mdw@git.distorted.org.uk Git - catacomb-python/blob - catacomb/pwsafe.py

   1 ### -*-python-*-
   2 ###
   3 ### Management of a secure password database
   4 ###
   5 ### (c) 2005 Straylight/Edgeware
   6 ###
   7
   8 ###----- Licensing notice ---------------------------------------------------
   9 ###
  10 ### This file is part of the Python interface to Catacomb.
  11 ###
  12 ### Catacomb/Python is free software; you can redistribute it and/or modify
  13 ### it under the terms of the GNU General Public License as published by
  14 ### the Free Software Foundation; either version 2 of the License, or
  15 ### (at your option) any later version.
  16 ###
  17 ### Catacomb/Python is distributed in the hope that it will be useful,
  18 ### but WITHOUT ANY WARRANTY; without even the implied warranty of
  19 ### MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  20 ### GNU General Public License for more details.
  21 ###
  22 ### You should have received a copy of the GNU General Public License along
  23 ### with Catacomb/Python; if not, write to the Free Software Foundation,
  24 ### Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
  25
  26 ###--------------------------------------------------------------------------
  27 ### Imported modules.
  28
  29 from __future__ import with_statement
  30
  31 import catacomb as _C
  32 import gdbm as _G
  33
  34 ###--------------------------------------------------------------------------
  35 ### Underlying cryptography.
  36
  37 class DecryptError (Exception):
  38   """
  39   I represent a failure to decrypt a message.
  40
  41   Usually this means that someone used the wrong key, though it can also
  42   mean that a ciphertext has been modified.
  43   """
  44   pass
  45
  46 class Crypto (object):
  47   """
  48   I represent a symmetric crypto transform.
  49
  50   There's currently only one transform implemented, which is the obvious
  51   generic-composition construction: given a message m, and keys K0 and K1, we
  52   choose an IV v, and compute:
  53
  54     * y = v || E(K0, v; m)
  55     * t = M(K1; y)
  56
  57   The final ciphertext is t || y.
  58   """
  59
  60   def __init__(me, c, h, m, ck, mk):
  61     """
  62     Initialize the Crypto object with a given algorithm selection and keys.
  63
  64     We need a GCipher subclass C, a GHash subclass H, a GMAC subclass M, and
  65     keys CK and MK for C and M respectively.
  66     """
  67     me.c = c(ck)
  68     me.m = m(mk)
  69     me.h = h
  70
  71   def encrypt(me, pt):
  72     """
  73     Encrypt the message PT and return the resulting ciphertext.
  74     """
  75     blksz = me.c.__class__.blksz
  76     b = _C.WriteBuffer()
  77     if blksz:
  78       iv = _C.rand.block(blksz)
  79       me.c.setiv(iv)
  80       b.put(iv)
  81     b.put(me.c.encrypt(pt))
  82     t = me.m().hash(b).done()
  83     return t + str(buffer(b))
  84
  85   def decrypt(me, ct):
  86     """
  87     Decrypt the ciphertext CT, returning the plaintext.
  88
  89     Raises DecryptError if anything goes wrong.
  90     """
  91     blksz = me.c.__class__.blksz
  92     tagsz = me.m.__class__.tagsz
  93     b = _C.ReadBuffer(ct)
  94     t = b.get(tagsz)
  95     h = me.m()
  96     if blksz:
  97       iv = b.get(blksz)
  98       me.c.setiv(iv)
  99       h.hash(iv)
 100     x = b.get(b.left)
 101     h.hash(x)
 102     if t != h.done(): raise DecryptError
 103     return me.c.decrypt(x)
 104
 105 class PPK (Crypto):
 106   """
 107   I represent a crypto transform whose keys are derived from a passphrase.
 108
 109   The password is salted and hashed; the salt is available as the `salt'
 110   attribute.
 111   """
 112
 113   def __init__(me, pp, c, h, m, salt = None):
 114     """
 115     Initialize the PPK object with a passphrase and algorithm selection.
 116
 117     We want a passphrase PP, a GCipher subclass C, a GHash subclass H, a GMAC
 118     subclass M, and a SALT.  The SALT may be None, if we're generating new
 119     keys, indicating that a salt should be chosen randomly.
