[tor-bugs] #17796 [Tor]: Make crypto_digest_t allocated using minimal memory

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#17796: Make crypto_digest_t allocated using minimal memory
--------------------+------------------------------------
 Reporter:  nickm   |          Owner:
     Type:  defect  |         Status:  needs_review
 Priority:  Low     |      Milestone:  Tor: 0.2.8.x-final
Component:  Tor     |        Version:
 Severity:  Normal  |     Resolution:
 Keywords:          |  Actual Points:
Parent ID:          |         Points:
  Sponsor:          |
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Comment (by teor):

 In crypto_digest_get_digest, this block is now unreachable:
 {{{
     default:
       log_warn(LD_BUG, "Called with unknown algorithm %d",
 digest->algorithm);
       /* If fragile_assert is not enabled, then we should at least not
        * leak anything. */
       memwipe(r, 0xff, sizeof(r));
       memwipe(&tmpenv, 0, sizeof(crypto_digest_t));
       tor_fragile_assert();
       break;
 }}}
 as this line will assert before the default case is reached:
 {{{
   const size_t alloc_bytes = crypto_digest_alloc_bytes(digest->algorithm);
 }}}

 If it wasn't unreachable, this one remaining instance of
 `sizeof(crypto_digest_t)` could write into unallocated memory:
 {{{
       memwipe(&tmpenv, 0, sizeof(crypto_digest_t));
 }}}

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/17796#comment:4>
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