SSL-Certificates with SHA256 signature
Monday, February 1. 2010, 23:23
At least since 2005 it's well known that the cryptographic hash function SHA1 is seriously flawed and it's only a matter of time until it will be broken. However, it's still widely used and it can be expected that it'll be used long enough to allow real world attacks (as it happened with MD5 before). The NIST (the US National Institute of Standards and Technology) suggests not to use SHA1 after 2010, the german BSI (Bundesamt für Sicherheit in der Informationstechnik) says they should've been fadet out by the end of 2009.
The probably most widely used encryption protocol is SSL. It is a protocol that can operate on top of many other internet protocols and is for example widely used for banking accounts.
As SSL is a pretty complex protocol, it needs hash functions at various places, here I'm just looking at one of them. The signatures created by the certificate authorities. Every SSL certificate is signed by a CA, even if you generate SSL certificates yourself, they are self-signed, meaning that the certificate itself is it's own CA. From what I know, despite the suggestions mentioned above no big CA will give you certificates signed with anything better than SHA1. You can check this with:
openssl x509 -text -in [your ssl certificate]
Look for "Signature Algorithm". It'll most likely say sha1WithRSAEncryption. If your CA is good, it'll show sha256WithRSAEncryption. If your CA is really bad, it may show md5WithRSAEncryption.
When asking for SHA256 support, you often get the answer that the software still has problems, it's not ready yet. When asking for more information I never got answers. So I tried it myself. On an up-to-date apache webserver with mod_ssl, it was no problem to install a SHA256 signed certificate based on a SHA256 signed test CA. All browsers I've tried (Firefox 3.6, Konqueror 4.3.5, Opera 10.10, IE8 and even IE6) had no problem with it. You can check it out at https://sha2.hboeck.de/. You will get a certificate warning (obviously, as it's signed by my own test CA), but you'll be able to view the page. If you want to test it without warnings, you can also import the CA certificate.
I'd be interested if this causes any problems (on server or on client side), so please leave a comment if you are aware of any incompatibilities.
Update: By request in the comments, I've also created a SHA512 testcase.
Update 2: StartSSL wrote me that they tried providing SHA256-certificates about a year ago and had too many problems - it wasn't very specific but they mentioned that earlier Windows XP and Windows 2003 Server versions may have problems.
The probably most widely used encryption protocol is SSL. It is a protocol that can operate on top of many other internet protocols and is for example widely used for banking accounts.
As SSL is a pretty complex protocol, it needs hash functions at various places, here I'm just looking at one of them. The signatures created by the certificate authorities. Every SSL certificate is signed by a CA, even if you generate SSL certificates yourself, they are self-signed, meaning that the certificate itself is it's own CA. From what I know, despite the suggestions mentioned above no big CA will give you certificates signed with anything better than SHA1. You can check this with:
openssl x509 -text -in [your ssl certificate]
Look for "Signature Algorithm". It'll most likely say sha1WithRSAEncryption. If your CA is good, it'll show sha256WithRSAEncryption. If your CA is really bad, it may show md5WithRSAEncryption.
When asking for SHA256 support, you often get the answer that the software still has problems, it's not ready yet. When asking for more information I never got answers. So I tried it myself. On an up-to-date apache webserver with mod_ssl, it was no problem to install a SHA256 signed certificate based on a SHA256 signed test CA. All browsers I've tried (Firefox 3.6, Konqueror 4.3.5, Opera 10.10, IE8 and even IE6) had no problem with it. You can check it out at https://sha2.hboeck.de/. You will get a certificate warning (obviously, as it's signed by my own test CA), but you'll be able to view the page. If you want to test it without warnings, you can also import the CA certificate.
I'd be interested if this causes any problems (on server or on client side), so please leave a comment if you are aware of any incompatibilities.
Update: By request in the comments, I've also created a SHA512 testcase.
Update 2: StartSSL wrote me that they tried providing SHA256-certificates about a year ago and had too many problems - it wasn't very specific but they mentioned that earlier Windows XP and Windows 2003 Server versions may have problems.
Tracked: Feb 06, 01:42