenticated network proxy that can be abused,
  2. If the servers you include in the whitelist can also serve active content they become useful to an attacker,
  3. Your server now has knowledge of which certificates are being validated by the client,

Neither of these approaches is perfect but they both allow you to get the information necessary to validate certificates using PKIjs within the browser.

Ryan

The PKCS#12 standard needs another update

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PKCS#12 is the defacto file format for moving private keys and certificates around. It was defined by RSA and Microsoft in the late 90s and is used by Windows extensively. It was also recently added to KIMP as a means to export key material.

As an older format, it was designed with support for algorithms like MD2, MD5, SHA1, RC2, RC4, DES and 3DES. It was recently standardized by IETF RFC 7292 and the IETF took this opportunity to add support for SHA2 but have not made an accommodation for any mode of AES.

Thankfully PKCS #12 is based on CMS which does support AES (see RFC 3565 and RFC 5959). In theory, even though RFC 7292 doesn’t specify a need to support AES,  there is enough information to use it in an interoperable way.

Another standard used by PKCS#12 is PKCS #5 (RFC 2898), this specifies the mechanics of password-based encryption. It provides two ways to do this PBES1 and PBES2, more on this later.

Despite these complexities and constraints, we wanted to see if we could provide a secure and interoperable implementation of PKCS#12 in PKIjs since it is one of the most requested features. This post documents our findings.

Observations

Lots of unused options

PKCS#12 is the swiss army knife of certificate and key transport. It can carry keys, certificates, CRLs and other metadata. The specification offers both password and certificate-based schemes for privacy and integrity protection.

This is accomplished by having an outer “integrity envelope” that may contain many “privacy envelopes”.

When using certificates to protect the integrity of its contents, the specification uses the CMS SignedData message to represent this envelope.

When using passwords it uses an HMAC placed into its own data structure.

The use of an outer envelope containing many different inner envelopes enables the implementor to mix and match various types of protection approaches into a single file. It also enables implementers to use different secrets for integrity and privacy protection.

Despite this flexibility, most implementations only support the following combination:

  1. Optionally using the HMAC mechanism for integrity protection of certificates
  2. Using password-based privacy protection for keys
  3. Using the same password for privacy protection of keys

NOTE: OpenSSL was the only implementation we found that supports the ability to use a different password for the “integrity envelope” and  “privacy envelope”. This is done using the “twopass” option of the pkcs12 command.

The formats flexibility is great. We can envision a few different types of scenarios one might be able to create with it, but there appears to be no documented profile making it clear what is necessary to interoperate with other implementations.

It would be ideal if a future incarnation of the specification provided this information for implementers.

Consequences of legacy

As mentioned earlier there are two approaches for password based encryption defined in PKCS#5 (RFC 2989).

The first is called PBES1, according to the RFC :

PBES1 is recommended only for compatibility with existing
applications, since it supports only two underlying encryption
schemes, each of which has a key size (56 or 64 bits) that may not be
large enough for some applications.

The PKCS#12 RFC reinforces this by saying:

The procedures and algorithms
defined in PKCS #5 v2.1 should be used instead.
Specifically, PBES2 should be used as encryption scheme, with PBKDF2
as the key derivation function.

With that said it seems, there is no indication in the format on which scheme is used which leaves an implementor forced with trying both options to “just see which one works”.

Additionally it seems none of the implementations we have encountered followed this advice and only support the PBES1 approach.

Cryptographic strength

When choosing cryptographic algorithm one of the things you need to be mindful of is the minimum effective strength. There are differing opinions on what each algorithm’s minimum effective strength is at different key lengths, but normally the difference not significant. These opinions also change as computing power increases along with our ability to break different algorithms.

When choosing which algorithms to use together you want to make sure all algorithms you use in the construction offer similar security properties If you do not do this the weakest algorithm is the weakest link in the construction. It seems there is no guidance in the standard for topic, as a result we have found:

  1. Files that use weak algorithms protection of strong  keys,
  2. Files that use a weaker algorithm for integrity than they do for privacy.

This first point is particularly important. The most recently available guidance for minimum effective key length comes from the German Federal Office for Information Security, BSI. These guidelines, when interpreted in the context of PKCS #12, recommend that a 2048-bit RSA key protection happens using SHA2-256 and a 128-bit symmetric key.

The strongest symmetric algorithm specified in the PKCS #12 standard is 3DES which only offers an effective strength of 112-bits.  This means, if you believe the BSI, it is not possible to create a standards compliant PKCS#12 that offer the effective security necessary to protect a 2048-bit RSA key.

ANSI on the other hand, currently recommends that 100-bit symmetric key is an acceptable minimum strength to protect a 2048-bit RSA key (though this recommendation was made a year earlier than the BSI recommendation).

So if you believe ANSI, the strongest suite offered by RFC 7292 is strong enough to “adequately” protect such a key, just not a larger one.

The unfortunate situation we are left with in is that it is not possible to create a “standards compliant” PKCS12 that support modern cryptographic recommendations.

