 TLS Forward Secrecy in Postfix

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Warning

Forward secrecy does not protect against active attacks such as forged DNS
replies or forged TLS server certificates. If such attacks are a concern, then
the SMTP client will need to authenticate the remote SMTP server in a
sufficiently-secure manner. For example, by the fingerprint of a (CA or leaf)
public key or certificate. Conventional PKI relies on many trusted parties and
is easily subverted by a state-funded adversary.

Overview

Postfix supports forward secrecy of TLS network communication since version
2.2. This support was adopted from Lutz Ja"nicke's "Postfix TLS patch" for
earlier Postfix versions. This document will focus on TLS Forward Secrecy in
the Postfix SMTP client and server. See TLS_README for a general description of
Postfix TLS support.

Topics covered in this document:

  * Give me some background on forward secrecy in Postfix

      o What is Forward Secrecy
      o Forward Secrecy in TLS
      o Forward Secrecy in the Postfix SMTP Server
      o Forward Secrecy in the Postfix SMTP Client

  * Never mind, just show me what it takes to get forward secrecy

      o Getting started, quick and dirty
      o How can I see that a connection has forward secrecy?
      o What ciphers provide forward secrecy?
      o What do "Anonymous", "Untrusted", etc. in Postfix logging mean?

  * Credits

What is Forward Secrecy

The term "Forward Secrecy" (or sometimes "Perfect Forward Secrecy") is used to
describe security protocols in which the confidentiality of past traffic is not
compromised when long-term keys used by either or both sides are later
disclosed.

Forward secrecy is accomplished by negotiating session keys using per-session
cryptographically-strong random numbers that are not saved, and signing the
exchange with long-term authentication keys. Later disclosure of the long-term
keys allows impersonation of the key holder from that point on, but not
recovery of prior traffic, since with forward secrecy, the discarded random key
agreement inputs are not available to the attacker.

Forward secrecy is only "perfect" when brute-force attacks on the key agreement
algorithm are impractical even for the best-funded adversary and the random-
number generators used by both parties are sufficiently strong. Otherwise,
forward secrecy leaves the attacker with the challenge of cracking the key-
agreement protocol, which is likely quite computationally intensive, but may be
feasible for sessions of sufficiently high value. Thus forward secrecy places
cost constraints on the efficacy of bulk surveillance, recovering all past
traffic is generally infeasible, and even recovery of individual sessions may
be infeasible given a sufficiently-strong key agreement method.

Forward Secrecy in TLS

Early implementations of the SSL protocol do not provide forward secrecy (some
provide it only with artificially-weakened "export" cipher suites, but we will
ignore those here). The client sends a random "pre-master secret" to the server
encrypted with the server's RSA public key. The server decrypts this with its
private key, and uses it together with other data exchanged in the clear to
generate the session key. An attacker with access to the server's private key
can perform the same computation at any later time. The TLS library in Windows
XP and Windows Server 2003 only supported cipher suites of this type, and
Exchange 2003 servers largely do not support forward secrecy.

Later revisions to the TLS protocol introduced forward-secrecy cipher suites in
which the client and server implement a key exchange protocol based on
ephemeral secrets. Sessions encrypted with one of these newer cipher suites are
not compromised by future disclosure of long-term authentication keys.

The key-exchange algorithms used for forward secrecy require the TLS server to
designate appropriate "parameters" consisting of a mathematical "group" and an
element of that group called a "generator". Presently, there are two flavors of
"groups" that work with PFS:

  * Prime-field groups (EDH): The server needs to be configured with a
    suitably-large prime and a corresponding "generator". The acronym for
    forward secrecy over prime fields is EDH for Ephemeral Diffie-Hellman (also
    abbreviated as DHE).

  * Elliptic-curve groups (EECDH): The server needs to be configured with a
    "named curve". These offer better security at lower computational cost than
    prime field groups, but are not as widely implemented. The acronym for the
    elliptic curve version is EECDH which is short for Ephemeral Elliptic Curve
    Diffie-Hellman (also abbreviated as ECDHE).

