Implementer's notes

The W3C's Web Security Context Working Group have just released the Last Call version of its "User Interface Guidelines" document, which is a set of recommendations for the security related UI in Web User Agents.

This specification deals with the trust decisions that users must make online, and with ways to support them in making safe and informed decisions where possible.

This document specifies user interactions with a goal toward making security usable, based on known best practice in this area. Subsequent testing of this specification will include conformance, interoperability, and usability testing.

If you want to comment on the document you are welcome to do so:

The W3C Membership and other interested parties are invited to review the document and send comments to public-usable-authentication@w3.org (with public archive) through 15 September 2008. We appreciate if comments follow these guidelines for writing good issues.

Last week at the W3C's Web Security Context Working Group's meeting at Opera HQ here in Oslo we discussed what should be the criteria for displaying the Extended Validation (EV) indicator, or the Augmented Assurance (AA) indicator, as the WG has decided to call this technology.

As I have said earlier, we are of the opinion that the EV indicator should not be displayed unless all content is loaded from EV servers.

The opposing view is that, so long as all content is loaded over secure connections, the displayed document is what the author of the main document intended (bugs, vulnerabilities, and all), and that it is therefore only necessary to verify that the main document is served from an EV server, as this will provide the information necessary to identify the author.

The decision of the WG was in favour of the less restrictive position: if an AA/EV document loads all resources over strongly TLS-protected connections, then the document can be displayed with an AA/EV indicator.

In the interest of providing a common user experience with respect to EV we have decided to follow this recommendation, and today's Kestrel snapshot include this policy change.

We have, however, left the old logic in place, controlled by a preference that can be updated remotely. This permits us to quickly change to a stricter mode if the consensus about what constitutes an AA/EV site changes in the future.

As a consequence of this policy change a large number of sites, such as Paypal, with mixed EV and non-EV content will now get the "Green Bar" in Opera:

Today we're releasing the first Kestrel Weekly with Extended Validation (EV) support.

As I've written before, Extended Validation is a new way to indicate, in a Web site certificate, that the identity of the company behind the Web site has been verified according to a rigorous standard. The guidelines for this process is defined by the CA/Browser forum, which is a group consisting of many certificate issuers (CAs), browser vendors, and others.

We are now ready to start public alpha testing.

So, how does it work?

When a Web site is recognized as an EV site, Opera will change the background color of the security toolbar to green, instead of yellow as it is for normal secure Web sites. There will be a few further adjustments done in this area, the most major being that we have stopped displaying the Organization Name field and country in the security toolbar for non-EV sites, because this name might not have been properly verified for non-EV certificates.


What is required to be permitted to issue EV certificates?

Root CAs that want to issue EV certificates must first pass a rigorous audit to prove that they have the proper procedures in place to identify accurately the company requesting a certificate, and that this company is in control of the given server. Further, an agreement between the Root CA and the browser vendor is required before the browser can recognize the CA's certificates as EV certificates.

How does Opera know it is an EV certificate?

All EV certificates contain an identifier (called an EV-OID, actually a Certificate Policy identifier). The CA will insert this identifier when it has verified all the information. Only certificates issued from specific Roots are allowed to use these identifiers.

How does Opera know that a Root is allowed to issue EV certificates?

Each Opera instance will regularly download a digitally-signed list identifying the CA, its certificate, and which EV-OID(s) it is permitted to use (different CAs can use different EV-OIDs). When Opera verifies a certificate issued from this Root, it will "sift" through the data where the EV-OIDs are stored to see if it contains one of the EV-OIDs recognized for this Root. If such an EV-OID is present, Opera will proceed to check if the other requirements (see below) for saying the Web site is an EV site are fulfilled.

What is required by a Web site to be considered an EV site?

The primary requirement is, of course, that it is eligible to get an EV certificate, and that it has purchased and installed one. The certificate, and all the intermediate certificates must (of course) still be valid, and cannot have been revoked by the issuer (we now check both OCSP and CRLs). Further, there must be no problems verifying the certificate; that is, there must be no unknown issuers, no mismatch of server name, and no weak encryption.

There are also a couple of specific encryption key requirements: The Web site's key should (for RSA) be at least 2048 bits long, but non-root certificates which expire before 23:59:59 UTC/GMT December 31, 2010 MAY use RSA keys of at least 1024 bits. Except for the Root key, which (for RSA) must be at least 2048 bit long, all signing keys must be at least as long as the key of the certificate it is used to sign.

And lastly, if the Web site includes content from other Web servers, those servers must *also* be hosting EV-sites. This is a point at which we are much stricter than the other EV-capable browsers currently. As I said earlier: "It ain't EV 'til it is EV, all EV".

The reason for this requirement is that these other servers may provide content that directly controls the appearance of the entire site, either through frames or external Ecmascript embedded in the page (and the latter has full control of the site's content).

Considering how many click-wrap licensed Web services (such as for Web statistics) there are, it is not likely that the Web designers signing their sites up for these services will have done anything close to good-enough legal identity check of the service. Also, don't forget all the liability disclaimers such contracts include. It is impossible to check the contracts, but we can check that the sites have been able to get an EV certificate.