Implementations

OpenSSL

A while ago OpenSSL was updated to support AES-CBC in PKCS#8 which is the format that PKCS#12 uses to represent keys.  In an ideal world, we would be using AES-GCM for our interoperability target but we will take what we can get.

To create such a file you would use a command similar to this:

rmh$ openssl genrsa 2048|openssl pkcs8 -topk8 -v2 aes-256-cbc -out key.pem

Generating RSA private key, 2048 bit long modulus

……………+++

……………….+++

e is 65537 (0x10001)

Enter Encryption Password:

Verifying – Enter Encryption Password:
rmh$ cat key.pem

—–BEGIN ENCRYPTED PRIVATE KEY—–

…

…

…

—–END ENCRYPTED PRIVATE KEY—–

If you look at the resulting file with an ASN.1 parser you will see the file says the Key Encryption Key (KEK) is aes-256-cbc.

It seems the latest OpenSSL (1.0.2d) will even let us do this with a PKCS#12, those commands would look something like this:

openssl genrsa 2048|openssl pkcs8 -topk8 -v2 aes-256-cbc -out key.pem

openssl req -new -key key.pem -out test.csr

openssl x509 -req -days 365 -in test.csr -signkey key.pem -out test.cer

openssl pkcs12 -export -inkey key.pem -in test.cer -out test.p12 -certpbe AES-256-CBC -keypbe AES-256-CBC

NOTE: If you do not specify explicitly specify the certpbe and keypbe algorithm this version defaults to using pbewithSHAAnd40BitRC2-CBC to protect the certificate and pbeWithSHAAnd3-KeyTripleDES-CBC to protect the key.

RC2 was designed in 1987 and has been considered weak for a very long time. 3DES is still considered by many to offer 112-bits of security though in 2015 it is clearly not an algorithm that should still be in use.

Since it supports it OpenSSL should really be updated to use aes-cbc-256 by default and it would be nice if  support for AES-GCM was also added.

NOTE: We also noticed if you specify “-certpbe NONE and -keypbe NONE” (which we would not recommend) that OpenSSL will create a PKCS#12 that uses password-based integrity protection and no privacy protection.

Another unfortunate realization is OpenSSL uses an iteration count of 2048 when deriving a key from a password, by today’s standards is far too small.

We also noticed the OpenSSL output of the pkcs12 command does not indicate what algorithms were used to protect the key or the certificate, this may be one reason why the defaults were never changed — users simply did not notice:

rmh$ openssl pkcs12 -in windows10_test.pfx

Enter Import Password:

MAC verified OK

Bag Attributes

   localKeyID: 01 00 00 00

   friendlyName: {4BC68C1A-28E3-41DA-BFDF-07EB52C5D72E}

   Microsoft CSP Name: Microsoft Base Cryptographic Provider v1.0

Key Attributes

   X509v3 Key Usage: 10

Enter PEM pass phrase:

Bag Attributes

   localKeyID: 01 00 00 00

subject=/CN=Test/O=2/OU=1/[email protected]/C=<\xD0\xBE

issuer=/CN=Test/O=2/OU=1/[email protected]/C=<\xD0\xBE

—–BEGIN CERTIFICATE—–

…

…

—–END CERTIFICATE—–

Windows

Unfortunately, it seems that all versions of Windows (even Windows 10) still produces PKCS #12’s using pbeWithSHAAnd3-KeyTripleDES-CBC for “privacy” of keys and privacy of certificates it uses pbeWithSHAAnd40BitRC2-CBC. It then relies on the HMAC scheme for integrity.

Additionally it seems it only supports PBES1 and not PBES2.

Windows also uses an iteration count of 2048 when deriving keys from passwords which is far too small.

It also seems unlike OpenSSL, Windows is not able to work with files produced with more secure encryption schemes and ciphers.

PKIjs

PKIjs has two, arguably conflicting goals, the first of which is to enable modern web applications to interoperate with traditional X.509 based applications. The second of which is to use modern and secure options when doing this.

WebCrypto has a similar set of guiding principles, this is why it does not support weaker algorithms like RC2, RC4 and 3DES.

Instead of bringing in javascript based implementations of these weak algorithms into PKIjs we have decided to only support the algorithms supported by webCrypto (aes-256-cbc, aes-256-gcm with SHA1 or SHA2 using PBES2.

This represents a tradeoff. The keys and certificates and exported by PKIjs will be protected with the most interoperable and secure pairing of algorithms available but the resulting files will still not work in any version of Windows (even the latest Windows 10 1511 build).

The profile of PKCS#12 PKIjs creates that will work with OpenSSL will only do so if you the -nomacver option:

openssl pkcs12 -in pkijs_pkcs12.pfx -nomacver


This is because OpenSSL uses the older PBKDF1 for integrity protection and PKIjs is using the newer PBKDF2, as a result of this command integrity will not be checked on the PKCS#12.

With that caveat, here is an example of how one would generate a PKCS#12 with PKIjs.

Summary

Despite its rocky start, PKCS#12 is still arguably one of the most important cryptographic message formats. An attempt has been made to modernize it somewhat, but they did not go far enough.

