ssh-keygen and writing user certificate in X.509 format?
I need to create a SSH certificate encoded as X.509 per RFC 6187, X.509v3 Certificates for Secure Shell Authentication. Base on the ssh-keygen (1) man page and a few online tutorials I am at the point I can create the CA, sign a user identity, and save it in OpenSSH certificate format.
Here are the instructions:
-
Create CA key
ssh-keygen -b 4096 -t rsa -f example-com-ca -C "CA key for example.com" -
Create User key
ssh-keygen -b 2048 -t rsa -f id_rsa -C [email protected] -
Create User certificate
ssh-keygen -s example-com-ca -n [email protected] -V +52w -I example.com-user ./id_rsa.pub
The resulting certificate is OpenSSH format:
$ cat id_rsa-cert.pub
[email protected] AAAAHHNzaC1yc2EtY2VydC12MDFAb3BlbnNzaC5jb20AAAAg0Ta
5nRrxKSB3k5sqCMH27W715uEUB54FKH44mBUMlCkAAAADAQABAAABAQDw8XuWbvSGsxyUdBY+KCfSRRz
G525MUN9/nbshWdl60ozMc4KU/Td44J8jKVq/hNHuyO7kqTaaeiO61FQmRPz3/vpmwUGEWkdhdQ5ujBj
1+X2/acnV+8Q2mXzxnvMvkcPh4T2jSXEMTJ8v5WG6cJkih+rJEbHHJF0tpxRSyxiNKfenDRRmGiyABd1
JDmkLLaNFLSSo4WeCFQ82nMi4Lod4QQMlakPgd76s6GNFflRklFWuhRadCdxrnz5e9ZMV8vDeAi+8IS8
9z+hWotYC4TVAb8is72Un6/1rwNQgWX5NcX3O1ocboJuEanFIh2QJTYq/UBwSQobH2+fXq06Qm4efAAA
AAAAAAAAAAAABAAAAEGV4YW1wbGUuY29tLXVzZXIAAAAUAAAAEGpkb2VAZXhhbXBsZS5jb20AAAAAXSv
+tAAAAABfC+ETAAAAAAAAAIIAAAAVcGVybWl0LVgxMS1mb3J3YXJkaW5nAAAAAAAAABdwZXJtaXQtYWd
lbnQtZm9yd2FyZGluZwAAAAAAAAAWcGVybWl0LXBvcnQtZm9yd2FyZGluZwAAAAAAAAAKcGVybWl0LXB
0eQAAAAAAAAAOcGVybWl0LXVzZXItcmMAAAAAAAAAAAAAAhcAAAAHc3NoLXJzYQAAAAMBAAEAAAIBAMG
bEpcMRTnu5ewWWytcQqzcnSDZbF5yRYpEyQsKTLrQmSux104OOsXQ/5ba30/li4uewK0sL7Qqb79ZCVh
9pH4dnZf6KYIEtkTV2mj4d11ZsRYMXAcBSpuua+O6CTdEGMtmWrQZ3mVUMvCy8UFqcePE/QjxgvUBYZN
jRxx+nPtbYf88AWF1/I6uwKODaPspwpwh07TTqNjmIMpvc6gTWx0RO1avzTveOEvTnapgJI0mRjUmyGX
vvncE9U0WC65kugIkZcjI/kkXBVrgYVfhDssF8bvX2cO7NQt4QH5yANWgm0HraGqmNRAPXd84vomI0gF
8W4xwC9gnfJzjheeXLUEzSXOwiZSU2ElF1kGUlYRtM59VUjsTvFxjrXA9pPtGO5RopXot1GB3Y7vxnkj
blDzSdMAeLms0jv8Um6ty9uDzmW8o5GHZudatL8CAEP6HIvLrI6zQeI8iIlLmCsNjxE8p7annldcrNsX
f8hpNeG7RPtgcU1pMmc/fc3UPBvToqnMVfjKi1n2kGYrhOiNbfcpW7nZUSfLcSZGryIYMb7IlTjsXm6v
E1rCzJKukcMuI8U3qUwRzRS/xgLF5msSOiJ9qFM/FERGa6zYvdgTV2aq/07fDedqleChMreCiYPxPp1n
DLJOBcC+nvx7tIGj05z9BJvuCfOQWVnjIjZc0tUitAAACDwAAAAdzc2gtcnNhAAACALBk/Z4b3Z4O35H
NpC91oLWJExk39tkjdgJBC7YelzfH7apA4em/rk8G219EIJ8elKifJzLze1t3bfWOx/dwUHuTfqZ/e1h
j6Q/iNDQ9Q/2ijWso1E6alSIapdGogWHCQ4IDBlWaF8xzlCACbo82js2uvSYmbbcMlXPKqLbPJwiNJt8
AhDz3/JqRqedN7tObDMgUz0O0PGkYwUXjVV77EaVpRZF3ffTvnBBw9vHSwriVOZgOk4l2iXbZU51A+QB
l8bgFj4QCGvobMul/0AV+QsQ20AqUQ/nEIM1rXuH+ki0PYVpKkXPhKf2ODkLZdmpuKZX5lM9FumkFf/s
VVPa5GsonJG5s2VVEz7L+Ed6KBaJ+kFQrXu4hDxwEUCd/y/gYSicOb7B7N0jkPaVwRoR6tb0mAXGKE44
tumvptu/AJjlB23QOgIIToARgqampzmPwAm8jbU2AU3RtWx+RZGPnJKsJPtADMZ7ByJnGY/mPoNpGqQc
H8h+tClb1Ihxhbh1RQSuJNdgNlNGJbSdsonS9/8fxyxt7ok06Z05N6dy3PLwTuub1EzKmeSwQhHLHWXA
SKILcUaMosak1ybQZz8kMMrsMMUA2ubjrtGA8oe5skhc9gbAurebO1iGg+asUSNycDXZypwl20wpMlzL
VkxXSGHIz6Cd9QsfmJtuQh4QXfFrE [email protected]
This is where I am having trouble. I don't see a ssh-keygen (1) option to write the certificate in X.509 format. I prefer PEM encoding, but I can probably work with ASN.1/DER encoding.
How does one write the certificate in X.509 format?
A related question is at Alternatives to SSH x509 logon.
OpenSSH does not support X.509 – it can neither generate such certificates nor consume them. (OpenSSH CAs are something entirely different, and there is no standard OpenSSH CA ⇆ X.509 translation specified. Besides the format, they intentionally don't support many PKI features like multi-level CA hierarchy, while being able to carry OpenSSH-specific "key options" and other metadata that doesn't have corresponding X.509 extensions.)
You probably want Roumen Petrov's PKIX-SSH, which is an actively maintained OpenSSH fork with extensive X.509 PKI support, including the RFC 6187 x509v3-* formats.
If you must generate an authorized_keys entry for an existing X.509 certificate, without having the necessary tools, the format is relatively simple and can be done by hand via any programming language. The authorized_keys data is always based on SSHv2 packet format (where each 'string' is implicitly preceded by uint32be indicating its length); the first item is always a string with the pubkey type itself; the remainder is as documented in page 5.
(The 'certificate' field holds DER-encoded data, but note that PEM encoding is literally just Base64 around the same ASN.1 DER, with "begin/end" headers slapped on.)