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SPKI verification in pseudocode

  You can find below a sketch of a pseudocode for verifying if `self' has
  authorised `requested_action' for `subject'. It returns true/false. It
  does this by reading the sequence construct `seq' that has been supplied
  by the subject or perhaps built by the verifier itself. All syntax 
  checks have been omitted.

  This reflects my current understanding of the draft. Here most of the
  intelligence needed has been transferred from the verifier to the
  subject. Note that the function fails if the sequence has irrelevant

  It does not support threshold subjects. We will probably need a more
  complex mechanism for that.

  I've used a Python-like pseudocode. I hope you can follow.

  - Markku-Juhani Saarinen <mjos@ssh.fi>

-- start of the pseudocode --

def permission(seq, self, subject, requested_action):

  i = 1                 # the 0th element should read 'sequence'
  last_obj = NULL       # the last object encountered 
  red_5tuple = NULL     # the current 5-tuple

  # walk trough the sequence 

  while i < len(seq):

    switch seq[i][0]:     # this is the first element in each sublist
                          # i.e. the object identifier
      case 'cert':          
      case 'pub-key':
        last_obj = seq[i]

      case 'do':
        if seq[i][1] == 'hash':
          hash_and_put_into_global_hashtree(last_obj, seq[i][2])
          # seq[i][2] should give the name of the hash algorithm

      case 'signature':
        if valid_signature_by_the_issuer(seq[i]): 
          this_5tuple = 5tupleize(related_certificate(seq[i]))

          # reduce the current 5 tuple

          if red_5tuple == NULL:
            red_5tuple = this_5tuple
            red_5tuple = 5_tuple_reduction(red_5tuple, this_5tuple)

# ok, proceed in the list

    i = i + 1

# ok, we have read the sequence.
# see if it permits the requested action for the subject

  if red_5tuple.issuer != self or red_5tuple.subject != subject:
    return false

  if time() not in red_5tuple.valid:
    return false  

  if intersection(red_5tuple.auth, requested_action) != requested_action:
    return false
  return true