[GHC] #7634: MD5 collision could lead to SafeHaskell violation

[GHC]

#7634: MD5 collision could lead to SafeHaskell violation
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Reporter:  shachaf           |          Owner:                  
    Type:  bug               |         Status:  new             
Priority:  normal            |      Component:  libraries/base  
 Version:  7.6.1             |       Keywords:                  
      Os:  Unknown/Multiple  |   Architecture:  Unknown/Multiple
 Failure:  Other             |      Blockedby:                  
Blocking:                    |        Related:                  
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 The current scheme for computing `TypeRep` fingerprints is: `md5sum
 (encodeUTF32BE (unwords [moduleName, packageName, typeName]))`.
 `SafeHaskell` doesn't allow custom-written `Typeable` instances, but this
 is more or less the code that `deriving Typeable` generates.

 MD5 is broken and not collision-resistant. If someone can make an MD5
 collision, they could use it to derive `unsafeCoerce` and execute
 arbitrary code. The constraints (UTF-32, names being alphanumeric, etc.)
 make it pretty tricky to find a valid collision by the standard methods,
 but I don't know enough about this to say how feasible it is.

 It seems to me that, especially with `new-typeable`, it might not be
 necessary to use hashing at all, if GHC can figure out fingerprints
 statically. Or maybe separate compilation requirements make that
 unworkable (in which case maybe using a hash of the package/module name
 along with a separate per-module counter, or something along those lines,
 might be better, since people are less likely to control those? I'm not
 sure). Maybe the solution is just switching to another hash function, or
 something else. At any rate, the issue should be considered -- using MD5
 isn't a good idea in cases where collisions could have security
 implications.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7634>
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