1 Repository

https://github.com/jlangenegger/ssh_certificate/

Sign client's public keys

To sign client's public keys there is the script 'generate_client_certificate.sh' to simplify the procedure.
The scripts does have the following options:

-g
- This takes a github user name as an argument and generates a certificate for each key stored in github.
-f
- Instead of the github user name, one can provide a file that contains all the keys.
- Nevertheless the flag '-g' is needed as the certificate holder's name.
-V
- Add the validity interval of a certificate
- Per default a certificate is valid for seven days.
- More information can be found here: validity_interval
-n
- This flag restricts the certificate to a list of principals that the client is allowed to log in.

The output of 'generate_client_certificate.sh' is a .tar archive that contains the certificate, the public key that is used to authenticate servers as well as an instruction to install the certificate on the client's machine. It is stored in the home directory '$HOME/signed_keys'.

2 Sign hosts's public keys

To sign host's public keys there is the script 'generate_host_certificate.sh' to simplify the procedure.
The scripts does have the following options:

-I
- This takes the HOST_ID of the server.
- e.g host.netdef.org
-f
- The file that contains all the keys.
-V
- Add the validity interval of a certificate
- Per default a certificate is valid for one year.
- More information can be found here: validity_interval
-n
- This flag restricts the certificate to a list of principals that the host is known by.

The output of 'generate_host_certificate.sh' is the certificate 'HOST_ID-cert.pub' that needs to be copied to the host. It is stored in the home directory '$HOME/signed_keys'.

3 Prepare Yubikey

This needs to be done on a offline machine!

Install libraries that are later used

To setup the yubikey the yubico-piv-tool is used. It must be installed from source to work correctly. For the installation the following packages are needed:

apt-get install autoconf automake build-essential libtool libssl-dev pkg-config check libpcsclite-dev gengetopt help2man

Install the tool

git clone https://github.com/Yubico/yubico-piv-tool.git

cd yubico-piv-tool

autoreconf --install
./configure  --disable-dependency-tracking
make
make install

Change default pins and management key of yubikey

To prepare the PIV applet in the YubiKey the management key, the pin and the punk needs to be set.

yubico-piv-tool -a set-mgm-key -n 010203040506070801020304050607080102030405060708
yubico-piv-tool -k $key -a change-pin -P 123456 -N 123456
yubico-piv-tool -k $key -a change-puk -P 12345678 -N 12345678

Generate RSA private keys for SSH Host CA

Then generate a RSA private key for the SSH Host CA, and generate a dummy X.509 certificate for that key. The only use for the X.509 certificate is to make PIV/PKCS#11 happy. They want to be able to extract the public-key from the smart-card, and do that through the X.509 certificate.

YUBIKEYNUM=0
PATH_TO_CERTIFICATE="/etc/ssh-ca"

# generate key directly on yubikey and self-sign the certificate
yubico-piv-tool -k 123456 -s 9c -a generate -o yubikey$YUBIKEYNUM.pem
yubico-piv-tool -k 123456 -a verify-pin -a selfsign-certificate -s 9c -S "/CN=yubikey`$YUBIKEYNUM`/" -i yubikey$YUBIKEYNUM.pem -o yubikey$YUBIKEYNUM-cert.pem
# import self-signed certificate
yubico-piv-tool -k 123456 -a import-certificate -s 9c -i yubikey$YUBIKEYNUM-cert.pem

# convert public key to RSA
ssh-keygen -f yubikey$YUBIKEYNUM.pem -i -mPKCS8 > yubikey$YUBIKEYNUM.pub

# move public key to correct place and remove leftovers
mv yubikey$YUBIKEYNUM.pub $PATH_TO_CERTIFICATE 
rm yubikey$YUBIKEYNUM-cert.pem yubikey$YUBIKEYNUM.pem

4 Sign server's RSA key

PATH_TO_CERTIFICATE="/etc/ssh-ca"
PATH_TO_YKCS11="/usr/local/lib/libykcs11.so"

ssh-keygen  -D $PATH_TO_YKCS11
            -s $PATH_TO_CERTIFICATE/yubikey$YUBIKEYNUM.pub
            -I server_name \
            -h \
            -n server.netdef.org \
            -V +52w \
            /etc/ssh-ca/ssh_host_rsa_key.pub

Options explanation:

-D
- is used to access the yubikey
-s
- provides the public certificate to access the yubikey
-I server_name
- The key identifier to include in the certificate.
-h
- Generate a host certificate (instead of a user certificate)
-n server.netdef.org
- The principal names to include in the certificate.
- For host certificates this is a list of all names that the system is known by.
- Note: Use the unqualified names carefully here in organizations where hostnames are not unique (ca.netdef.org vs. ca.dev.netdef.org)
-V +52w
- The validity period.
- For host certificates, you’ll probably want them pretty long lived.
- This setting sets the validity period from now until 52 weeks hence.
/etc/ssh-ca/ssh_host_rsa_key.pub
- The path to the host RSA public key to sign.
- Our signed host key certificate will be /etc/ssh-ca/ssh_host_rsa_key-cert.pub.

5 Sign client's RSA key

PATH_TO_CERTIFICATE="/etc/ssh-ca"
PATH_TO_YKCS11="/usr/local/lib/libykcs11.so"

ssh-keygen  -D $PATH_TO_YKCS11
            -s $PATH_TO_CERTIFICATE/yubikey$YUBIKEYNUM.pub
            -I client_name \
            -n root \
            -V +24h \
            /etc/ssh_ca/id_rsa.pub