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https://issues.apache.org/jira/browse/HADOOP-15235?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16364906#comment-16364906
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Robert Kanter commented on HADOOP-15235:
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The 002 patch replaces the call to {{Base64(0).encodeAsString(sig);}} with 
{{Base64(0).encodeToString(sig);}}, which works in branch-2 and branch-3.

> 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
>         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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