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Posts tagged with "Not In Kestrel"

New^W NOT in Kestrel #5: As many certificate warnings (if you don't want them)

Friday, 21. December 2007, 16:01:34

SSL, Not In Kestrel, browsers, Opera ...

As I explained in "NOT in Kestrel #4", the certificate is the passport of the website. What was not significatly mentioned in that article is that if there is any non-fatal problem with the verification of the certificate, or with other related information, Opera (and other clients) will display
a certificate warning.

Problems that cause certificate warnings to be displayed can for example be:

The certificate issuer is unknown (and it is not possible to discover a link to a known Root).
The certificate is expired
The name of the server does not match any of the servers named in the
certificate.
Weak encryption keys (now only public keys)

These warnings are displayed the first time in a session that you connect to a given server, and would not be displayed again for the rest of the session if the user accepted the certificate.

This has (understandably) caused some irritation when a user is frequently visiting a site causing such warnings to be displayed, and there has been frequent requests to be able to accept such certificates more permanently.

I have been, and still am, skeptical to such an ability, because I think a serious and secure website should not trigger security warnings.

I have, however, decided to meet the requests halfway. In Opera 9.50 it is now possible from the security panel of the certificate warning to "permanently" accept a certificate for the given SSL/TLS server (and port). Although accepted by the user, Opera will (same as before) not display a padlock for these sites because Opera has not been able to properly establish the server's credentials.

The acceptance isn't, however, quite "permanent" (therefore the quotes). A certificate that has not expired will be accepted until it expires (at which time the webmaster SHOULD replace it), and for periods of 90 days at a time after expiration.

Enjoy, but use with caution.

Seasons Greetings! See you in the new year.

2 comments

New^W NOT in Kestrel #4: As many warnings about unknown certificate issuers

Friday, 21. December 2007, 15:18:36

Not In Kestrel, browser statistics, protocols, TLS ...

One of the most important parts of SSL/TLS, as it is used in browsers, is the use of certificates to identify the website you are visiting, and to identify the keys used to agree on the encryption keys used for the connection.

The certificate is the website's passport, and like a passport it is issued by an authority trusted by the people checking them. For passports this is the government of a country, for certificates it is a trusted Certificate Authority (CA).

As with passports, there are a lot of things that have to be checked to make sure the certificate is not forged.

Each certificate contain, among other things the name of who the certificate is issued to (the website); the public key of the website; who issued it (the CA); and a digital signature of the data in the certificate, which is signed by the CA.

To verify the signature the browser must use the public key of the CA to decrypt the signature (which could only be created by using the associated private key), and compare the result with the calculated "checksum" of the certificate. The public key is stored in another certificate issued to the CA - the CA certificate.

There are two types of CA certificates, the ones that are signed by yet another CA, called Intermediate CA certificates, and certificates signed by the CA itself, which are known as the Root Certificates or Self Signed Certificates.

The Intermediates CA certificates are verified using the public key of the signing CA, while the Root CA certificates are verified using the public key in the certificate itself.

The Root CA certificate is a special case. By itself it only verifies that somebody claiming to be the named issuer issued the Root Certificate, and by extension vouches for the named identity in the certificates issued from it. But that does not carry any real assurances.

To be sure that the Root CA is valid the signature on the certificate have to be verified by a trusted copy of the Root Certificate, stored in the client's list of such certificates. These certificates are usually shipped with the client, and are accepted and trusted by the vendor to be reasonably careful about who they issue certificates to and how they store the private key. The user can elect to not trust some of these roots, and may add his own list of certificates.

Only a certificate and its associated certificate chain that can be verified and traced back to a Root in the Trusted Root store on the client is accepted automatically. If a certificate cannot be verified because the signature does not validate this is a fatal error; if we can't trace the certificate to a known root we display a certificate warning.

A problem often encountered is that a web site gets a certificate from a CA that is issued from an Intermediate CA (Most CAs do this now, for convenience and security reasons, as well as for selling CA certificates to CAs without their own root or to cross-sign roots) but the web site administrator forgets to install the Intermediate CA certificate on the server. This means that the client will not be able to trace the certificate to a known Root, even if it has the Root in its repository, because clients usually do not, and are not expected to, know specific Intermediate CA certificates; the result (for the user) is an unexpected Unknown Issuer certificate warning.

SSL/TLS servers that do not send intermediate certificates are actually not operating in compliance with the SSL/TLS standard. The standard requires the server to send any CA certificates it cannot reasonably expect the client to have already, and the only thing it can expect the client to have is the root certificate, and not any intermediates.

There is however a mechanism (called Authority Information Access, AIA) defined in the specification of the certificate format that let the CA specify a "CA Issuer" download location for the Intermediate CA certificates. This mechanism is already used by at least one other browser (which neatly explains why so many badly configured sites get past testing), and now Opera 9.50 also use this mechansism.

