[OpenID] OpenId Association Timeout Recommendations

David Fuelling sappenin at gmail.com
Mon Feb 5 10:24:08 PST 2007

Hey List,

I'm wondering if anyone has an opinion on a "recommended" association
timeout for OpenId OP/RP implementations?

I think it takes something like 2^80 operations to brute force SHA1 (the
least secure OpenId HMAC Association type).  Supposedly, in 2005 SHA1 was
"sort of" broken by a Chinese researcher (see here: 
http://www.schneier.com/blog/archives/2005/02/sha1_broken.html) but
according to Bruce Schneier, HMAC is not affected by this development (only
digital signatures are).

All that to say, it seems like it would still take a long time to brute
force an SHA1 association (SHA256 even longer), so I'm wondering what
people's thoughts are where OpenId implementation should set this number by

For example, one of the most popular Java OpenId 2.0 implementations
currently uses a 30 minute expiration.  What about 3 days?  7 days? Longer?

I guess I'm trying to figure out where the "balance between security and
convenience" decision should be made.

Thanks for your input!


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