EV is intended to give you better information about who provided the content with which you are viewing and interacting. If not all the servers providing the content are providing this kind of information, do you really know who provided the important part of the content? Our answer is that you don't, therefore such sites, which at the moment include Paypal.com, will not get the EV indication unless they either remove all references to the non-EV content, or those providers upgrade to provide EV content, as well.

Which Root CAs are currently recognized as issuing EV certificates?

During the test period starting with this release we have provisonally configured three Roots as EV Roots (in alphabetic order)

• Entrust's EV Root
  
• GlobalSign's EV Root
  
• VeriSign's G5 Class 3 (EV) Root
  

Other CAs and Roots may be added later.

About the online repository

As mentioned above, Opera retrieves information about which CAs are recognized as EV-issuers from an online repository. This repository, hosted at https://certs.opera.com/, contains a digitally-signed file listing all the EV issuers and a (separately) signed list of the EV-OIDs they are using.

This repository is refreshed every 6 hours, so we can start updating clients very quickly. At present Opera will check this repository immediately when it starts, but the final version will only check once a week at the same time as we check for other updates.

Examples of EV sites

• https://www.dnbnor.no/
  
• https://nettbank.sparebank1.no/
  
• https://healthcare.searchamerica.com/
  
• https://taxcube.kpmgabogados.es/
  
• https://meine.deutsche-bank.de/mod/WebObjects/dbpbc.woa?lang=en
  
• https://ev.globalsign.com/
  

The most important security pillar in SSL/TLS is the strength of the method used when agreeing on the encryption keys. If the method used for this is inherently weak, then it doesn't really matter how secure the rest of the encryption we are using really is.

The primary method used for this is the Public Key encryption methods, the most famous of which is RSA. These methods work by breaking the encryption key into two parts; one secret/private key and one public which is known by all (certificates are used to confirm who the public key belongs to). The relationship between these is that a message encrypted by one key can only be decrypted by the other. This means that anything encrypted with the private key can be read by everyone, but one will know that only the secret key could have created the message, and this property is used in digital signatures. Similarly a message encrypted with the public key can only be read using the private key to decrypt, and this is used in SSL/TLS to agree on the encryption keys used for the connection.

The security of this step depends on how difficult it is to break the encryption used to protect the encryption keys. For these methods breaking the private key means you can break all messages protected by it, and that when you have broken the key you can impersonate the owner. The difficulty of breaking an RSA key is generally determined by how long the key is. Given present technology, I estimate that the work doubles for every 25-30 bits that are added to the key length (as opposed to work doubling for every additional bit when attacking keys for a symmetric method, like AES). At present, 640 bit RSA keys have been broken in 5 months using less than 100 workstations, but even the commonly used 1024 bit keys are threatened now and are not recommended for messages that need to be secure past 2010. As a result, Opera will warn (and is the only browser that does) when these keys are shorter than 900 bits.

A way to make exposure of past messages more difficult is to change the key used to protect the messages very often. This means that there are more keys to break if you want access to all messages. With sufficiently strong private keys, massive attack becomes infeasible and even attacking a single key becomes impracticable.

In SSL/TLS these kind of rapidly changing keys are implemented using the Public Key method, also known as the Diffie-Hellman (DH) key agreement. The system most commonly works by having the server send a temporary (ephemeral) DH key (or DHE key) to the client, which then confirms the authenticity of the key by digitally signing it with its RSA key. The client then uses this DHE key to agree with the server on the encryption keys. Given that these keys are changed every few minutes there will be hundreds of keys used by a server every day, making the task of breaking the keys infeasible if the tasks take even a short time. But best of all, even if the RSA key of the site is broken, the attacker won't get the secret parts of the DHE keys. To be able to do this, they would have to break each key.

Given that the RSA and Diffie-Hellman algorithms are based on the same math, they are equally strong for a given key-size. This means that to provide the same level of security for a given connection as the RSA key, the DHE key has to be as long as the RSA keys. This is where an increasing number of secure servers fall short.

Both RSA and DHE secret key operations are very time consuming and therefore reduce the number of connections a server can handle. While most sites are using RSA keys that are sufficiently long, the fact that there are lots of DHE keys have led some vendors to mistakenly think that they can reduce the length of the DH keys so that more transactions can be performed. Most of these reduced keys are either 512 bits or 768 bits long (which Opera warns about), but I have actually seen servers sending 256(!) bit DHE keys (Hint: I estimate that these can be broken in minutes or hours).

What these vendors seem to forget is that not all attackers are interested in every transaction performed at a site. An attacker might just be interested in one individual visiting the site, and in such cases the size of the DHE key becomes significant. If the key is too short it may become economically feasible for the attacker to break the DHE key. This is why Opera also warns about weak DHE keys, which are shorter than 900 bits.

As reports about weak DHE keys seem to increase, I found it necessary to take a few steps to counter this problem.

The first step was to ask the TLS Work Group to specify clearer how these keys are created. In addition, specify what steps a client should take to ensure the DHE keys are adequately secure. This is currently being worked on for TLS 1.2.

