OpenSSH public key serialization

[reaperhulk]

We should support serialization of public keys to OpenSSH format.

[adiroiban]

This is my code based on Twisted SSH keys https://github.com/chevah/chevah-keycert

it no longer depends on Twisted and on top of twisted it add support for SSH.Com (Tectia) and Putty keys... fixed for parsing invalid keys

If you want I can help integrate the code with python cryptography.

[reaperhulk]

This issue boils down to essentially reversing the code here: https://github.com/pyca/cryptography/blob/master/src/cryptography/hazmat/primitives/serialization.py#L35 (plus adding a new serialization.PublicFormat type and supporting it in the public_bytes methods of the public keys).

[reaperhulk]

One challenge here: our public_bytes method takes an encoding and a format, but for OpenSSH only the format is really meaningful (there's no DER/PEM for openssh public keys). Ideas on how to handle this? We could allow/require None for encoding in this case potentially, but meh.

[alex]

Encoding=SSH(), format=SSH()?

On Mon, Mar 14, 2016 at 11:10 PM, Paul Kehrer notifications@github.com
wrote:

One challenge here: our public_bytes method takes an encoding and a
format, but for OpenSSH only the format is really meaningful (there's no
DER/PEM for openssh public keys). Ideas on how to handle this? We could
allow/require None for encoding in this case potentially, but meh.

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"I disapprove of what you say, but I will defend to the death your right to
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[reaperhulk]

We can do that. We'll need to audit every place we currently use the Encoding enum to make sure we're erroring properly when you pass that value, but we'd have to do that with None as well.

[alex]

I think the worst case scenario is an assertionerror

On Mon, Mar 14, 2016 at 11:15 PM, Paul Kehrer notifications@github.com
wrote:

We can do that. We'll need to audit every place we currently use the
Encoding enum to make sure we're erroring properly when you pass that
value, but we'd have to do that with None as well.

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#1744 (comment)

"I disapprove of what you say, but I will defend to the death your right to
say it." -- Evelyn Beatrice Hall (summarizing Voltaire)
"The people's good is the highest law." -- Cicero
GPG Key fingerprint: D1B3 ADC0 E023 8CA6

[reaperhulk]

I'm working a bit on this. Will hopefully have time to put together a full PR tonight. Here's roughly what serializing an RSA public key looks like:

            e = self._bn_to_int(cdata.e)
            n = self._bn_to_int(cdata.n)
            e_bytes = utils.int_to_bytes(e)
            if ord(e_bytes[0]) & 0x80 != 0:
                e_bytes = b"\x00" + e_bytes
            n_bytes = utils.int_to_bytes(n)
            if ord(n_bytes[0]) & 0x80 != 0:
                n_bytes = b"\x00" + n_bytes

            payload = base64.b64encode(b"{0}{1}{2}{3}{4}{5}".format(
                struct.pack(">I", 7),
                b"ssh-rsa",
                struct.pack(">I", len(e_bytes)),
                e_bytes,
                struct.pack(">I", len(n_bytes)),
                n_bytes,
            ))
            return b"ssh-rsa {0}".format(payload)

[adiroiban]

Latest Twisted code was ported to use pyca/cryptography

twisted.conch.ssh.keys was also recently ported to py3 and has pretty good test coverage.

Are you interested in re-using that code in pyca/cryptography ?

Once you have decided on an API I can work on implementing that API based on twisted.conch.ssh.keys.

Once that code is in pyca/cryptography I can updated Twisted to use the shard code from pyca/cryptography.

[reaperhulk]

If you'd like to work on this I'm happy to define the API. It's just an expansion of enums for our current public_bytes method. So public_bytes(encoding, format) where right now Encoding is an enum of PEM/DER and PublicFormat is an enum of subjectPublicKeyInfo/PKCS1 and we'd add "OpenSSH" to each of them (since PEM/DER has no meaning for OpenSSH). So a public key would be serialized via:

public_key.public_bytes(Encoding.OpenSSH, PublicFormat.OpenSSH)

I'm not sure if this work will make it into 1.3 (as that's going out this week) but we can definitely land it for 1.4.

[alex]

@adiroiban Were you interested in contributing this, or shall we go ahead and port the code ourselves?

[adiroiban]

I am interested into SSH key format support, but I was not aware of the scope of this ticket.

I was looking to see if for example we could completely replace twisted.conch.ssh.keys (https://github.com/twisted/twisted/blob/trunk/twisted/conch/ssh/keys.py) with something implemented here.

From the current feedback, it looks like the scope of this ticket is very specific and target only the OpenSSH key encoding on disk, without support the extra comment field or supporting multiple formats.

At this stage, I think that is better for someone with a good understanding of the scope of this ticket to implement it. Later we can discuss if it make sense to have a high level API in cryptography to manage SSH keys.