diff --git a/client-webserver/client.py b/client-webserver/client.py
index 8d562b9aa..7b739e67d 100755
--- a/client-webserver/client.py
+++ b/client-webserver/client.py
@@ -4,8 +4,14 @@ from chocolate_protocol_pb2 import chocolatemessage
 from Crypto.Hash import SHA256
 import CSR
 from CSR import M2Crypto
-import urllib2, os, sys, time, random, sys
+import urllib2, os, sys, time, random, sys, hashcash
 # CSR.py here should be a symlink to ../server-ca/CSR.py
+# hashcash.py here should be a symlink to ../server-ca/hashcash.py
+
+difficulty = 20
+# TODO: unfortunately, the C hashcash implementation seems to be about
+#       2^6 times faster than the native Python implementation, so
+#       calibrating the difficulty is a bit of a problem.
 
 def sha256(m):
     return SHA256.new(m).hexdigest()
@@ -29,9 +35,11 @@ def init(m):
     m.session = ""
 
 def make_request(m, csr):
-    m.request.recipient = os.environ["CHOCOLATESERVER"]
+    server = os.environ["CHOCOLATESERVER"]
+    m.request.recipient = server
     m.request.timestamp = int(time.time())
     m.request.csr = csr
+    m.request.clientpuzzle = hashcash.mint(server, difficulty)
 
 def sign(k, m):
     m.request.sig = CSR.sign(k, ("(%d) (%s) (%s)" % (m.request.timestamp, m.request.recipient, m.request.csr)))
@@ -42,6 +50,8 @@ init(k)
 init(m)
 make_request(m, csr=open("req.pem").read())
 sign(open("key.pem").read(), m)
+print m
+assert False
 r=decode(do(m))
 print r
 while r.proceed.IsInitialized():
diff --git a/client-webserver/hashcash.py b/client-webserver/hashcash.py
new file mode 120000
index 000000000..3335450d4
--- /dev/null
+++ b/client-webserver/hashcash.py
@@ -0,0 +1 @@
+../server-ca/hashcash.py
\ No newline at end of file
diff --git a/server-ca/chocolate.py b/server-ca/chocolate.py
index b73f69162..c06e72a68 100755
--- a/server-ca/chocolate.py
+++ b/server-ca/chocolate.py
@@ -4,6 +4,7 @@ import web, redis, time
 import CSR
 import hashlib
 import hmac
+import hashcash
 from CSR import M2Crypto
 from Crypto import Random
 from chocolate_protocol_pb2 import chocolatemessage
@@ -12,6 +13,8 @@ from google.protobuf.message import DecodeError
 MaximumSessionAge = 100   # seconds, to demonstrate session timeout
 MaximumChallengeAge = 600 # to demonstrate challenge timeout
 
+difficulty = 20           # bits of hashcash required with new requests
+
 try:
     chocolate_server_name = open("SERVERNAME").read().rstrip()
 except IOError:
@@ -201,6 +204,12 @@ class session(object):
             # It is mandatory to make a signing request at the outset of a session.
             self.die(r, r.BadRequest, uri="https://ca.example.com/failures/missingrequest")
             return
+        # Check hashcash before doing any crypto or database access.
+        if not m.request.clientpuzzle or not hashcash.check(m.request.clientpuzzle, chocolate_server_name, difficulty):
+            # TODO: should enforce hashcash expiry and use the database to store valid
+            #       ones in order to prevent double-spending.
+            self.die(r, r.NeedClientPuzzle, uri="https://ca.example.com/failures/hashcash")
+            return
         if self.request_made():
             # Can't make new signing requests if there have already been requests in
             # this session.  (All signing requests should occur together at the
@@ -208,7 +217,6 @@ class session(object):
             self.die(r, r.BadRequest, uri="https://ca.example.com/failures/priorrequest")
             return
         # Process the request.
-        # TODO: check client puzzle before processing request
         timestamp = m.request.timestamp
         recipient = m.request.recipient
         csr = m.request.csr
diff --git a/server-ca/hashcash.py b/server-ca/hashcash.py
new file mode 100644
index 000000000..8a660bc50
--- /dev/null
+++ b/server-ca/hashcash.py
@@ -0,0 +1,208 @@
+#!/usr/bin/env python2.3
+"""Implement Hashcash version 1 protocol in Python
++-------------------------------------------------------+
+| Written by David Mertz; released to the Public Domain |
++-------------------------------------------------------+
+
+Double spend database not implemented in this module, but stub
+for callbacks is provided in the 'check()' function
+
+The function 'check()' will validate hashcash v1 and v0 tokens, as well as
+'generalized hashcash' tokens generically.  Future protocol version are
+treated as generalized tokens (should a future version be published w/o
+this module being correspondingly updated).