 120     """
 121     if not salt: salt = _C.rand.block(h.hashsz)
 122     tag = '%s\0%s' % (pp, salt)
 123     Crypto.__init__(me, c, h, m,
 124                     h().hash('cipher:' + tag).done(),
 125                     h().hash('mac:' + tag).done())
 126     me.salt = salt
 127
 128 ###--------------------------------------------------------------------------
 129 ### Backend storage.
 130
 131 class StorageBackend (object):
 132   """
 133   I provide a backend for password and metadata storage.
 134
 135   Backends are responsible for storing and retrieving stuff, but not for the
 136   cryptographic details.  Backends need to store two kinds of information:
 137
 138     * metadata, consisting of a number of property names and their values;
 139       and
 140
 141     * password mappings, consisting of a number of binary labels and
 142       payloads.
 143   """
 144
 145   FAIL = ['FAIL']
 146
 147   ## Life cycle methods.
 148
 149   @classmethod
 150   def create(cls, file):
 151     """Create a new database in the named FILE, using this backend."""
 152     return cls(writep = True, _magic = lambda me: me._create(file))
 153   def _create(me, file):
 154     me._db = _G.open(file, 'n', 0600)
 155
 156   def __init__(me, file = None, writep = False, _magic = None, *args, **kw):
 157     """
 158     Main constructor.
 159     """
 160     super(StorageBackend, me).__init__(*args, **kw)
 161     if _magic is not None: _magic(me)
 162     elif file is None: raise ValueError, 'missing file parameter'
 163     else: me._db = _G.open(file, writep and 'w' or 'r')
 164     me._writep = writep
 165     me._livep = True
 166
 167   def close(me):
 168     """
 169     Close the database.
 170
 171     It is harmless to attempt to close a database which has been closed
 172     already.
 173     """
 174     if me._livep:
 175       me._livep = False
 176       me._db.close()
 177
 178   ## Utilities.
 179
 180   def _check_live(me):
 181     """Raise an error if the receiver has been closed."""
 182     if not me._livep: raise ValueError, 'database is closed'
 183
 184   def _check_write(me):
 185     """Raise an error if the receiver is not open for writing."""
 186     me._check_live()
 187     if not me._writep: raise ValueError, 'database is read-only'
 188
 189   def _check_meta_name(me, name):
 190     """
 191     Raise an error unless NAME is a valid name for a metadata item.
 192
 193     Metadata names may not start with `$': such names are reserved for
 194     password storage.
 195     """
 196     if name.startswith('$'):
 197       raise ValueError, "invalid metadata key `%s'" % name
 198
 199   ## Context protocol.
 200
 201   def __enter__(me):
 202     """Context protocol: make sure the database is closed on exit."""
 203     return me
 204   def __exit__(me, exctype, excvalue, exctb):
 205     """Context protocol: see `__enter__'."""
 206     me.close()
 207
 208   ## Metadata.
 209
 210   def get_meta(me, name, default = FAIL):
 211     """
 212     Fetch the value for the metadata item NAME.
 213
 214     If no such item exists, then return DEFAULT if that was set; otherwise
 215     raise a `KeyError'.
 216     """
 217     me._check_meta_name(name)
 218     me._check_live()
 219     try: value = me._db[name]
 220     except KeyError: value = default
 221     if value is StorageBackend.FAIL: raise KeyError, name
 222     return value
 223
 224   def put_meta(me, name, value):
 225     """Store VALUE in the metadata item called NAME."""
 226     me._check_meta_name(name)
 227     me._check_write()
 228     me._db[name] = value
 229
 230   def del_meta(me, name):
 231     """Forget about the metadata item with the given NAME."""
 232     me._check_meta_name(name)
 233     me._check_write()
 234     del me._db[name]
 235
 236   def iter_meta(me):
 237     """Return an iterator over the name/value metadata items."""
