ssh key failures: "Input strings must be a multiple of 8 in length"

[Antoine Martin]

This happens on some distributions that ship old versions of Python-Twisted (older than 10.x?) - the user will not be able to use their SSH key with Window Switch because of this.

This causes stacktraces like this one:

[II] 2011/08/09 17:41:05 ConchUserAuth.ok(################,False)
[EE] 2011/08/09 17:41:05 ConchUserAuth.ok((...),False) cannot load private key
[ee] 2011/08/09 17:41:05 Traceback (most recent call last):
[ee] 2011/08/09 17:41:05   File "/usr/lib/pymodules/python2.6/winswitch/net/conch_util.py", line 228, in ok
[ee] 2011/08/09 17:41:05     key = privateKeyFromFile(self.factory.private_key, new_passphrase)
[ee] 2011/08/09 17:41:05   File "/usr/lib/pymodules/python2.6/winswitch/net/conch_util.py", line 207, in privateKeyFromFile
[ee] 2011/08/09 17:41:05     return     keys.Key.fromFile(filename, passphrase=passphrase).keyObject
[ee] 2011/08/09 17:41:05   File "/usr/lib/python2.6/dist-packages/twisted/conch/ssh/keys.py", line 62, in fromFile
[ee] 2011/08/09 17:41:05     return Class.fromString(file(filename, 'rb').read(), type, passphrase)
[ee] 2011/08/09 17:41:05   File "/usr/lib/python2.6/dist-packages/twisted/conch/ssh/keys.py", line 90, in fromString
[ee] 2011/08/09 17:41:05     return method(data, passphrase)
[ee] 2011/08/09 17:41:05   File "/usr/lib/python2.6/dist-packages/twisted/conch/ssh/keys.py", line 216, in _fromString_PRIVATE_OPENSSH
[ee] 2011/08/09 17:41:05     keyData = DES3.new(decKey, DES3.MODE_CBC, iv).decrypt(b64Data)
[ee] 2011/08/09 17:41:05 ValueError: Input strings must be a multiple of 8 in length

It will occur more often now that distros are using the AES-128 cypher when generating SSH keys.

This bug is here just as a reminder that there is absolutely nothing we can do about this: it is the distribution's responsibility to fix their buggy libraries. Once again, Ubuntu is shipping buggy versions in its "stable" release... sigh

More info ​at zenoss.org and ​their ticket

[Antoine Martin]

If you this and are wondering how to fix this, just ask your distro to fix Twisted.

This cannot be fixed in Window Switch.

[Ryanofsky]

It's pretty easy to work around this issue by converting your key from AES to DES3:

openssl rsa -in id_rsa -out newkey_id_rsa -des3
mv newkey_id_rsa id_rsa
chmod 600 id_rsa
truncate --size=-1 id_rsa

The truncate command is needed to remove a trailing newline in the OpenSSL output which causes conch to parse the key incorrectly.

For reference, here's the error I was seeing with my AES key:

[EE] 2012/19/08 14:41:04 ConchUserAuth.privateKeyFromFile(/home/ryanofsky/.ssh/id_rsa,#######) unsupported key file format?
[ee] 2012/19/08 14:41:04 Traceback (most recent call last):
[ee] 2012/19/08 14:41:04   File "/usr/lib/pymodules/python2.6/winswitch/net/conch_util.py", line 298, in privateKeyFromFile
[ee] 2012/19/08 14:41:04     key =	keys.Key.fromFile(filename, passphrase=passphrase).keyObject
[ee] 2012/19/08 14:41:04   File "/usr/lib/python2.6/dist-packages/twisted/conch/ssh/keys.py", line 62, in fromFile
[ee] 2012/19/08 14:41:04     return Class.fromString(file(filename, 'rb').read(), type, passphrase)
[ee] 2012/19/08 14:41:04   File "/usr/lib/python2.6/dist-packages/twisted/conch/ssh/keys.py", line 90, in fromString
[ee] 2012/19/08 14:41:04     return method(data, passphrase)
[ee] 2012/19/08 14:41:04   File "/usr/lib/python2.6/dist-packages/twisted/conch/ssh/keys.py", line 216, in _fromString_PRIVATE_OPENSSH
[ee] 2012/19/08 14:41:04     keyData = DES3.new(decKey, DES3.MODE_CBC, iv).decrypt(b64Data)
[ee] 2012/19/08 14:41:04 ValueError: IV must be 8 bytes long

Here's the error I was seeing my DES3 key + newline:

[EE] 2012/19/08 14:43:43 ConchUserAuth.privateKeyFromFile(/home/ryanofsky/.ssh/id_rsa,#######) unsupported key file format?
[ee] 2012/19/08 14:43:43 Traceback (most recent call last):
[ee] 2012/19/08 14:43:43   File "/usr/lib/pymodules/python2.6/winswitch/net/conch_util.py", line 298, in privateKeyFromFile
[ee] 2012/19/08 14:43:43     key =	keys.Key.fromFile(filename, passphrase=passphrase).keyObject
[ee] 2012/19/08 14:43:43   File "/usr/lib/python2.6/dist-packages/twisted/conch/ssh/keys.py", line 62, in fromFile
[ee] 2012/19/08 14:43:43     return Class.fromString(file(filename, 'rb').read(), type, passphrase)
[ee] 2012/19/08 14:43:43   File "/usr/lib/python2.6/dist-packages/twisted/conch/ssh/keys.py", line 90, in fromString
[ee] 2012/19/08 14:43:43     return method(data, passphrase)
[ee] 2012/19/08 14:43:43   File "/usr/lib/python2.6/dist-packages/twisted/conch/ssh/keys.py", line 216, in _fromString_PRIVATE_OPENSSH
[ee] 2012/19/08 14:43:43     keyData = DES3.new(decKey, DES3.MODE_CBC, iv).decrypt(b64Data)
[ee] 2012/19/08 14:43:43 ValueError: Input strings must be a multiple of 8 in length