[dane] (draft-ietf-dane-smime) SHA-224 not in CryptoAPI

Sean Leonard <dev+ietf@seantek.com> Wed, 07 January 2015 05:21 UTC

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From: Sean Leonard <dev+ietf@seantek.com>
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Subject: [dane] (draft-ietf-dane-smime) SHA-224 not in CryptoAPI
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I would like to point out that SHA-224 is not a good choice for a fixed 
hash algorithm. SHA-224 is not implemented in Microsoft CryptoAPI / 
Cryptography Next Generation, which means that Windows apps (clients and 
servers) will have a more difficult time implementing this thing. 
Reference: <http://msdn.microsoft.com/library/bb931357>. I suggest 
sticking with SHA-256.

Looking back at the archives, it appears that a motivator for SHA-224 is 
that it is "short" and therefore fits in a DNS label with hex encoding. 
I do not buy this argument. Base32 encoding works great for SHA-256.

Since DNS labels are octets anyway, if you want "short" I do not see a 
big deal with putting 32 octets in the label. Pushback on the grounds 
that "it's not a hostname" and "watch out for case folding" are moot 
because "_smimecert" is not a valid hostname either: that's why "_" was 
chosen for SRV records and their kin. It's not a vanity contest. Maybe 
there are other considerations; I am not a DNS ops expert. Suffice to 
say, base32 would work just fine.

(As an implementer, I have other various opinions about this work but 
don't feel like bringing them up at this time.)