 238     me._check_live()
 239     k = me._db.firstkey()
 240     while k is not None:
 241       if not k.startswith('$'): yield k, me._db[k]
 242       k = me._db.nextkey(k)
 243
 244   ## Passwords.
 245
 246   def get_passwd(me, label):
 247     """
 248     Fetch and return the payload stored with the (opaque, binary) LABEL.
 249
 250     If there is no such payload then raise `KeyError'.
 251     """
 252     me._check_live()
 253     return me._db['$' + label]
 254
 255   def put_passwd(me, label, payload):
 256     """
 257     Associate the (opaque, binary) PAYLOAD with the (opaque, binary) LABEL.
 258
 259     Any previous payload for LABEL is forgotten.
 260     """
 261     me._check_write()
 262     me._db['$' + label] = payload
 263
 264   def del_passwd(me, label):
 265     """
 266     Forget any PAYLOAD associated with the (opaque, binary) LABEL.
 267
 268     If there is no such payload then raise `KeyError'.
 269     """
 270     me._check_write()
 271     del me._db['$' + label]
 272
 273   def iter_passwds(me):
 274     """Return an iterator over the stored password label/payload pairs."""
 275     me._check_live()
 276     k = me._db.firstkey()
 277     while k is not None:
 278       if k.startswith('$'): yield k[1:], me._db[k]
 279       k = me._db.nextkey(k)
 280
 281 ###--------------------------------------------------------------------------
 282 ### Password storage.
 283
 284 class PW (object):
 285   """
 286   I represent a secure (ish) password store.
 287
 288   I can store short secrets, associated with textual names, in a way which
 289   doesn't leak too much information about them.
 290
 291   I implement (some of) the Python mapping protocol.
 292
 293   I keep track of everything using a StorageBackend object.  This contains
 294   password entries, identified by cryptographic labels, and a number of
 295   metadata items.
 296
 297   cipher        Names the Catacomb cipher selected.
 298
 299   hash          Names the Catacomb hash function selected.
 300
 301   key           Cipher and MAC keys, each prefixed by a 16-bit big-endian
 302                 length and concatenated, encrypted using the master
 303                 passphrase.
 304
 305   mac           Names the Catacomb message authentication code selected.
 306
 307   magic         A magic string for obscuring password tag names.
 308
 309   salt          The salt for hashing the passphrase.
 310
 311   tag           The master passphrase's tag, for the Pixie's benefit.
 312
 313   Password entries are assigned labels of the form `$' || H(MAGIC || TAG);
 314   the corresponding value consists of a pair (TAG, PASSWD), prefixed with
 315   16-bit lengths, concatenated, padded to a multiple of 256 octets, and
 316   encrypted using the stored keys.
 317   """
 318
 319   def __init__(me, file, writep = False):
 320     """
 321     Initialize a PW object from the database in FILE.
 322
 323     If WRITEP is false (the default) then the database is opened read-only;
 324     if true then it may be written.  Requests the database password from the
 325     Pixie, which may cause interaction.
 326     """
 327
 328     ## Open the database.
 329     me.db = StorageBackend(file, writep)
 330
 331     ## Find out what crypto to use.
 332     c = _C.gcciphers[me.db.get_meta('cipher')]
 333     h = _C.gchashes[me.db.get_meta('hash')]
 334     m = _C.gcmacs[me.db.get_meta('mac')]
 335
 336     ## Request the passphrase and extract the master keys.
 337     tag = me.db.get_meta('tag')
 338     ppk = PPK(_C.ppread(tag), c, h, m, me.db.get_meta('salt'))
 339     try:
 340       b = _C.ReadBuffer(ppk.decrypt(me.db.get_meta('key')))
 341     except DecryptError:
 342       _C.ppcancel(tag)
 343       raise
 344     me.ck = b.getblk16()
 345     me.mk = b.getblk16()
 346     if not b.endp: raise ValueError, 'trailing junk'
 347
 348     ## Set the key, and stash it and the tag-hashing secret.
