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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 that other document to your ISP and have them check
their server configuration against it.
Unfortunately, fetchmail confuses 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. For compatibility reasons, this cannot
be fixed in a bugfix release.
-- Matthias Andree, 2009-05-09
Quickstart
----------
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 (in the rcfile,
omit the leading "--"):
--sslproto tls1 --sslcertck
For use of SSL or TLS on a separate port, if the whole TCP connection is
SSL-encrypted from the very beginning, use these command line options (in the
rcfile, omit the leading "--"):
--ssl --sslproto ssl3 --sslcertck
Background and use (long version :-))
------------------
Using fetchmail's "ssl" option, 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 following: the server has a key pair (a secret and a public key),
and it sends the client it's 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 don't use a challenge based authentication
mechanism (because he is thought to be the real server, fetchmail will try to
authenticate against it by telling it your password).
So, not only 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". When it
is specified, the signature of 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's 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 isn't 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
reasonable sure that you talk to the original server, because only that knows
the secret key, and it is very hard to generate a matching secret key from the
public key. If it doesn't, it might be an attack, but keep in mind that the
server key may also have changed legitimately before panicking ;)
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 byes (i.e. a ":" 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).
*CAVEAT*: OpenSSL must be at least version 0.9.7 to be able to check CRLs.
- Thomas Moestl <tmoestl@gmx.net>
- Matthias Andree
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