man: document pkcs#11 hookup in /etc/crypttab

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diff --git a/man/yubikey-crypttab.sh b/man/yubikey-crypttab.sh
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+# Make sure noone can read the files we generate but us
+umask 077
+
+# Destroy any old key on the Yubikey (careful!)
+ykman piv reset
+
+# Generate a new private/public key pair on the device, store the public key in 'pubkey.pem'.
+ykman piv generate-key -a RSA2048 9d pubkey.pem
+
+# Create a self-signed certificate from this public key, and store it on the device.
+ykman piv generate-certificate --subject "Knobelei" 9d pubkey.pem
+
+# Check if the newly create key on the Yubikey shows up as token in PKCS#11. Have a look at the output, and
+# copy the resulting token URI to the clipboard.
+p11tool --list-tokens
+
+# Generate a (secret) random key to use as LUKS decryption key.
+dd if=/dev/urandom of=plaintext.bin bs=128 count=1
+
+# Encode the secret key also as base64 text (with all whitespace removed)
+base64 < plaintext.bin | tr -d '\n\r\t ' > plaintext.base64
+
+# Encrypt this newly generated (binary) LUKS decryption key using the public key whose private key is on the
+# Yubikey, store the result in /etc/encrypted-luks-key.bin, where we'll look for it during boot.
+openssl rsautl -encrypt -pubin -inkey pubkey.pem -in plaintext.bin -out /etc/encrypted-luks-key.bin
+
+# Configure the LUKS decryption key on the LUKS device. We use very low pbkdf settings since the key already
+# has quite a high quality (it comes directly from /dev/urandom after all), and thus we don't need to do much
+# key derivation.
+cryptsetup luksAddKey /dev/sda1 plaintext.base64 --pbkdf=pbkdf2 --pbkdf-force-iterations=1000
+
+# Now securely delete the plain text LUKS key, we don't need it anymore, and since it contains secret key
+# material it should be removed from disk thoroughly.
+shred -u plaintext.bin plaintext.base64
+
+# We don't need the public key anymore either, let's remove it too. Since this one is not security
+# sensitive we just do a regular "rm" here.
+rm pubkey.pem
+
+# Test: Let's run systemd-cryptsetup to test if this all worked. The option string should contain the full
+# PKCS#11 URI we have in the clipboard, it tells the tool how to decypher the encrypted LUKS key.
+systemd-cryptsetup attach mytest /dev/sda1 /etc/encrypted-luks-key.bin 'pkcs11-uri=pkcs11:…'
+
+# If that worked, let's now add the same line persistently to /etc/crypttab, for the future.
+echo "mytest /dev/sda1 /etc/encrypted-luks-key 'pkcs11-uri=pkcs11:…' >> /etc/crypttab