It is not essential to know what these are, but one does need to know that
OpenSSL supports EECDH with version 1.0.0 or later. Thus the configuration
parameters related to Elliptic-Curve forward secrecy are available when Postfix
is linked with OpenSSL >= 1.0.0 (provided EC support has not been disabled by
the vendor, as in some versions of RedHat Linux).

Elliptic curves used in cryptography are typically identified by a "name" that
stands for a set of well-known parameter values, and it is these "names" (or
associated ASN.1 object identifiers) that are used in the TLS protocol. On the
other hand, with TLS there are no specially designated prime field groups, so
each server is free to select its own suitably-strong prime and generator.

Forward Secrecy in the Postfix SMTP Server

The Postfix >= 2.2 SMTP server supports forward secrecy in its default
configuration. If the remote SMTP client prefers cipher suites with forward
secrecy, then the traffic between the server and client will resist decryption
even if the server's long-term authentication keys are later compromised.

Some remote SMTP clients may support forward secrecy, but prefer cipher suites
without forward secrecy. In that case, Postfix >= 2.8 could be configured to
ignore the client's preference with the main.cf setting "tls_preempt_cipherlist
= yes". However, this will likely cause interoperability issues with older
Exchange servers and is not recommended for now.

EDH Server support

Postfix >= 2.2 support 1024-bit-prime EDH out of the box, with no additional
configuration, but you may want to override the default prime to be 2048 bits
long, and you may want to regenerate your primes periodically. See the quick-
start section for details. With Postfix >= 3.1 the out of the box (compiled-in)
EDH prime size is 2048 bits.

With prime-field EDH, OpenSSL wants the server to provide two explicitly-
selected (prime, generator) combinations. One for the now long-obsolete
"export" cipher suites, and another for non-export cipher suites. Postfix has
two such default combinations compiled in, but also supports explicitly-
configured overrides.

  * The "export" EDH parameters are used only with the obsolete "export"
    ciphers. To use a non-default prime, generate a 512-bit DH parameter file
    and set smtpd_tls_dh512_param_file to the filename (see the quick-start
    section for details). With Postfix releases after the middle of 2015 the
    default opportunistic TLS cipher grade (smtpd_tls_ciphers) is "medium" or
    stronger, and export ciphers are no longer used.

  * The non-export EDH parameters are used for all other EDH cipher suites. To
    use a non-default prime, generate a 1024-bit or 2048-bit DH parameter file
    and set smtpd_tls_dh1024_param_file to the filename. Despite the name this
    is simply the non-export parameter file and the prime need not actually be
    1024 bits long (see the quick-start section for details).

As of mid-2015, SMTP clients are starting to reject TLS handshakes with primes
smaller than 2048 bits. Each site needs to determine which prime size works
best for the majority of its clients. See the quick-start section for the
recommended configuration to work around this issue.

EECDH Server support

Postfix >= 2.6 support NIST P-256 EECDH when built with OpenSSL >= 1.0.0. When
the remote SMTP client also supports EECDH and implements the P-256 curve,
forward secrecy just works.

    Note: With Postfix 2.6 and 2.7, enable EECDH by setting the main.cf
    parameter smtpd_tls_eecdh_grade to "strong".

The elliptic curve standards are evolving, with new curves introduced in RFC
8031 to augment or replace the NIST curves tarnished by the Snowden
revelations. Fortunately, TLS clients advertise their list of supported curves
to the server so that servers can choose newer stronger curves when mutually
supported. OpenSSL 1.0.2 released in January 2015 was the first release to
implement negotiation of supported curves in TLS servers. In older OpenSSL
releases, the server is limited to selecting a single widely supported curve.

With Postfix prior to 3.2 or OpenSSL prior to 1.0.2, only a single server-side
curve can be configured, by specifying a suitable EECDH "grade":

        smtpd_tls_eecdh_grade = strong | ultra
        # Underlying curves, best not changed:
        # tls_eecdh_strong_curve = prime256v1
        # tls_eecdh_ultra_curve = secp384r1

Postfix >= 3.2 supports the curve negotiation API of OpenSSL >= 1.0.2. When
using this software combination, the default setting of "smtpd_tls_eecdh_grade"
changes to "auto", which selects a curve that is supported by both the server
and client. The list of candidate curves can be configured via
"tls_eecdh_auto_curves", which can be used to configure a prioritized list of
supported curves (most preferred first) on both the server and client. The
default list is suitable for most users.