It also seems OpenSSL has made an attempt to work around this gap by adding support for AES-CBC to their implementation.

Windows on the other hand still only appear to support only the older encryption construct with the weaker ciphers.

That said even when strong, modern cryptography are in use we must remember any password-based encryption scheme will only be as secure as the password that is used with it. As a proof point, consider these tools for PKCS#12 and PKCS#8 that make password cracking trivial for these formats.

We believe the storage and transport of encrypted private keys is an important enough topic that it deserves a modern and secure standard. With that in mind recommend the following changes to RFC 7292 be made:

  • Deprecate the use of all weaker algorithms,
  • Make it clear both AES-CBC and AES-GCM should be supported,
  • Make it clear what the minimal profile of this fairly complex standard is,
  • Require the support of PBES2,
  • Be explicit and provide modern key stretching guidance for use with PBKDF2,
  • Clarify how one uses PBMAC1 for integrity protection,
  • Require that the certificates and the keys are both protected with the same or equally secure mechanisms.

As for users, there are a few things you can do to protect yourself from the associated issues discussed here, some of which include:

  • Do not use passwords to protect your private keys. Instead generated symmetric keys or generated passwords of an appropriate lengths (e.g. “openssl rand -base64 32”),
  • When using OpenSSL always specify which algorithms are being used when creating your PKCS#12 files and double check those are actually the algorithms being used,
  • Ensure that the algorithms you are choosing to protect your keys offer a minimum effective key length equal to or greater than the keys you will protect,
  • Securely delete any intermediate copies of keys or inputs to the key generation or export process.

Ryan & Yury

 

Browser Bound Certificates

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The addition of WebCrypto to the browser enables a number of interesting client server opportunities that did not exist prior. One of which I think is interesting is what I have been calling browser bound certificates.

In-fact at least two such scenarios were included in the charter of the W3C WebCrypto working group – Document Signing and Encrypted Mail.

Now neither of these scenarios necessarily prescribe the use of X.509 certificates but considering signed PDF’s are the defacto-standard for signed documents and S/MIME is supported by Android, IOS, Windows Phones and Outlook it seems its not totally silly to say this approach has at least some merit.

To implement both of these one needs to have support for X.509 and its concepts within the browser, this is where Browser Bound certificates and PKIjs comes in. Imagine a client authenticating a user and over that authenticated session the client submits a certificate request bound to that session that is passed to an API on the server side that issues the client a X.509 certificate.

With that the client now has all the material that is necessary to sign and/or encrypt messages on the client side using the formats already in use. The web can interoperate with the desktop.

In our theoretical application need to take all the traditional precautions for both web and crypto-aware applications some of which include:

  1. Not mixing content from other domains,
  2. Loading the site and all of its resources over SSL,
  3. Segmenting the signing and verification code with postMessage,
  4. Using crypto primitives in safe ways,
  5. Using non-exportable keys,
  6. Keeping the keys short-lived.

But we can with these Browser Bound certificates build modern PKI aware applications that have great user experience that can even work without the server being present once provisioned.

Certificate Path Building in PKIjs

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Now that its possible to decode and verify the signature on X.509 certificates within the browser the natural question to ask is what can I do with that?

Well first off to build an interesting application you will need to have the ability to validate that a certificate is trusted the first step in doing that is building the certificate path associated with the certificate.

The defacto standard for path building libraries is the NIST PKITS tests our goal is to create a library that will be able to pass the sane tests from this suite (some are odd for sure).

This is a pretty high bar and will take some time. At the time of writing this blog post we pass 1-33 of this test suite in with flying colors these tests cover all of the basic certificate validation rules. We also think the library will pass all Policy Constraints and Name Constraints but more testing is needed to confirm.

So how does building a chain look like today with this library?

var certs = new Array();

// Load cert to be validated, its intermediates and root
for(var i = 0; i < cert_buffers.length; i++)
{
    var asn1 = org.pkijs.fromBER(cert_buffers[i]);
    certs.push(new org.pkijs.simpl.CERT({ schema: asn1.result }));
}

var crls = new Array();

// Load any CRLs we have
for(var i = 0; i < crl_buffers.length; i++)
{
    var asn1 = org.pkijs.fromBER(crl_buffers[i]);
    crls.push(new org.pkijs.simpl.CRL({ schema: asn1.result }));
}

var cert_chain_simpl = new org.pkijs.simpl.CERT_CHAIN({
    certs: certs,
    crls: crls
});

cert_chain_simpl.verify().then(
    function(result)
    {
        alert("Good result");
    },
    function(error)
    {
        alert("Error: " + error);
    }
);

The current incarnation of the API expects that the bag of certificates that is passed in will include all intermediates as well as all trust anchors. We will be changing this in a future release so that trust anchors are passed in another bag.

This will help ensure that the certificate inputs to be validated don’t contain anything that might accidentally result in the certificate being treated as valid when it should not be. With that said as it is currently structured we can begin developing automated testing which is great.

Note: Updated the post to indicate the goal is to pass the sane PKITS tests, some of which are not and some are not possible to pass in a web environment.