The second step was to evaluate the size of the DHE key when we receive it. If the key is shorter than 1024 bits, we close down the connection after sending an Insufficient Security (71) fatal error to the server, and automatically try to establish a new connection where the DHE methods are listed as less preferred than they normally are. By doing this we will most likely be able to establish a sufficiently secure connection using the RSA-only methods instead. If the server still selects a DHE method, then a warning may be displayed if necessary. This extra round trip will usually not take extra time because it will be handled as part of our usual TLS feature testing of the server (watch this space for news about that). A known issue we are working to fix is that for mail servers where this dialog have been shown previously, this will not take effect until the second time you check email, and that the first attempt will fail.

The end result is that (normally) you will no longer see the weak public key warning, except if the site is using a weak key in the certificate. We can't do anything about the certificate keys because the size of that key is selected directly by the Webmaster. If you get any of those warnings, please notify the webmaster!

Further reading about crypto on Wikipedia:

Public Key Cryptography
RSA
Diffie-Hellman (DH)
SSL and TLS
About Keysizes

In the past couple of weeks I have been sewing up most of the loose threads in Opera's support for Extended Validation (EV) certificates, and have actually started testing something that resembles the final system.

The work to add Extended Validation has been going on for a while, the basic functionality was implemented last year, but there hasn't been time to build the surrounding infrastructure until this year.

Now, we are finally starting to see the end of the road.

However, I have started to notice something that could become a problem: Not all EV sites, such as Verisign's and PayPal's sites, are coded the way I think they should be coded, or the way Opera's EV support need to be able to recognize them as EV sites.

The sites themselves and the content they serve are not the problem. The thirdparty content they have chosen to include on their site is; Content that is NOT served from EV servers. Apparently, MSIE 7 does not take this external content into consideration when deciding to show the EV indication.

One of the most prominent examples of this third-party content is Google Analytics, which is added to a web page by an external Javascript from a non-EV secure server hosted by Google. Since Google's servers are not using EV certificates, Opera's EV support does not consider the site to be an EV site.

This is due to how Opera's security evaluation system works. Each URL loaded is evaluated based on which protocol was used, what kind of encryption, keylengths, and whether or not the certificate validated correctly. All of this is summed up as a Level in the range of 0 to 3 as follows

• 0: No encryption (example, plain HTTP)
  
• 1: Weak encryption (40 and 56 bit symmetric methods), weak keys (less than 900 bits RSA/DH/DSA), certificate errors, authentiction only. Additionally a combination of problems that would result in Level 2 security being displayed if only one of them were encountered.
  
• 2: One, and only one, of these: medium strength keys (900-999 bit RSA/DH/DSA) or failure to get a valid OCSP response. A combination of both results in Level 1 being chosen.
  
• 3: Only when using 128 bit encryption, 1000 bit or more RSA/DH/DSA keys, no certificate validation problems.
  

EV is implemented as Level 4 in this ranking, and a Level 3 resource is only escalated to Level 4 if it contains the necessary EV identificator.

For a group of URLs, such as a web page with redirects, images, CSS, scripts, frames, and so on, the URL with the weakest security level determines the security level for the entire document. This means that a web page with a single unencrypted 1-by-1 image gets Level 0, and that an EV website which includes a 1-by-1 image from a Level 3 non-EV site does not get the EV status. The only exception to this rule is that, on certain conditions, the first request may be a HTTP to HTTPS redirect.

The reason for this design is simple:

• It is difficult, and in most cases impossible, for a web browser to determine how sensitive a particular element of a page is.
  
• Some elements can affect the meaning of the site (for example, a big image, a script, or a frame).
  
• In some cases, such as unencrypted requests, analyzing that traffic may reveal what the user is working on (for example, evaluating a stock transaction).
  

One can make the argument that since the page is secure we can assume that the owners of the site know what they are doing, and that any use of less secure resources is intentional and does not represent a security problem. That assumption is wrong. They may know what they are doing, and have no intention of lowering the security level, but they can still make a mistake somewhere. In at least two cases I have seen, misconfiguration of proxies or redirects caused content that was not secure to be included in otherwise secure banking/payment pages. In one of the cases this content was an external JavaScript used in a payment page, and an attacker could have gotten full access to the secure page if he had changed the JavaScript in transit.

With respect to EV one can make the same argument, but it is flawed in this case, too:

• The primary point of using an EV certificate on a site is to know the identity of the site's owner.
  
• The EV site's owner may or may not know who owns a non-EV third-party site, the client has no way of determining that. We do not know, and cannot know, if the agreement is just a click-wrap or the result of months long lawyer negotiations.
  
• In several of the observed cases the third-party content was external JavaScript, which means the third-party is essentially granted full access to manipulate the site's content, and all of it.
  
• By allowing non-EV third-party content in a EV site it becomes possible to set up EV "redirector" sites that let non-EV sites look like they are using EV by embedding their content in a frame.
  

Part of the reason for creating Extended Validation certificates was raising the bar for identification on the Internet, so that is what we will do: Raise the bar.

When Opera ships a version with EV, probably in Kestrel, a site will not be considered as having EV unless all resources used in the rendered page are using Extended Validation certificates.

"It ain't EV 'til it's EV, all EV"