mailinabox/setup/ssl.sh

#!/bin/bash
#
# RSA private key, SSL certificate, Diffie-Hellman bits files
# -------------------------------------------

# Create an RSA private key, a self-signed SSL certificate, and some
# Diffie-Hellman cipher bits, if they have not yet been created.
#
# The RSA private key and certificate are used for:
#
#  * DNSSEC DANE TLSA records
#  * IMAP
#  * SMTP (opportunistic TLS for port 25 and submission on port 587)
#  * HTTPS
#
# The certificate is created with its CN set to the PRIMARY_HOSTNAME. It is
# also used for other domains served over HTTPS until the user installs a
# better certificate for those domains.
#
# The Diffie-Hellman cipher bits are used for SMTP and HTTPS, when a
# Diffie-Hellman cipher is selected during TLS negotiation. Diffie-Hellman
# provides Perfect Forward Secrecy. 

source setup/functions.sh # load our functions
source /etc/mailinabox.conf # load global vars

echo "Creating initial SSL certificate and perfect forward secrecy Diffie-Hellman parameters..."
apt_install openssl

mkdir -p $STORAGE_ROOT/ssl

# Generate a new private key.
#
# The key is only as good as the entropy available to openssl so that it
# can generate a random key. "OpenSSL’s built-in RSA key generator ....
# is seeded on first use with (on Linux) 32 bytes read from /dev/urandom,
# the process ID, user ID, and the current time in seconds. [During key
# generation OpenSSL] mixes into the entropy pool the current time in seconds,
# the process ID, and the possibly uninitialized contents of a ... buffer
# ... dozens to hundreds of times." /dev/urandom is, in turn, seeded from
# "the uninitialized contents of the pool buffers when the kernel starts,
# the startup clock time in nanosecond resolution, input event and disk
# access timings, and entropy saved across boots to a local file" as well
# as the order of execution of concurrent accesses to /dev/urandom.
# (Heninger et al 2012, https://factorable.net/weakkeys12.conference.pdf)
#
# /dev/urandom draws from the same entropy sources as /dev/random, but
# doesn't block or issue any warnings if no entropy is actually available.
# (http://www.2uo.de/myths-about-urandom/) Thus eventually /dev/urandom
# can be expected to have been seeded with the "input event and disk access
# timings", but there's no guarantee that this has even ocurred.
#
# Some of these seeds are obviously not helpful for us: There are no input
# events on severs (keyboard/mouse), and the user ID of this process is
# always the same (we're root). And the seeding of /dev/urandom with the
# time and a seed from a previous boot is handled by *during boot* by
# /etc/init.d/urandom, which, in principle, may not have occurred yet!
#
# A perfect storm of issues can cause the generated key to be not very random:
#
#   * zero'd memory (plausible on embedded systems, cloud VMs?)
#   * a predictable process ID (likely on an embedded/virtualized system)
#   * a system clock reset to a fixed time on boot
#   * one CPU or no concurrent processes on /dev/urandom (so no concurrent accesses)
#   * no hard disk (so no disk access timings - but is this possible for us?)
#   * early run (no entry yet, boot not finished)
#   * first boot (no entropy saved from previous boot)
#
if [ ! -f $STORAGE_ROOT/ssl/ssl_private_key.pem ]; then
	# Set the umask so the key file is never world-readable.
	(umask 077; hide_output \
		openssl genrsa -out $STORAGE_ROOT/ssl/ssl_private_key.pem 2048)
fi

# Generate a certificate signing request.
if [ ! -f $STORAGE_ROOT/ssl/ssl_cert_sign_req.csr ]; then
	hide_output \
	openssl req -new -key $STORAGE_ROOT/ssl/ssl_private_key.pem -out $STORAGE_ROOT/ssl/ssl_cert_sign_req.csr \
	  -sha256 -subj "/C=$CSR_COUNTRY/ST=/L=/O=/CN=$PRIMARY_HOSTNAME"
fi

# Generate a SSL certificate by self-signing.
if [ ! -f $STORAGE_ROOT/ssl/ssl_certificate.pem ]; then
	hide_output \
	openssl x509 -req -days 365 \
	  -in $STORAGE_ROOT/ssl/ssl_cert_sign_req.csr -signkey $STORAGE_ROOT/ssl/ssl_private_key.pem -out $STORAGE_ROOT/ssl/ssl_certificate.pem
fi

# Generate some Diffie-Hellman cipher bits.
# openssl's default bit length for this is 1024 bits, but we'll create
# 2048 bits of bits per the latest recommendations.
if [ ! -f $STORAGE_ROOT/ssl/dh2048.pem ]; then
	openssl dhparam -out $STORAGE_ROOT/ssl/dh2048.pem 2048
fi