Forward Secrecy in the Postfix SMTP Client

The Postfix >= 2.2 SMTP client supports forward secrecy in its default
configuration. All supported OpenSSL releases support EDH key exchange. OpenSSL
releases >= 1.0.0 also support EECDH key exchange (provided elliptic-curve
support has not been disabled by the vendor as in some versions of RedHat
Linux). If the remote SMTP server supports cipher suites with forward secrecy
(and does not override the SMTP client's cipher preference), then the traffic
between the server and client will resist decryption even if the server's long-
term authentication keys are later compromised.

Postfix >= 3.2 supports the curve negotiation API of OpenSSL >= 1.0.2. The list
of candidate curves can be changed via the "tls_eecdh_auto_curves"
configuration parameter, which can be used to select a prioritized list of
supported curves (most preferred first) on both the Postfix SMTP server and
SMTP client. The default list is suitable for most users.

The default Postfix SMTP client cipher lists are correctly ordered to prefer
EECDH and EDH cipher suites ahead of similar cipher suites that don't implement
forward secrecy. Administrators are strongly discouraged from changing the
cipher list definitions.

The default minimum cipher grade for opportunistic TLS is "medium" for Postfix
releases after the middle of 2015, "export" for older releases. Changing the
minimum cipher grade does not change the cipher preference order. Note that
cipher grades higher than "medium" exclude Exchange 2003 and likely other MTAs,
thus a "high" cipher grade should be chosen only on a case-by-case basis via
the TLS policy table.

Getting started, quick and dirty

EECDH Client support (Postfix >= 2.2 with OpenSSL >= 1.0.0)

This works "out of the box" with no need for additional configuration.

Postfix >= 3.2 supports the curve negotitation API of OpenSSL >= 1.0.2. The
list of candidate curves can be changed via the "tls_eecdh_auto_curves"
configuration parameter, which can be used to select a prioritized list of
supported curves (most preferred first) on both the Postfix SMTP server and
SMTP client. The default list is suitable for most users.

EECDH Server support (Postfix >= 2.6 with OpenSSL >= 1.0.0)

With Postfix 2.6 and 2.7, enable elliptic-curve support in the Postfix SMTP
server. This is the default with Postfix >= 2.8. Note, however, that elliptic-
curve support may be disabled by the vendor, as in some versions of RedHat
Linux.

    /etc/postfix/main.cf:
        # Postfix 2.6 & 2.7 only. EECDH is on by default with Postfix >= 2.8.
        # The default grade is "auto" with Postfix >= 3.2.
        smtpd_tls_eecdh_grade = strong

EDH Client support (Postfix >= 2.2, all supported OpenSSL versions)

This works "out of the box" without additional configuration.

EDH Server support (Postfix >= 2.2, all supported OpenSSL versions)

Optionally generate non-default Postfix SMTP server EDH parameters for improved
security against pre-computation attacks and for compatibility with Debian-
patched Exim SMTP clients that require a >= 2048-bit length for the non-export
prime.

Execute as root (prime group generation can take a few seconds to a few
minutes):

    # cd /etc/postfix
    # umask 022
    # openssl dhparam -out dh512.tmp 512 && mv dh512.tmp dh512.pem
    # openssl dhparam -out dh1024.tmp 1024 && mv dh1024.tmp dh1024.pem
    # openssl dhparam -out dh2048.tmp 2048 && mv dh2048.tmp dh2048.pem
    # chmod 644 dh512.pem dh1024.pem dh2048.pem

The Postfix SMTP server EDH parameter files are not secret, after all these
parameters are sent to all remote SMTP clients in the clear. Mode 0644 is fine.

You can improve security against pre-computation attacks further by
regenerating the Postfix SMTP server EDH parameters periodically (an hourly or
daily cron job running the above commands as root can automate this task).