+
+A 'generalized hashcash' is implemented in the '_mint()' function, with the
+public function 'mint()' providing a wrapper for actual hashcash protocol.
+The generalized form simply finds a suffix that creates zero bits in the
+hash of the string concatenating 'challenge' and 'suffix' without specifying
+any particular fields or delimiters in 'challenge'.  E.g., you might get:
+
+    >>> from hashcash import mint, _mint
+    >>> mint('foo', bits=16)
+    '1:16:040922:foo::+ArSrtKd:164b3'
+    >>> _mint('foo', bits=16)
+    '9591'
+    >>> from sha import sha
+    >>> sha('foo9591').hexdigest()
+    '0000de4c9b27cec9b20e2094785c1c58eaf23948'
+    >>> sha('1:16:040922:foo::+ArSrtKd:164b3').hexdigest()
+    '0000a9fe0c6db2efcbcab15157735e77c0877f34'
+
+Notice that '_mint()' behaves deterministically, finding the same suffix
+every time it is passed the same arguments.  'mint()' incorporates a random
+salt in stamps (as per the hashcash v.1 protocol).
+"""
+import sys
+from string import ascii_letters
+from math import ceil, floor
+from sha import sha
+from random import choice
+from time import strftime, localtime, time
+
+ERR = sys.stderr            # Destination for error messages
+DAYS = 60 * 60 * 24         # Seconds in a day
+tries = [0]                 # Count hashes performed for benchmark
+
+def mint(resource, bits=20, now=None, ext='', saltchars=8, stamp_seconds=False):
+    """Mint a new hashcash stamp for 'resource' with 'bits' of collision
+
+    20 bits of collision is the default.
+
+    'ext' lets you add your own extensions to a minted stamp.  Specify an
+    extension as a string of form 'name1=2,3;name2;name3=var1=2,2,val'
+    FWIW, urllib.urlencode(dct).replace('&',';') comes close to the
+    hashcash extension format.
+
+    'saltchars' specifies the length of the salt used; this version defaults
+    8 chars, rather than the C version's 16 chars.  This still provides about
+    17 million salts per resource, per timestamp, before birthday paradox
+    collisions occur.  Really paranoid users can use a larger salt though.
+
+    'stamp_seconds' lets you add the option time elements to the datestamp.
+    If you want more than just day, you get all the way down to seconds,
+    even though the spec also allows hours/minutes without seconds.
+    """
+    ver = "1"
+    now = now or time()
+    if stamp_seconds: ts = strftime("%y%m%d%H%M%S", localtime(now))
+    else:             ts = strftime("%y%m%d", localtime(now))
+    challenge = "%s:"*6 % (ver, bits, ts, resource, ext, _salt(saltchars))
+    return challenge + _mint(challenge, bits)
+
+def _salt(l):
+    "Return a random string of length 'l'"
+    alphabet = ascii_letters + "+/="
+    return ''.join([choice(alphabet) for _ in [None]*l])
+
+def _mint(challenge, bits):
+    """Answer a 'generalized hashcash' challenge'
+
+    Hashcash requires stamps of form 'ver:bits:date:res:ext:rand:counter'
+    This internal function accepts a generalized prefix 'challenge',
+    and returns only a suffix that produces the requested SHA leading zeros.
+
+    NOTE: Number of requested bits is rounded up to the nearest multiple of 4
+    """
+    counter = 0
+    hex_digits = int(ceil(bits/4.))
+    zeros = '0'*hex_digits
+    while 1:
+        digest = sha(challenge+hex(counter)[2:]).hexdigest()
+        if digest[:hex_digits] == zeros:
+            tries[0] = counter
+            return hex(counter)[2:]
+        counter += 1
+
+def check(stamp, resource=None, bits=None,
+                 check_expiration=None, ds_callback=None):
+    """Check whether a stamp is valid
+
+    Optionally, the stamp may be checked for a specific resource, and/or
+    it may require a minimum bit value, and/or it may be checked for
+    expiration, and/or it may be checked for double spending.
