Suggested by Manfred Kaiser, who also wrote most of this patch
(although outlying parts, like documentation and SSH-1 support, are by
me).
This is a second line of defence against the kind of spoofing attacks
in which a malicious or compromised SSH server rushes the client
through the userauth phase of SSH without actually requiring any auth
inputs (passwords or signatures or whatever), and then at the start of
the connection phase it presents something like a spoof prompt,
intended to be taken for part of userauth by the user but in fact with
some more sinister purpose.
Our existing line of defence against this is the trust sigil system,
and as far as I know, that's still working. This option allows a bit of
extra defence in depth: if you don't expect your SSH server to
trivially accept authentication in the first place, then enabling this
option will cause PuTTY to disconnect if it unexpectedly does so,
without the user having to spot the presence or absence of a fiddly
little sigil anywhere.
Several types of authentication count as 'trivial'. The obvious one is
the SSH-2 "none" method, which clients always try first so that the
failure message will tell them what else they can try, and which a
server can instead accept in order to authenticate you unconditionally.
But there are two other ways to do it that we know of: one is to run
keyboard-interactive authentication and send an empty INFO_REQUEST
packet containing no actual prompts for the user, and another even
weirder one is to send USERAUTH_SUCCESS in response to the user's
preliminary *offer* of a public key (instead of sending the usual PK_OK
to request an actual signature from the key).
This new option detects all of those, by clearing the 'is_trivial_auth'
flag only when we send some kind of substantive authentication response
(be it a password, a k-i prompt response, a signature, or a GSSAPI
token). So even if there's a further path through the userauth maze we
haven't spotted, that somehow avoids sending anything substantive, this
strategy should still pick it up.
(cherry picked from commit 5f5c710cf3)
Since the previous commit is causing an RC2 build of 0.75 anyway,
let's take the opportunity to bring in updates to the docs from main,
so that the release will have the most up-to-date version available.
This is a combined cherry-pick of:
f6142ba29b7c1bea59a3f5d1d4ce4b
I've just spent the afternoon playing with it (rather belatedly - this
is the first time I've tried it out since it was first announced!),
and quickly decided that on the one hand it looks quite useful, but on
the other hand, running it in a Windows console is not for me and I'd
prefer to talk to it via PuTTY and psusan, for nicer copy-paste
controls and the ability to forward Pageant into it.
That turns out to be very easy and (I think) useful, so in it goes as
another psusan use case.
Suggested by Jacob: if this dialog box is going to pop up
_unexpectedly_ - perhaps when people have momentarily forgotten
they're even running Pageant, or at least forgotten they added a key
encrypted,, or maybe haven't found out yet that their IT installed it
- then it could usefully come with a help button that pops up further
explanation of what the dialog box means, and from which you can find
your way to the rest of the help.
If we're publishing the server, then we should say something about the
fact that this option exists to talk to it. Also, if the option exists
on the front page at all in a released version of PuTTY, it behooves
us to document it slightly more usefully than just a handwave at 'this
is specialist and experimental'.
SUPDUP came, at my insistence, with a history section in the docs
for people who hadn't heard of it. It seems only fair that the
other obsolete network protocols (or, at least, the ones we *wish*
were obsolete :-) should have the same kind of treatment.
Moved the Raw protocol to below Serial, so that the first two
sections are SSH and Serial, matching the (now very emphatic)
priority order in the config UI.
Similarly, reordered the bullet points in \k{config-hostname}.
I've filled in some text about prime generation methods and Ed448,
which were all the things marked as 'review before release'.
While I'm at it, also filled in a reasonable enough DSA key length
recommendation, because the FIXME comment in that section was within
sight of one of the places I was editing. FIPS 186-4 seemed to think
that RSA and DSA had comparable relationships between the key length
and practical security level, so I see no reason not to use the same
recommendation for both key types.
This seems more useful than the previous behaviour of not prompting for
a passphrase and only emitting the public part; if we want that back
I suppose we could invent a "-O text-public".
Also, document the text dump format a bit in the man page.
This removes both uses of SHA-1 in the file format: it was used as the
MAC protecting the key file against tamperproofing, and also used in
the key derivation step that converted the user's passphrase to cipher
and MAC keys.
The MAC is simply upgraded from HMAC-SHA-1 to HMAC-SHA-256; it is
otherwise unchanged in how it's applied (in particular, to what data).
The key derivation is totally reworked, to be based on Argon2, which
I've just added to the code base. This should make stolen encrypted
key files more resistant to brute-force attack.
Argon2 has assorted configurable parameters for memory and CPU usage;
the new key format includes all those parameters. So there's no reason
we can't have them under user control, if a user wants to be
particularly vigorous or particularly lightweight with their own key
files. They could even switch to one of the other flavours of Argon2,
if they thought side channels were an especially large or small risk
in their particular environment. In this commit I haven't added any UI
for controlling that kind of thing, but the PPK loading function is
all set up to cope, so that can all be added in a future commit
without having to change the file format.
While I'm at it, I've also switched the CBC encryption to using a
random IV (or rather, one derived from the passphrase along with the
cipher and MAC keys). That's more like normal SSH-2 practice.
When I transcribed the code into this document, I misread 'put_data'
as 'put_string' in several places, and documented SSH-style string
length headers that do not actually exist in the format.
Somebody on comp.security.ssh asked about it recently, and I decided
that storing it in a comment in the key file was not really good
enough. Also, that comment was incomplete (it listed the private key
formats for RSA and DSA but not any of the newer ECC key types, simple
as their private-key formats may be).
A user wrote in to point out the one in winhandl.c, and out of sheer
curiosity, I grepped the whole source base for '([a-zA-Z])\1\1' to see
if there were any others. Of course there are a lot of perfectly
sensible ones, like 'www' or 'Grrr', not to mention any amount of
0xFFFF and the iiii/bbbb emphasis system in Halibut code paragraphs,
but I did spot one more in the recently added udp.but section on
traits, and another in a variable name in uxagentsock.c.
A user mentioned having found this confusing recently, and fair
enough, because it's done in a way that doesn't quite match the
built-in OO system of any language I know about. But after the
rewriting in recent years, I think pretty much everything in PuTTY
that has a system of interchangeable implementations of the same
abstract type is now done basically the same way, so this seems like a
good moment to document the idiom we use and explain all its ins and
outs.