 349     me.k = Crypto(c, h, m, me.ck, me.mk)
 350     me.magic = me.k.decrypt(me.db.get_meta('magic'))
 351
 352   @classmethod
 353   def create(cls, file, tag, c, h, m):
 354     """
 355     Create and initialize a new database FILE.
 356
 357     We want a GCipher subclass C, a GHash subclass H, and a GMAC subclass M;
 358     and a Pixie passphrase TAG.
 359
 360     This doesn't return a working object: it just creates the database file
 361     and gets out of the way.
 362     """
 363
 364     ## Set up the cryptography.
 365     pp = _C.ppread(tag, _C.PMODE_VERIFY)
 366     ppk = PPK(pp, c, h, m)
 367     ck = _C.rand.block(c.keysz.default)
 368     mk = _C.rand.block(c.keysz.default)
 369     k = Crypto(c, h, m, ck, mk)
 370
 371     ## Set up and initialize the database.
 372     kct = ppk.encrypt(_C.WriteBuffer().putblk16(ck).putblk16(mk))
 373     with StorageBackend.create(file) as db:
 374       db.put_meta('tag', tag)
 375       db.put_meta('salt', ppk.salt)
 376       db.put_meta('cipher', c.name)
 377       db.put_meta('hash', h.name)
 378       db.put_meta('mac', m.name)
 379       db.put_meta('key', kct)
 380       db.put_meta('magic', k.encrypt(_C.rand.block(h.hashsz)))
 381
 382   def keyxform(me, key):
 383     """Transform the KEY (actually a password tag) into a password label."""
 384     return me.k.h().hash(me.magic).hash(key).done()
 385
 386   def changepp(me):
 387     """
 388     Change the database password.
 389
 390     Requests the new password from the Pixie, which will probably cause
 391     interaction.
 392     """
 393     tag = me.db.get_meta('tag')
 394     _C.ppcancel(tag)
 395     ppk = PPK(_C.ppread(tag, _C.PMODE_VERIFY),
 396               me.k.c.__class__, me.k.h, me.k.m.__class__)
 397     kct = ppk.encrypt(_C.WriteBuffer().putblk16(me.ck).putblk16(me.mk))
 398     me.db.put_meta('key', kct)
 399     me.db.put_meta('salt', ppk.salt)
 400
 401   def pack(me, key, value):
 402     """Pack the KEY and VALUE into a ciphertext, and return it."""
 403     b = _C.WriteBuffer()
 404     b.putblk16(key).putblk16(value)
 405     b.zero(((b.size + 255) & ~255) - b.size)
 406     return me.k.encrypt(b)
 407
 408   def unpack(me, ct):
 409     """
 410     Unpack a ciphertext CT and return a (KEY, VALUE) pair.
 411
 412     Might raise DecryptError, of course.
 413     """
 414     b = _C.ReadBuffer(me.k.decrypt(ct))
 415     key = b.getblk16()
 416     value = b.getblk16()
 417     return key, value
 418
 419   ## Mapping protocol.
 420
 421   def __getitem__(me, key):
 422     """Return the password for the given KEY."""
 423     try: return me.unpack(me.db.get_passwd(me.keyxform(key)))[1]
 424     except KeyError: raise KeyError, key
 425
 426   def __setitem__(me, key, value):
 427     """Associate the password VALUE with the KEY."""
 428     me.db.put_passwd(me.keyxform(key), me.pack(key, value))
 429
 430   def __delitem__(me, key):
 431     """Forget all about the KEY."""
 432     try: me.db.del_passwd(me.keyxform(key))
 433     except KeyError: raise KeyError, key
 434
 435   def __iter__(me):
 436     """Iterate over the known password tags."""
 437     for _, pld in me.db.iter_passwds():
 438       yield me.unpack(pld)[0]
 439
 440   ## Context protocol.
 441
 442   def __enter__(me):
 443     return me
 444   def __exit__(me, excty, excval, exctb):
 445     me.db.close()
 446
 447 ###----- That's all, folks --------------------------------------------------