Once the parameters are in place, update main.cf as follows:

    /etc/postfix/main.cf:
        smtpd_tls_dh1024_param_file = ${config_directory}/dh2048.pem
        smtpd_tls_dh512_param_file = ${config_directory}/dh512.pem

If some of your MSA clients don't support 2048-bit EDH, you may need to adjust
the submission entry in master.cf accordingly:

    /etc/postfix/master.cf:
        submission inet n       -       n       -       -       smtpd
            # Some submission clients may not yet do 2048-bit EDH, if such
            # clients use your MSA, configure 1024-bit EDH instead.  However,
            # as of mid-2015, many submission clients no longer accept primes
            # with less than 2048-bits.  Each site needs to determine which
            # type of client is more important to support.
            -o smtpd_tls_dh1024_param_file=${config_directory}/dh1024.pem
            -o smtpd_tls_security_level=encrypt
            -o smtpd_sasl_auth_enable=yes
            ...

How can I see that a connection has forward secrecy?

Postfix can be configured to report information about the negotiated cipher,
the corresponding key lengths, and the remote peer certificate or public-key
verification status.

  * With "smtp_tls_loglevel = 1" and "smtpd_tls_loglevel = 1", the Postfix SMTP
    client and server will log TLS connection information to the maillog file.
    The general logfile format is shown below. With TLS 1.3 there may be
    additional properties logged after the cipher name and bits.

        postfix/smtp[process-id]: Untrusted TLS connection established
        to host.example.com[192.168.0.2]:25: TLSv1 with cipher cipher-name
        (actual-key-size/raw-key-size bits)

        postfix/smtpd[process-id]: Anonymous TLS connection established
        from host.example.com[192.168.0.2]: TLSv1 with cipher cipher-name
        (actual-key-size/raw-key-size bits)

  * With "smtpd_tls_received_header = yes", the Postfix SMTP server will record
    TLS connection information in the Received: header in the form of comments
    (text inside parentheses). The general format depends on the
    smtpd_tls_ask_ccert setting. With TLS 1.3 there may be additional
    properties logged after the cipher name and bits.

        Received: from host.example.com (host.example.com [192.168.0.2])
                (using TLSv1 with cipher cipher-name
                (actual-key-size/raw-key-size bits))
                (Client CN "host.example.com", Issuer "John Doe" (not
        verified))

        Received: from host.example.com (host.example.com [192.168.0.2])
                (using TLSv1 with cipher cipher-name
                (actual-key-size/raw-key-size bits))
                (No client certificate requested)

    TLS 1.3 examples. Some of the new attributes may not appear when not
    applicable or not available in older versions of the OpenSSL library.

        Received: from localhost (localhost [127.0.0.1])
                (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256
        bits)
                 key-exchange X25519 server-signature RSA-PSS (2048 bits)
        server-digest SHA256)
                (No client certificate requested)

        Received: from localhost (localhost [127.0.0.1])
                (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256
        bits)
                 key-exchange X25519 server-signature RSA-PSS (2048 bits)
        server-digest SHA256
                 client-signature ECDSA (P-256) client-digest SHA256)
                (Client CN "example.org", Issuer "example.org" (not verified))

      o The "key-exchange" attribute records the type of "Diffie-Hellman" group
        used for key agreement. Possible values include "DHE", "ECDHE",
        "X25519" and "X448". With "DHE", the bit size of the prime will be
        reported in parentheses after the algorithm name, with "ECDHE", the
        curve name.

      o The "server-signature" attribute shows the public key signature
        algorithm used by the server. With "RSA-PSS", the bit size of the
        modulus will be reported in parentheses. With "ECDSA", the curve name.
        If, for example, the server has both an RSA and an ECDSA private key
        and certificate, it will be possible to track which one was used for a
        given connection.

      o The new "server-digest" attribute records the digest algorithm used by
        the server to prepare handshake messages for signing. The Ed25519 and
        Ed448 signature algorithms do not make use of such a digest, so no
        "server-digest" will be shown for these signature algorithms.

      o When a client certificate is requested with "smtpd_tls_ask_ccert" and
        the client uses a TLS client-certificate, the "client-signature" and
        "client-digest" attributes will record the corresponding properties of
        the client's TLS handshake signature.

