Key.privateBlob is wrong for ECDSA

[twisted-trac]

	@cjwatson reported
Trac ID	trac#9682
Type	defect
Created	2019-07-31 13:48:44Z

In ce1b014 and #1053, I successfully argued that the encoding of ECDSA keys expected by Key._fromString_PRIVATE_BLOB was wrong: this is currently only used by the agent protocol, and the de-facto implementation of that in OpenSSH differed from that previously implemented in Twisted. We changed Twisted to match.

However, when I was fixing the decoder, I missed that Key.privateBlob implements a corresponding encoder, so now round-tripping fails for ECDSA keys:

>>> from twisted.conch.ssh import keys
>>> from twisted.conch.test import keydata
>>> ecdsaKey = keys.Key._fromECComponents(**keydata.ECDatanistp256)
>>> keys.Key._fromString_PRIVATE_BLOB(ecdsaKey.privateBlob())
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/cjwatson/src/python/twisted/twisted/build/py36-alldeps-nocov/lib/python3.6/site-packages/twisted/conch/ssh/keys.py", line 292, in _fromString_PRIVATE_BLOB
    'type %r' % (curveName, keyType))
twisted.conch.ssh.keys.BadKeyError: ECDSA curve name b'\x00\xa8\xa6_P\xd9\xef\xe4#\xd1*_x\xf3\x1b\xa3Z[\xeb!\xa4\x01\xbee\xed\xdb\x82\xd6_9\xe7 \xfb' does not match key type b'ecdsa-sha2-nistp256'

This doesn't make much difference today because Key.privateBlob is unused outside tests, but failing to round-trip is clearly unhelpful. The most obvious use case for this method is in writing the OpenSSH v1 private key format, and I discovered this discrepancy in the process of doing that.

Searchable metadata

trac-id__9682 9682
type__defect defect
reporter__cjwatson cjwatson
priority__normal normal
milestone__None None
branch__ 
branch_author__ 
status__closed closed
resolution__fixed fixed
component__conch conch
keywords__None None
time__1564580924631536 1564580924631536
changetime__1571655809872531 1571655809872531
version__None None
owner__Colin_Watson__cjwatson_____ Colin Watson <cjwatson@...>

[twisted-trac]

	@cjwatson commented

#1172

[twisted-trac]

	@hawkowl commented

lgtm

[twisted-trac]

	@cjwatson set owner to @cjwatson @cjwatson set status to closed

In changeset 1f9d14b

#!CommitTicketReference repository="" revision="1f9d14b1b32b1bc88d035f0994832172122a30bb"
Merge cjwatson:9682-conch-fix-ecdsa-private-blob: Fix writing of ECDSA private blob format (#1172)

Author: cjwatson
Reviewer: hawkowl
Fixes: ticket:9682