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Friday, 20 February 2009
SHA-3 Round 1: Buffer Overflows
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NIST is currently holding a competition to choose a design for the SHA-3 algorithm (Bruce Schneier has a good description of secure hashing algorithms and why this is important). The reference implementations of a few of the contestants have bugs in them that could cause crashes, performance problems, or security problems if they are used in their current state. Based on our bug reports, some of those bugs have already been fixed. Here's the full story: The main idea behind the competition is to have the cryptographic community weed out the less secure algorithms and choose from the remainder. A couple of us at Fortify (thanks to Doug Held for his help) decided to do our part. We're not hard-core cryptographers, so we decided to take a look at the reference implementations. This competition is to pick an algorithm, but all of the submissions had to include a C implementation, to demonstrate how it works and test the speed, which will be a factor in the final choice. We used Fortify SCA to audit the 42 projects accepted into Round 1. We were impressed with the overall quality of the code, but we did find significant issues in a few projects, including buffer overflows in two of the projects. We have emailed the submission teams with our findings and one team has already corrected their implementation.
Confirmed issues:
Buffer Overflow
Out-of-bounds Read
Memory Leak
Null Dereference

One of the projects with buffer issues was MD6, the implementation provided Professor Ron Rivest and his team. All of the problems came back to the hashval field of the md6_state struct:

     unsigned char hashval[ (md6_c/2)*(md6_w/8) ];
The buffer size is determined by two constants:

     #define w md6_w     /* # bits in a word                   (64) */
     #define c md6_c     /* # words in compression output      (16) */
At several points, this buffer is read or written to using a different bound:

     if (z==1) /* save final chaining value in st->hashval */
          { memcpy( st->hashval, C, md6_c*(w/8) );
            return MD6_SUCCESS;
Further analysis showed that ANSI standard layout rules would make incorrect behavior unlikely, but other compilers may have allowed it to be exploited. The MD6 team has doubled the size of the vulnerable buffer, which eliminated the risk. In this case, Fortify SCA found an issue that would have been difficult to catch otherwise. The other buffer overflow was found in the Blender implementation, from Dr. Colin Bradbury. This issue was a classic typo:

DataLength sourceDataLength2[3]; // high order parts of data length
     if (ss.sourceDataLength < (bcount | databitlen)) // overflow
if (++ss.sourceDataLength2[0] == 0) // increment higher order count if (++ss.sourceDataLength2[1] == 0) // and the next higher order ++ss.sourceDataLength2[3]; // and the next one, etc. The developer simply mistyped, using 3 instead of 2 for the array access. This issue was probably not caught because it would not be exposed without a very large input. The other issues we found were memory leaks and null dereferences from memory allocation. This just emphasizes what we already knew about C, even the most careful, security conscious developer messes up memory management. Some of you are saying, so what? These are reference implementations and this is only Round 1. There are a few problems with that thought. Reference implementations don't disappear, they serve as a starting point for future implementations or are used directly. A bug in the RSA reference implementation was responsible for vulnerabilities in OpenSSL and two seperate SSH implementations. They can also be used to design hardware implementations, using buffer sizes to decide how much silicon should be used. The other consideration is speed, which will be a factor in the choice of algorithm. The fix for the MD6 buffer issues was to double the size of a buffer, which could degrade the performance. On the other hand, memory leaks could slow an implementation. A correct implementation is an accurate implementation.
We will put out a more detailed report on all the results soon.
Technorati Tags: sha-3 buffer overflow
Posted by jforsythe at 5:41 PM in crypto

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