The National Institute of Standards and Technology (NIST) is still using SSL certificates signed with the SHA-1 signature algorithm, despite issuing a Special Publication disallowing the use of this algorithm for digital signature generation after 2013.
"SHA-1 shall not be used for digital signature generation after December 31, 2013."
— NIST recommendation
The SSL certificate for www.nist.gov is signed using the SHA-1 hashing algorithm, and was issued by VeriSign on 23 January 2014, more than three weeks after NIST's own ban came into effect. Also issued this year, NIST's "Secure File Transfer Service" at xnfiles.nist.gov uses a SHA-1 certificate.
An attacker able to find SHA-1 collisions could carefully construct a pair of certificates with colliding SHA-1 hashes: one a conventional certificate to be signed by a trusted CA, the other a sub-CA certificate able to be used to sign arbitrary SSL certificates. By substituting the signature from the CA-signed certificate into the sub-CA certificate, certificate chains containing the attacker-controlled sub-CA certificate will pass browser verification checks. This attack is, however, made more difficult by path constraints and the inclusion of unpredictable data into the certificate before signing it.
The increasing practicality of finding SHA-1 hash collisions could make it possible for a well-funded attacker to impersonate any HTTPS website. With a practical attack against SHA-1 (using cloud computing resources) estimated to cost $2.77M in 2012, falling to $700k by 2015, it may attract government agencies.
The SSL certificate for www.nist.gov with the signature algorithm and issuance date highlighted.
Along with NIST itself, many US Government institutions have continued to generate new SSL certificates with SHA-1 signatures. Examples include the certificate for donogc.navy.mil, issued on 3 January 2014, and several United States Bankruptcy Court document filing systems (each state has its own site and uses its own SHA-1-signed SSL certificate). Despite receiving widespread criticism for a number of other security problems last year, the ObamaCare exchange at healthcare.gov also saw fit to deploy a new SSL certificate in January which uses the SHA-1 hashing algorithm.
NIST and the rest of the US government are far from alone, however, as more than 92% of all certificates issued this year use the SHA-1 hashing algorithm.
Although the recommendations from the National Institute of Standards and Technology have been prepared for US federal agencies, the cryptographic weaknesses of SHA-1 should concern anyone who relies on SHA-1 to generate or validate digital signatures. Microsoft shares these concerns and has announced plans to deprecate the use of SHA-1 in both SSL and code signing certificates by the end of 2016.
The NSA-designed SHA-2 family (which includes SHA-224, SHA-256, SHA-384 and SHA-512) now provides the only cryptographic hash functions approved by NIST for digital signature generation. Whilst SHA-2 shares some similarities with SHA-1, there are significant structural differences: SHA-2 does not share the SHA-1's mathematical weakness. All of the SHA-2 algorithms have much longer digests: SHA-1 only produces a 160-bit digest, whereas the most common digest length for SHA-2 is 256-bits.
Other signature algorithms with negligible shares (e.g. MD5 and SHA-512) are not displayed.
In total, more than 98% of all SSL certificates in use on the Web are still using SHA-1 signatures. Netcraft's February 2014 SSL Survey found more than 256,000 of these certificates would otherwise be valid beyond the start of 2017 and, due to the planned deprecation of SHA-1, will need to be replaced before their natural expiry dates.
SHA-256 is the most commonly used signature algorithm from the SHA-2 family, but it is used by only 1.86% of the valid certificates in Netcraft's February 2014 SSL Survey; nonetheless, this share has more than doubled in the space of 4 months, suggesting that some certificate authorities are starting to take the issue seriously. So far in 2014, more than 61% of the new certificates signed with SHA-256 were issued by a single certificate authority, Go Daddy. SHA-512 is the only other SHA-2 family algorithm to be seen used in SSL certificates, albeit deployed on only 4 websites so far.
The CA/B Forum – which comprises of both certificate authorities and web browser vendors – put forward Ballot 111 last year, which motions to take advantage of the deprecation of SHA-1 by accelerating the forum's planned move to shorter maximum certificate lifetimes. The deprecation alone will mean that some five-year certificates that are valid today will not be usable for their entire lifetime.
In practice, it is likely to be Microsoft's plans to deprecate the use of SHA-1 signatures by the end of 2016 which will force the mass adoption of SHA-2 by certificate authorities, although allowing three years for this to happen might seem generous. The majority of end users are unlikely to be affected by the change, and most website administrators will probably have to renew their SSL certificates within this period anyway, but certificates which are reissued with SHA-1 signatures run the risk of being unsupported by other browsers in the future.
Cryptographic weaknesses in SHA-1 have been discussed for several years. A notable better-than-brute-force attack was announced nine years ago, demonstrating a SHA-1 hash collision that could be achieved within 269 calculations, as opposed to the 280 that would be required by a brute-force approach.
More recently, the best public cryptanalysis of SHA-1 estimated that a full collision can be achieved with a complexity of around 261, while a near-collision can be achieved in 257.5. These attacks have been implemented in the HashClash framework, which provides differential path construction attacks against SHA-1, as well as chosen prefix collisions against the even-weaker MD5 algorithm. The CA/B Forum recommends that all certificate serial numbers should exhibit at least 20 bits of entropy, which would mitigate chosen-prefix collision attacks for non collision resistant hash functions, although such measures are not thought to be necessary for SHA-2 at the current time.
Windows XP has supported SHA-256, SHA-384 and SHA-512 since the release of Service Pack 3 in 2008, and Windows Server 2003 can also support SHA-2 if the KB938397 hotfix has been installed. Deprecating SHA-1 could therefore also have some other indirect security benefits: anyone still using Windows XP before Service Pack 3 will be unable to make effective use of the web as SHA-2 certificates gain prominence.
The SHA-1 algorithm is also used in all versions of the TLS cryptographic protocol, and only the latest version (TLS 1.2) introduces SHA-256 as a replacement for the MD5/SHA-1 combination for both the pseudorandom function and the finished message hash. Microsoft's SHA-1 deprecation policy will only apply to applications which call the CertGetCertificateChain API to build and validate a certificate chain, so older browsers and hardware devices which do not yet support TLS 1.2 will be unaffected.
Update 5 Feb 2014: Following the publication of this article, NIST today replaced the SHA-1 certificate on www.nist.gov with a new one which uses SHA-256 as a signature algorithm. At the time of writing, the certificate used by xnfiles.nist.gov is still signed with SHA-1.