The next sections will explain what cipher-name, key-size, and peer
verification status information to expect.

What ciphers provide forward secrecy?

There are dozens of ciphers that support forward secrecy. What follows is the
beginning of a list of 51 ciphers available with OpenSSL 1.0.1e. The list is
sorted in the default Postfix preference order. It excludes null ciphers that
only authenticate and don't encrypt, together with export and low-grade ciphers
whose encryption is too weak to offer meaningful secrecy. The first column
shows the cipher name, and the second shows the key exchange method.

    $ openssl ciphers -v \
            'aNULL:-aNULL:kEECDH:kEDH:+RC4:!eNULL:!EXPORT:!LOW:@STRENGTH' |
        awk '{printf "%-32s %s\n", $1, $3}'
    AECDH-AES256-SHA                 Kx=ECDH
    ECDHE-RSA-AES256-GCM-SHA384      Kx=ECDH
    ECDHE-ECDSA-AES256-GCM-SHA384    Kx=ECDH
    ECDHE-RSA-AES256-SHA384          Kx=ECDH
    ECDHE-ECDSA-AES256-SHA384        Kx=ECDH
    ECDHE-RSA-AES256-SHA             Kx=ECDH
    ECDHE-ECDSA-AES256-SHA           Kx=ECDH
    ADH-AES256-GCM-SHA384            Kx=DH
    ADH-AES256-SHA256                Kx=DH
    ADH-AES256-SHA                   Kx=DH
    ADH-CAMELLIA256-SHA              Kx=DH
    DHE-DSS-AES256-GCM-SHA384        Kx=DH
    DHE-RSA-AES256-GCM-SHA384        Kx=DH
    DHE-RSA-AES256-SHA256            Kx=DH
    ...

To date, all ciphers that support forward secrecy have one of five values for
the first component of their OpenSSL name: "AECDH", "ECDHE", "ADH", "EDH" or
"DHE". Ciphers that don't implement forward secrecy have names that don't start
with one of these prefixes. This pattern is likely to persist until some new
key-exchange mechanism is invented that also supports forward secrecy.

The actual key length and raw algorithm key length are generally the same with
non-export ciphers, but may they differ for the legacy export ciphers where the
actual key is artificially shortened.

Starting with TLS 1.3 the cipher name no longer contains enough information to
determine which forward-secrecy scheme was employed, but TLS 1.3 always uses
forward-secrecy. On the client side, up-to-date Postfix releases log additional
information for TLS 1.3 connections, reporting the signature and key exchange
algorithms. Two examples below (the long single line messages are folded across
multiple lines for readability):

    postfix/smtp[process-id]:
      Untrusted TLS connection established to 127.0.0.1[127.0.0.1]:25:
      TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits)
      key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest
    SHA256
      client-signature ECDSA (P-256) client-digest SHA256

    postfix/smtp[process-id]:
      Untrusted TLS connection established to 127.0.0.1[127.0.0.1]:25:
      TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits)
      key-exchange ECDHE (P-256) server-signature ECDSA (P-256) server-digest
    SHA256

In the above connections, the "key-exchange" value records the "Diffie-Hellman"
algorithm used for key agreement. The "server-signature" value records the
public key algoritm used by the server to sign the key exchange. The "server-
digest" value records any hash algorithm used to prepare the data for signing.
With "ED25519" and "ED448", no separate hash algorithm is used.

Examples of Postfix SMTP server logging:

    postfix/smtpd[process-id]:
      Untrusted TLS connection established from localhost[127.0.0.1]:25:
      TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits)
      key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest
    SHA256
      client-signature ECDSA (P-256) client-digest SHA256

    postfix/smtpd[process-id]:
      Anonymous TLS connection established from localhost[127.0.0.1]:
      TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits)
      server-signature RSA-PSS (2048 bits) server-digest SHA256

    postfix/smtpd[process-id]:
      Anonymous TLS connection established from localhost[127.0.0.1]:
      TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits)
      server-signature ED25519

What do "Anonymous", "Untrusted", etc. in Postfix logging mean?

