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   1 Cryptographic hash functions
   2
   3
   4         Hash functions are very useful cryptographic primitives.
   5         Catacomb provides a few of the best-known cryptographic hashes.
   6
   7
   8 Hash function interface
   9
  10         Hash functions share a regular interface.  I'll take Ron
  11         Rivest's MD5 as an example.
  12
  13         Using a hash function is a three-stage process.  You initialize
  14         a context, you hash some data, and you get the result out of the
  15         end.  The data to be hashed need not be contiguous, and you
  16         don't have to have it all before you start.  Hash function
  17         contexts don't use up lots of memory.
  18
  19         An MD5 context is called `md5_ctx'.  You initialize it with
  20         `md5_init'.  You hash a block of data using `md5_hash' giving it
  21         the context, the pointer to the data and its length.  Finally,
  22         you extract the data using `md5_done', giving it the address of
  23         a buffer of MD5_HASHSZ bytes for it to write the result.
  24
  25         There are some other standard operations as well, but they're
  26         not often used: see the header files for details.
  27
  28         The hash functions supported are:
  29
  30         MD4             By Ron Rivest -- returns a 128-bit hash.  MD4 is
  31                         not collision-resistant, and may not even be
  32                         second-preimage-resistant.  Don't depend on its
  33                         security.  On the other hand, MD4 is *very*
  34                         fast.
  35
  36         MD5             Also by Rivest, also returns a 128-bit hash.
  37                         MD5 is slower than MD4, and more conservative,
  38                         but there are still grave doubts about its
  39                         security.
  40
  41         SHA1            Designed by the US National Security Agency.
  42                         Returns a 160-bit hash.  Slower than MD5.  Looks
  43                         strong.  Fixes a problem in the earlier SHA.
  44
  45         RIPEMD-160      Designed by the people who broke MD4 and MD5.
  46                         Returns a 160-bit hash.  Slower than SHA1.  My
  47                         personal preference.
  48
  49
  50 HMAC interface
  51
  52         It's possible to construct a `keyed hash' or `message
  53         authentication code' from a hash function.  Most methods for
  54         doing this are insecure.  HMAC is a good method, with rigorously
  55         proven security properties.
  56
  57         Each hash function above has an HMAC mode defined for it.  This
  58         works much the same way as block cipher modes.
  59
  60         Using HMAC is a two-step process.  First, you initialize a MAC
  61         key block `mackey' to contain the key you'll use, and then you
  62         initialize MAC contexts `macctx' which to actually hash the
  63         data.  Hashing works just the same as the basic hash function,
  64         except that you use `macinit', `machash' and `macdone' functions
  65         rather than the plain `init', `hash' and `done'.
  66
  67
  68 Generic interfaces
  69
  70         Generic interfaces to hash functions and MACs are provided.  See
  71         README.cipher to get an idea for how a similar generic interface
  72         works -- I'll only explain the differences here.
  73
  74         The generic hash object, `ghash' contains an `ops' member
  75         referring to:
  76
  77         h->ops->b->name         The name of the hash function.
  78
  79         h->ops->b->hashsz       The output size of the hash function.
  80
  81         h->ops->hash(h, p, sz)  Hash sz bytes of data starting at
  82                                 address p.
  83
  84         h->ops->done(h, b)      Stop hashing, write the result to buffer
  85                                 b with size `hashsz'.
  86
  87         h->ops->destroy(h)      Destroy the generic hash object.
  88
  89         The generic hash class, `gchash', contains a base which has the
  90         same members as `ops->b' above, and an `init' function which
  91         takes no arguments and returns a pointer to a `ghash'.
  92
  93         There's a generic MAC interface too.  A MAC class, `gcmac'
  94         contains a hash base (exactly the same, but with a different
  95         name), and a function `key' which takes a pointer to some key
  96         data and the key size, and returns a pointer to a `generic mac'
  97         object, `gmac'.
  98
  99         A `gmac' contains an `ops' member:
 100
 101         m->ops->b->name, m->ops->b->hashsz
 102                                 As above.
 103
 104         m->ops->init(m)         Returns a generic *hash* object to
 105                                 actually compute a MAC over some data.
 106
 107         m->ops->destroy(m)      Destroys the generic MAC block.
 108
 109
 110         That was quite complex.  Here's an example of using a generic
 111         MAC.
 112
 113                 void compute_mac(gcmac *gcm, const void *k, size_t ksz,
 114                                  const void *p, size_t sz,
 115                                  void *hash)
 116                 {
 117                   gmac *m = gcm->init(k, ksz);
 118                   ghash *h = m->ops->init(m);
 119                   m->ops->destroy(m);
 120                   h->ops->hash(h, p, sz);
 121                   h->ops->done(h, hash);
 122                   h->ops->destroy(h);
 123                 }
 124
 125         Note that the hash doesn't depend on the MAC object continuing
 126         to exist.
 127
 128 -- [mdw]
 129
 130 \f
 131 Local variables:
 132 mode: text
 133 End: