ssh does nut support brainpool curves and thus GnuPG does not know how to map its internal name of the curve to the name as specified by ssh. GnuPG supports these curves:

We had this idea to have a label: or similar item in the extended-key-format which is displayed in addition to the other info. The user can then use an editor to put whatever she likes into this field.

there should be clearer labelling of smartcards so that users can tell them apart more easily

@werner I was hoping to make a modified gpg-agent build that would let me walk through what's going on after the nonce is sent but it looks like the gpg4win process only takes in a package of pre-built gpg binaries which rules that out. As far as I can figure out, after the nonce is read and accepted, libassuan creates a stream object out of the socket and then finding nothing in the stream terminates the ssh handler. We send the actual client request immediately after the nonce but in a separate call to send() so I now wonder if by not having anything read in at the same time as the nonce gpg-agent or libassuan thinks that it's a 0-length stream.

I just took a look through assuan-socket.c and it appears that we just need to send the nonce and don't need to read anything back. We also found a bug on our side that was preventing the nonce from being sent, which has been fixed. The error message logged above no longer happens.

The nonce is a string of octets thus it needs to be passed verbatim. I would need to study the code in libassun/src/assuan-socket.c to tell more.

@werner After sending the nonce value from the socket file, does anything need to be read back before ssh-agent commands can be sent? Are there any byte ordering requirements for sending the nonce or can they be sent in the same order as they are in the file?

I've been working with one of Microsoft's developers on a temporary tool that should bridge the connection between named pipes and the Unix sockets emulation used by gpg-agent but things appear to trip up with sending the nonce. From the position of the tool, the nonce value is successfully sent (send returns 16), but never seems to be picked up by gpg-agent. Instead both gpg-agent and the bridge sit there until whatever tool is using them (I test using ssh-add -l) is terminated, at which point gpg-agent immediately spits up the message

Rhat's for the client, right. I never used it. We used to run a Windows 8 instance in a VM to run tests via ssh on it. That worked most not really stable. For obvious reasons I am more interested in the server part ;-)

Thanks. I took these patches and simplified them. Not test tested, though,.

I would argue that the Windows port of OpenSSH is not unstable at this point, especially given that Microsoft is even providing it as an installable feature in the next regular Windows 10 release. The fact that the port is now using actual OpenSSH version numbers instead of their own 0.x versions lends credence to this as well.

Thanks for the fix! however, the fix only addresses the two flags we currently know about. I've pushed a branch T3880-fix that tries to implement the If the agent does not support the requested flags […] It must reply with a SSH_AGENT_FAILURE message part of the spec.

It is in 2.2.6

Thanks for the pointer. But as long as the Windows ssh server is that instable I see no urgent need to add this to GnuPG.

Fixed in 273964798592cd479c111f47e8ce46d5b1999d6a.

Well, can you then please fix it?

Fixed as of 525f2c482abb6bc2002eb878b03558fb43e6b004.

Applied as ebe12be034f0.

While I can't reproduce this problem myself, I think I found an issue of gpg-agent passphrase caching.
Double free may happen when multiple threads enter agent_put_cache, for example.

In 2.1.19, gpg-agent uses getpeerucred for macOS. I changed it (since it seemed not working). In 2.1.20, gpg-agent now uses getsockopt with LOCAL_PEERPID.
It seems for me that the crash occurs by ucred_free. If this is the case, 2.1.20 fixes this issue.

I can reproduce this. Our test indeed feeds a passphrase to the agent.

Any thoughts or progress on this?