The verification levels below are subject to man-in-the-middle attacks to
different degrees. If such attacks are a concern, then the SMTP client will
need to authenticate the remote SMTP server in a sufficiently-secure manner.
For example, by the fingerprint of a (CA or leaf) public key or certificate.
Remember that conventional PKI relies on many trusted parties and is easily
subverted by a state-funded adversary.

Anonymous (no peer certificate)
    Postfix SMTP client: With opportunistic TLS (the "may" security level) the
    Postfix SMTP client does not verify any information in the peer
    certificate. In this case it enables and prefers anonymous cipher suites in
    which the remote SMTP server does not present a certificate (these ciphers
    offer forward secrecy of necessity). When the remote SMTP server also
    supports anonymous TLS, and agrees to such a cipher suite, the verification
    status will be logged as "Anonymous".

    Postfix SMTP server: This is by far most common, as client certificates are
    optional, and the Postfix SMTP server does not request client certificates
    by default (see smtpd_tls_ask_ccert). Even when client certificates are
    requested, the remote SMTP client might not send a certificate. Unlike the
    Postfix SMTP client, the Postfix SMTP server "anonymous" verification
    status does not imply that the cipher suite is anonymous, which corresponds
    to the server not sending a certificate.

Untrusted (peer certificate not signed by trusted CA)
    Postfix SMTP client: The remote SMTP server presented a certificate, but
    the Postfix SMTP client was unable to check the issuing CA signature. With
    opportunistic TLS this is common with remote SMTP servers that don't
    support anonymous cipher suites.

    Postfix SMTP server: The remote SMTP client presented a certificate, but
    the Postfix SMTP server was unable to check the issuing CA signature. This
    can happen when the server is configured to request client certificates
    (see smtpd_tls_ask_ccert).

Trusted (peer certificate signed by trusted CA, unverified peer name)
    Postfix SMTP client: The remote SMTP server's certificate was signed by a
    CA that the Postfix SMTP client trusts, but either the client was not
    configured to verify the destination server name against the certificate,
    or the server certificate did not contain any matching names. This is
    common with opportunistic TLS (smtp_tls_security_level is "may" or else
    "dane" with no usable TLSA DNS records) when the Postfix SMTP client's
    trusted CAs can verify the authenticity of the remote SMTP server's
    certificate, but the client is not configured or unable to verify the
    server name.

    Postfix SMTP server: The remote SMTP client certificate was signed by a CA
    that the Postfix SMTP server trusts. The Postfix SMTP server never verifies
    the remote SMTP client name against the names in the client certificate.
    Since the client chooses to connect to the server, the Postfix SMTP server
    has no expectation of a particular client hostname.

Verified (peer certificate signed by trusted CA and verified peer name; or:
peer certificate with expected public-key or certificate fingerprint)
    Postfix SMTP client: The remote SMTP server's certificate was signed by a
    CA that the Postfix SMTP client trusts, and the certificate name matches
    the destination or server name(s). The Postfix SMTP client was configured
    to require a verified name, otherwise the verification status would have
    been just "Trusted".

    Postfix SMTP client: The "Verified" status may also mean that the Postfix
    SMTP client successfully matched the expected fingerprint against the
    remote SMTP server public key or certificate. The expected fingerprint may
    come from smtp_tls_policy_maps or from TLSA (secure) DNS records. The
    Postfix SMTP client ignores the CA signature.

    Postfix SMTP server: The status is never "Verified", because the Postfix
    SMTP server never verifies the remote SMTP client name against the names in
    the client certificate, and because the Postfix SMTP server does not expect
    a specific fingerprint in the client public key or certificate.

Credits

  * TLS support for Postfix was originally developed by Lutz Ja"nicke at
    Cottbus Technical University.
  * Wietse Venema adopted and restructured the code and documentation.
  * Viktor Dukhovni implemented support for many subsequent TLS features,
    including EECDH, and authored the initial version of this document.