+
+    If 'check_expiration' is specified, it should contain the number of
+    seconds old a date field may be.  Indicating days might be easier in
+    many cases, e.g.
+
+      >>> from hashcash import DAYS
+      >>> check(stamp, check_expiration=28*DAYS)
+
+    NOTE: Every valid (version 1) stamp must meet its claimed bit value
+    NOTE: Check floor of 4-bit multiples (overly permissive in acceptance)
+    """
+    if stamp.startswith('0:'):          # Version 0
+        try:
+            date, res, suffix = stamp[2:].split(':')
+        except ValueError:
+            ERR.write("Malformed version 0 hashcash stamp!\n")
+            return False
+        if resource is not None and resource != res:
+            return False
+        elif check_expiration is not None:
+            good_until = strftime("%y%m%d%H%M%S", localtime(time()-check_expiration))
+            if date < good_until:
+                return False
+        elif callable(ds_callback) and ds_callback(stamp):
+            return False
+        elif type(bits) is not int:
+            return True
+        else:
+            hex_digits = int(floor(bits/4))
+            return sha(stamp).hexdigest().startswith('0'*hex_digits)
+    elif stamp.startswith('1:'):        # Version 1
+        try:
+            claim, date, res, ext, rand, counter = stamp[2:].split(':')
+        except ValueError:
+            ERR.write("Malformed version 1 hashcash stamp!\n")
+            return False
+        if resource is not None and resource != res:
+            return False
+        elif type(bits) is int and bits > int(claim):
+            return False
+        elif check_expiration is not None:
+            good_until = strftime("%y%m%d%H%M%S", localtime(time()-check_expiration))
+            if date < good_until:
+                return False
+        elif callable(ds_callback) and ds_callback(stamp):
+            return False
+        else:
+            hex_digits = int(floor(int(claim)/4))
+            return sha(stamp).hexdigest().startswith('0'*hex_digits)
+    else:                               # Unknown ver or generalized hashcash
+        ERR.write("Unknown hashcash version: Minimal authentication!\n")
+        if type(bits) is not int:
+            return True
+        elif resource is not None and stamp.find(resource) < 0:
+            return False
+        else:
+            hex_digits = int(floor(bits/4))
+            return sha(stamp).hexdigest().startswith('0'*hex_digits)
+
+def is_doublespent(stamp):
+    """Placeholder for double spending callback function
+
+    The check() function may accept a 'ds_callback' argument, e.g.
+      check(stamp, "mertz@gnosis.cx", bits=20, ds_callback=is_doublespent)
+
+    This placeholder simply reports stamps as not being double spent.
+    """
+    return False
+
+if __name__=='__main__':
+    # Import Psyco if available
+    try:
+        import psyco
+        psyco.bind(_mint)
+    except ImportError:
+        pass
+    import optparse
+    out, err = sys.stdout.write, sys.stderr.write
+    parser = optparse.OptionParser(version="%prog 0.1",
+                      usage="%prog -c|-m [-b bits] [string|STDIN]")
+    parser.add_option('-b', '--bits', type='int', dest='bits', default=20,
+                      help="Specify required collision bits" )
+    parser.add_option('-m', '--mint', help="Mint a new stamp",
+                      action='store_true', dest='mint')
+    parser.add_option('-c', '--check', help="Check a stamp for validity",
+                      action='store_true', dest='check')
+    parser.add_option('-s', '--timer', help="Time the operation performed",
+                      action='store_true', dest='timer')
+    parser.add_option('-n', '--raw', help="Suppress trailing newline",
+                      action='store_true', dest='raw')
+    (options, args) = parser.parse_args()
+    start = time()
+    if options.mint:    action = mint
+    elif options.check: action = check
+    else:
+        out("Try: %s --help\n" % sys.argv[0])
+        sys.exit()
+    if args: out(str(action(args[0], bits=options.bits)))
+    else:    out(str(action(sys.stdin.read(), bits=options.bits)))
+    if not options.raw: sys.stdout.write('\n')
+    if options.timer:
+        timer = time()-start
+        err("Completed in %0.4f seconds (%d hashes per second)\n" %
+            (timer, tries[0]/timer))
+