SSH certificate encoding/parsing incompatibility with OpenSSH

[lkubb]

Description
There is an encoding mismatch regarding critical options with values between ssh-keygen and cryptography:

1. Loading an SSH certificate generated by ssh-keygen with cryptography does not yield expected values.
2. Reading an SSH certificates generated by SSHCertificateBuilder with ssh-keygen fails.

Steps
Re 1:

1. Generate a key to be signed: ssh-keygen -t ed25519
2. Generate a CA signing key: ssh-keygen -t ecdsa
3. Sign a certificate: ssh-keygen -s id_ed25519 -n cryptouser,testuser -I test@cryptography.io -O "critical:force-command=echo aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa" -O "critical:verify-required" -V 0x63b2b264:0x767eb5c0 id_ecdsa.pub
4. Load the certificate with cryptography:

from cryptography.hazmat.primitives.serialization import (
    load_ssh_public_identity,
)

cert = load_ssh_public_identity(
    b"ecdsa-sha2-nistp256-cert-v01@openssh.com AAAAKGVjZHNhLXNoYTItbm"
    b"lzdHAyNTYtY2VydC12MDFAb3BlbnNzaC5jb20AAAAgLfsFv9Gbc6LZSiJFWdYQl"
    b"IMNI50GExXW0fBpgGVf+Y4AAAAIbmlzdHAyNTYAAABBBIzVyRgVLR4F38bIOLBN"
    b"8CNm8Nf+eBHCVkKDKb9WDyLLD61CEmzjK/ORwFuSE4N60eIGbFidBf0D0xh7G6o"
    b"TNxsAAAAAAAAAAAAAAAEAAAAUdGVzdEBjcnlwdG9ncmFwaHkuaW8AAAAaAAAACm"
    b"NyeXB0b3VzZXIAAAAIdGVzdHVzZXIAAAAAY7KyZAAAAAB2frXAAAAAWAAAAA1mb"
    b"3JjZS1jb21tYW5kAAAALAAAAChlY2hvIGFhYWFhYWFhYWFhYWFhYWFhYWFhYWFh"
    b"YWFhYWFhYWFhYWFhAAAAD3ZlcmlmeS1yZXF1aXJlZAAAAAAAAACCAAAAFXBlcm1"
    b"pdC1YMTEtZm9yd2FyZGluZwAAAAAAAAAXcGVybWl0LWFnZW50LWZvcndhcmRpbm"
    b"cAAAAAAAAAFnBlcm1pdC1wb3J0LWZvcndhcmRpbmcAAAAAAAAACnBlcm1pdC1wd"
    b"HkAAAAAAAAADnBlcm1pdC11c2VyLXJjAAAAAAAAAAAAAAAzAAAAC3NzaC1lZDI1"
    b"NTE5AAAAICH6csEOmGbOfT2B/S/FJg3uyPsaPSZUZk2SVYlfs0KLAAAAUwAAAAt"
    b"zc2gtZWQyNTUxOQAAAEDz2u7X5/TFbN7Ms7DP4yArhz1oWWYKkdAk7FGFkHfjtY"
    b"/YfNQ8Oky3dCZRi7PnSzScEEjos7723dhF8/y99WwH"
)

print(cert.critical_options[b"force-command"])
assert cert.critical_options == {
    b"force-command": b"echo aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa",
    b"verify-required": b"",
}

b'\x00\x00\x00(echo aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa'
Traceback (most recent call last):
  File "/home/user/test.py", line 21, in <module>
    assert cert.critical_options == {b"force-command": b"echo aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa", b"verify-required": b""}
AssertionError

Re 2:

1. Generate an SSH certificate with cryptography

from datetime import datetime, timedelta

from cryptography.hazmat.primitives.asymmetric import ed25519
from cryptography.hazmat.primitives.serialization import (
    SSHCertificateBuilder,
    SSHCertificateType,
)

ca_key = ed25519.Ed25519PrivateKey.generate()
sign_key = ed25519.Ed25519PrivateKey.generate()


cert = (
    SSHCertificateBuilder()
    .public_key(sign_key.public_key())
    .type(SSHCertificateType.USER)
    .valid_for_all_principals()
    .valid_before((datetime.now() + timedelta(days=1)).timestamp())
    .valid_after(datetime.now().timestamp())
    .add_critical_option(
        b"force-command", b"echo aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
    )
    .sign(ca_key)
)

with open("/tmp/sshcert.pub", "wb") as f:
    f.write(cert.public_bytes())

2. Try to read it using ssh-keygen

$ cat /tmp/sshcert.pub
ssh-ed25519-cert-v01@openssh.com AAAAIHNzaC1lZDI1NTE5LWNlcnQtdjAxQG9wZW5zc2guY29tAAAAICgEmbEZIZTs/K0UREvoo+eBXJGJhfZFzm+Q2CeCdBQbAAAAIJ5oypqQoA7gt+MJER8tlWwJgdpocFnOcdJHT2rPXvdwAAAAAAAAAAAAAAABAAAAAAAAAAAAAAAAZKv6HwAAAABkrUufAAAAPQAAAA1mb3JjZS1jb21tYW5kAAAAKGVjaG8gYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWEAAAAAAAAAAAAAADMAAAALc3NoLWVkMjU1MTkAAAAgGaQ+IQWx7X2AbbbWE4Ua2N7aW/uT16yt8n1bcr2BytwAAABTAAAAC3NzaC1lZDI1NTE5AAAAQGCWJZWlpr6/m/xwxQOcCfg73CUgjvqD1jsTDeepemxZRWhByJ44qdASQ0ykiuQjzGrigYaynuRbA4tuap0K3A4=
$ ssh-keygen -Lf /tmp/sshcert.pub
/tmp/sshcert.pub:
        Type: ssh-ed25519-cert-v01@openssh.com user certificate
        Public key: ED25519-CERT SHA256:oxxO0zve78LqnyWIf3DRrbIzVoWXDK/FuBRerotDrHY
        Signing CA: ED25519 SHA256:HHnlyz4SqRKnCKwUVgN3hvJyKA8BQ5Hmy0SOsjgfnGQ (using ssh-ed25519)
        Key ID: ""
        Serial: 0
        Valid: from 2023-07-10T12:31:27 to 2023-07-11T12:31:27
        Principals: (none)
        Critical Options:
show_options: parse critical: string is too large

Background
The specification is a bit confusing in regards to how critical options/extensions with values should be encoded:

The critical options section of the certificate specifies zero or more
options on the certificates validity. The format of this field
is a sequence of zero or more tuples:

string       name
string       data

So that's the first header for the data. You then have to look at the
table to determine the format of data's contents. E.g.

force-command string Specifies a command that is executed
...

So, for name=force-command, the contents of the data buffer are a string.
That's the second header.

The intent is to support options/extensions with data types other
than string, e.g. integers or arrays of string.

Source

golang/crypto had the same issue: golang/go#10569

[reaperhulk]

A few more steps before we close this:

• Backport to 41.0.x with warning for parsing the incorrect encoding (Backport ssh cert fix #9211)
• Changelog entry to 42 stating we've dropped support (concurrent with forward porting the 41.0.2 changelog)

[reaperhulk]

Okay this is now released in 41.0.2 😄