Add doc on SSH User certificates

Makefile.am

@@ -31,7 +31,7 @@ ACLOCAL_AMFLAGS = -I m4
 
 EXTRA_DIST = windows.mk mac.mk tool/tests/basic.sh tools/fasc.pl
 
-EXTRA_DIST += doc/Android_code_signing.adoc doc/Attestation.adoc doc/Certificate_Authority.adoc doc/OS_X_code_signing.adoc doc/SSH_with_PIV_and_PKCS11.adoc doc/Windows_certificate.adoc doc/YKCS11_release_notes.adoc doc/YubiKey_PIV_introduction.adoc
+EXTRA_DIST += doc/Android_code_signing.adoc doc/Attestation.adoc doc/Certificate_Authority.adoc doc/OS_X_code_signing.adoc doc/SSH_User_certificates.adoc doc/SSH_with_PIV_and_PKCS11.adoc doc/Windows_certificate.adoc doc/YKCS11_release_notes.adoc doc/YubiKey_PIV_introduction.adoc
 
 
 if ENABLE_COV

doc/SSH_User_certificates.adoc

@@ -0,0 +1,72 @@
+Using SSH User Certificates with PIV keys
+-----------------------------------------
+
+This is a step-by-step on how to setup SSH user certificates using PIV
+for hardware-backed keys. There guide is primarily for an OS X or
+Linux system.
+
+Prerequisites
+-------------
+
+* a YubiKey with the PIV application loaded
+* the yubico-piv-tool software
+* the ykcs11 PKCS#11 module
+* OpenSSH
+
+[NOTE]
+This has been tested as working with OpenSSH version 7.2p2.
+It has also been tested as *not working* with OpenSSH version 6.9p1.
+
+
+Steps
+-----
+
+1. GENERATE an "ssh user CA" key and trust it for this account on this
+host
+
+  $ ssh-keygen -N '' -C user-ca -f ~/.ssh/ca
+  $ sed 's/^/cert-authority /' ~/.ssh/ca.pub > ~/.ssh/authorized_keys
+
+2. Generate a key in PIV slot 9c that requires touch. (Note the second
+operation will require touch)
+
+  $ yubico-piv-tool -a generate -s 9c -A RSA2048 --pin-policy=never --touch-policy=always -o public.pem
+  $ yubico-piv-tool -a selfsign-certificate -s 9c -S "/CN=SSH key/" -i public.pem -o cert.pem
+  $ yubico-piv-tool -a import-certificate -s 9c -i cert.pem
+
+3. Double check that the key appears in slot 9c (optional)
+
+  $ yubico-piv-tool -a status
+
+4. Clear the SSH agent
+
+  $ ssh-add -D
+  $ ssh-add -e /PATH/TO/libykcs11.so
+
+  * Make sure to use the correct path (e.g. `/usr/local/lib`) and extension (`.so` for Linux, `.dylib` for Mac OS X) for libykcs11.
+
+  * Beware that `ssh-add -D` doesn't seem to clear PKCS#11 libraries, only keys
+(http://lists.mindrot.org/pipermail/openssh-unix-dev/2016-July/035154.html[link]).
+
+5. Add PIV to the SSH agent (will prompt for PIV PIN)
+
+  $ ssh-add -s /PATH/TO/libykcs11.so
+
+6. Get the public key from PIV and sign it using the CA key. This produces `~/.ssh/id_rsa-cert.pub`
+
+  $ ssh-add -L > ~/.ssh/id_rsa.pub
+  $ ssh-keygen -s ~/.ssh/ca -I identity -n "${LOGNAME}" ~/.ssh/id_rsa.pub
+
+6. Authenticate to the target system using the new key:
+
+   $ ssh user@remote.example.com
+
+If you have followed these steps to the letter, you will not be asked for the PIV PIN, but your YubiKey will start blinking, waiting for touch.
+
+
+Thanks
+------
+
+Thanks to Christopher Harrell and Dean Sutherland from the
+https://yahoo-security.tumblr.com/[Yahoo Paranoids] for providing the
+instruction and the testing.