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Fetchmail client-side SSL support
=================================
Preface
-------
Note: there is a separate document "README.SSL-SERVER" describing the server-
side requirements for proper SSL support. It has checklist-style and is not
specific to fetchmail.
In case of troubles, mail the README.SSL-SERVER file to your ISP and
have them check their server configuration against it.
Note that fetchmail up to version 6.3.26 confused SSL/TLS protocol levels with
whether a service needs to use in-band negotiation (STLS/STARTTLS for
POP3/IMAP4) or is totally SSL-wrapped on a separate port.
Also, fetchmail 6.4.0 and newer releases changed some of the semantics
as the result of a bug-fix, and will auto-negotiate TLSv1 or newer only.
If your server does not support this, you may have to specify --sslproto
ssl3+. This is in order to prefer the newer TLS protocols, because SSLv2
and v3 are broken.
-- Matthias Andree, 2019-08-18
Quickstart
----------
Use an up-to-date release of OpenSSL 1.0.2 or newer, so as to get
TLSv1.2 support. If possible, use a TLS1.3-enabled OpenSSL v1.1.1 or newer.
Older OpenSSL versions are unsupported upstream, and fetchmail rejects them.
In all four examples below, the (--)sslcertck has become redunant
since fetchmail v6.4.0 but since fetchmail 6.3 releases will be in circulation
for a while, we'll leave it here to be safe.
For use of SSL or TLS with in-band negotiation on the regular service's port,
i. e. with STLS or STARTTLS, use these command line options
--sslproto auto --sslcertck
or these options in the rcfile (after the respective "user"... options)
sslproto auto sslcertck
For use of SSL or TLS on a separate port, if the whole TCP connection is
SSL-encrypted from the very beginning (SSL- or TLS-wrapped), use these
command line options (in the rcfile, omit all leading "--"):
--ssl --sslproto auto --sslcertck
or these options in the rcfile (after the respective "user"... options)
ssl sslproto auto sslcertck
Background and use (long version :-))
------------------
Using fetchmail's "ssl" and since 6.4.0 "sslproto" options,
you can have the data transferred between you and the server in an encrypted
form, so that eavesdropping should become practically impossible.
This works as follows: the server has a key pair (a secret and a public key),
and it sends the client its public key. Messages encrypted with the public key
can be decrypted using the private one and vice versa.
A symmetric session key (symmetric means that the same key is used for
encryption and decryption) can now be agreed upon by the two parties using the
secure channel the key pair builds. The session key is now used to encrypt the
traffic.
In the fetchmail case, the client can now authenticate itself to the server by
using the usual POP/IMAP/whatever authentication mechanisms.
However, so called man-in-the-middle attacks are still possible: in such
a setting, an attacker pretends to be the server, and thus can e.g. get your
authentication information if you do not use a challenge based authentication
mechanism (because it is thought to be the real server, fetchmail will try to
authenticate against it by telling it your password).
So, not only do you need to prove your identity to the server, the
server likewise needs to prove (authenticate) its identity to you.
In the standard setting, the server has a certificate (the client can have
a certificate too to prove its identity, but this is not covered by this
document). This certificate contains the server's public key, some data about
the server, and a digital signature and data about the signer.
Digital signatures can also be made using a key pair as described earlier.
To check this certificate, you should use the new option "sslcertck" (fetchmail
6.4.0 and newer enable it by default). When it is specified, the signature of
the server certificate is checked against local trusted certificates to see
whether the owner of one of the certificates has signed that server
certificate, and if so, whether the signature is valid.
So, if the server certificate is signed by a Certification Authority (CA),
you put the CA's certificate into a directory where you keep trusted
certificates, and point fetchmail to it. Fetchmail will then accept
certificates signed by the owner of that certificate with the private key
belonging to the public key in the certificate.
You can specify this path using the "sslcertpath" option if it is
different from the one OpenSSL uses by default.
The idea is that the CA only gives certificates to entities whose identity it
has checked and verified (and in this case, that the server name you specify
does belong to it). So, if you trust the CA's verification process, you can be
reasonably sure that if a certificate is signed by the CA, it really belongs to
the server and owner that it claims to.
Certificates are only valid in a certain time window, so your system clock
should be reasonably accurate when checking certificates.
Additionally, CAs keep Certificate Revocation Lists (CRLs) in which they note
the certificates that are to be treated as invalid (e.g. because the server
name has changed, another certificate was granted, or even because the
certificate was not granted to the rightful owner).
The certificate directory must be hashed in a way OpenSSL expects it: each time
you modify a file in that directory or add a file to it, you need to use the
"c_rehash" perl script that comes with OpenSSL (in the tools/ subdirectory, in
case that it is not installed). Additionally, you might need to convert the
certificates to different formats (the PEM format is expected and usually is
available, DER is another one; you can convert between both using the
openssl(1) utility's x509 sub-mode).
The really paranoid (who chose to not trust a CA) can check the fingerprint of
the public key that is used by the server. The fingerprint is a hash of that
key that (hopefully) has few collisions and is hard to attack using a "birthday
attack", i.e. nobody can generate a second key that hashes to the same value of
the original key in reasonable time. So, if the fingerprint matches, you can be
reasonably sure that you are talking to the original server, because
only that server knows the secret key, and it is very hard to generate a
matching secret key from the public key. If the fingerprint does not
match, there might be an attack, but, before panicking, keep in mind
that the server key may also have changed legitimately.
Fetchmail will present the fingerprint to you in verbose mode. You can have
fetchmail check the fingerprint (using the "sslfingerprint" option, and giving
the desired fingerprint as an argument).
The fingerprints fetchmail uses are MD5 sums. You can generate them e.g. using
openssl(1)'s "x509 -fingerprint" command. The format is a hexadecimal string
with a ":" separating two bytes (i.e. a ":" between every two hex "digits"). The
match is case insensitive since release 6.3.10 (upper-case digits A to F were
required up to and including release 6.3.9).
- Thomas Moestl <tmoestl@gmx.net>
- Matthias Andree
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