When Opera 9.50 encounters a site with a certificate chain that is missing an Intermediate CA certificate linking it to the Root, but the certificate specifies an AIA issuer download location, Opera will download the specified certificate. If the verification then succeeds (without other incidents) to link the certificate with a known root, the certificate will be accepted as normal, and the intermediate CA certificate will be "cached" in the new Intermediate CA certificate store in Opera's certificate manager for future reference, to avoid downloading it again later.

What this means is that you will no longer get any certificate warnings about unknown CAs when the certificate is issued from a root and a CA that also includes an AIA issuer download location for the intermediate certificate(s) for the Intermediate CA(s) that issued the certificate.

Special thanks to Gary and Les in Verisign for helping me debug this new functionality.

1 comment

New^W NOT in Kestrel #3: As many warnings about weak temporary public keys

Monday, 22. October 2007, 15:00:52

Opera, Not In Kestrel, SSL, standards ...

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

No comments

New^W NOT in Kestrel #2: "Export"-grade encryption is junk(ed)

Monday, 8. October 2007, 14:48:50

TLS, Opera, Security, encryption ...

SSL v2 is not the only old encryption capability that is no longer supported by Opera 9.5, also known as Kestrel. We have also removed support for 40 and 56-bit encryption. Like SSL v2 these methods have been disabled by default since v9.0

There are several reasons for this move:

40 and 56-bit encryption methods are "soooooooo" last century. These methods were defined as a way for US based browser vendors to be able to distribute their SSL enabled client outside the US. The reason this was necessary is that the US government and several allies defined (and I believe still define) encryption as a weapon, but it was possible to get permission to export software that only supported 40 bit encryption (and later 56 bit) and that included a couple of other restrictions (there was an exception for financial services). The reason for the limitations was, presumably, that the intelligence communities of these countries had the technology needed to break these keys. In 1999/2000 the restrictions were partially lifted, at least for mass market products like browsers and email clients.
40 and 56-bit methods are dismally weak given today's technology. In mid-1998 56-bit DES was broken in less than a 56 hours, this was reduced to 24 hours less than half a year later. Given today's technology I expect that the same can be done in less than 30 minutes with enough hardware. And 40-bit can probably be broken in less than a second. What this means is that these methods no longer provide any protection at all.
Any server that only supports these methods is more than 8 years old, which means the actual security of the server, even ignoring the lack of encryption strength, is .... questionable. To top it off, a number of Certificate Authorities sell "SGC" certificates that will permit most US produced and exported servers (and clients) to enable 128 bit encryption. These certificates were originally reserved financial institutions, but after the crypto export restrictions were lifted they have become available to all.

While I believe servers that only support 40/56 bit encryption are a bit more common, in absolute numbers (perhaps a few thousand), than SSL v2 servers, I can't remember hearing about any such sites for over a year, despite the fact the the methods have been disabled by default about that long in several browsers. That indicates that the servers are not visited by a lot of people, if any. I think it is time to signal quite clearly to the sites that may be left that the technology they are using is obsolete.

If you do encounter a "secure" site that require 40 or 56 bit encryption, what can you do? Well, I don't recommend it, but you can go back to Opera 9.2x and enable the weak ciphers. But before you do, perhaps you should ask the system administrator this question: "Why are you running the site with 8 year old software?"

6 comments

New^W NOT in Kestrel: The death of SSL v2

Wednesday, 3. October 2007, 14:05:18

TLS, Security, Opera, encryption ...

As I've written earlier, in Opera 9.0 we disabled SSL v2 by default, but if necessary a user could enable it.

In Opera 9.5 (Kestrel) we've taken this one step further, and completly disabled the support for SSL v2. That is, as of Opera 9.5 Opera is no longer able to connect to servers that only supports SSL v2.

There are several reasons for this:

SSL v2 is OLD. It was added in Netscape 2, back in 1994! SSL v3 replaced it 1996, which means that any service that only wants to use SSLv2 was designed in 1996, or earlier. And it hasn't received a significant upgrade since! Think about what that means about the technology used, and the security of the site...
SSL v2 is binary incompatible with SSL v3 and TLS; you cannot send a modern TLS handshake to a SSL v2-only server, it won't understand it. In fact, one of the very few things SSL v2 and SSL v3 have in common is the name! Given new TLS functionality Opera 9.x had already put SSL v2 as the last thing it will try before giving up.
SSL v2 itself is known to have at least one major security vulnerability. This particular vulnerability is not present in SSL v3
There are few, if any, publicly accessible SSL v2 servers left. A major reason for this is Gerv from Mozilla's campaign two years ago; he managed to convince the hosting company with almost 90% of the servers to upgrade. According to my information Netcraft stopped counting them last year, because there were so few left.

If you do encounter a "secure" site that requires SSL v2, what can you do? Well, I don't recommend it, but you can go back to Opera 9.2x and enable SSL v2. But before you do, perhaps you should ask the system administrator this question: "Why are you running the site with 12 year old software?"