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From "Alex Karasulu (JIRA)" <j...@apache.org>
Subject [jira] Commented: (DIRTSEC-4) Offset value set to 0 when extracting DBC from hmac-sha1 output
Date Tue, 22 May 2007 23:06:16 GMT

    [ https://issues.apache.org/jira/browse/DIRTSEC-4?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12498053
] 

Alex Karasulu commented on DIRTSEC-4:
-------------------------------------

Javier could you supply a patch as attachment ... I will review and apply it.  Furthermore
if you are interested in working on Tsec then please continue contributing to become a committer.
 We could use a few good developers on this front.

> Offset value set to 0 when extracting DBC from hmac-sha1 output
> ---------------------------------------------------------------
>
>                 Key: DIRTSEC-4
>                 URL: https://issues.apache.org/jira/browse/DIRTSEC-4
>             Project: Triplesec
>          Issue Type: Bug
>         Environment: JavaME mobile phone platform and validation server.
>            Reporter: javier tellez
>
> Hotp.java class implements RFC4226 "HOTP: An HMAC-Based One-Time Password Algorithm".
> In that class,
>    hotp.generate(secret, counter, digits) uses 
>    int offset = 0; 
> Section 5.4 from RFC
> "The following code example describes the extraction of a dynamic
> binary code given that hmac_result is a byte array with the HMACSHA-1 result:
>    int offset = hmac_result[19] & 0xf ;
>    int bin_code = (hmac_result[offset] & 0x7f) << 24
>                         | (hmac_result[offset+1] & 0xff) << 16
>                         | (hmac_result[offset+2] & 0xff) << 8
>                         | (hmac_result[offset+3] & 0xff)
> "
> that is, the offset is the least significant nibble from the last byte of 
> hotp.stepOne() output (the 20 bytes from hmac-sha1(k,c))
> Solved by setting offset to this value
> int offset = hmac_result[19] & 0xf; 

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