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https://issues.apache.org/jira/browse/HADOOP-15235?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16384124#comment-16384124
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Robert Kanter commented on HADOOP-15235:
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You're right! I've filed HADOOP-15282 to fix it.
> Authentication Tokens should use HMAC instead of MAC
> ----------------------------------------------------
>
> Key: HADOOP-15235
> URL: https://issues.apache.org/jira/browse/HADOOP-15235
> Project: Hadoop Common
> Issue Type: Improvement
> Components: security
> Affects Versions: 2.10.0, 3.2.0
> Reporter: Robert Kanter
> Assignee: Robert Kanter
> Priority: Major
> Fix For: 2.10.0, 3.2.0
>
> Attachments: HADOOP-15235.001.patch, HADOOP-15235.002.patch
>
>
> We currently use {{MessageDigest}} to compute a "SHA" MAC for signing
> Authentication Tokens. Firstly, what "SHA" maps to is dependent on the JVM
> and Cryptography Provider. While they _should_ do something reasonable, it's
> probably a safer idea to pick a specific algorithm. It looks like the Oracle
> JVM picks SHA-1; though something like SHA-256 would be better.
> In any case, it would also be better to use an HMAC algorithm instead.
> Changing from SHA-1 to SHA-256 or MAC to HMAC won't generate equivalent
> signatures, so this would normally be an incompatible change because the
> server wouldn't accept previous tokens it issued with the older algorithm.
> However, Authentication Tokens are used as a cheaper shortcut for Kerberos,
> so it's expected for users to also have Kerberos credentials; in this case,
> the Authentication Token will be rejected, but it will silently retry using
> Kerberos, and get an updated token. So this should all be transparent to the
> user.
> And finally, the code where we verify a signature uses a non-constant-time
> comparison, which could be subject to timing attacks. I believe it would be
> quite difficult in this case to do so, but we're probably better off using a
> constant-time comparison.
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