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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. 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 or minor release. -- Matthias Andree, 2009-05-09 Also, fetchmail 6.4.0 and newer releases (this is also true for this release, as the changes were backported from upstream - noted by Red Hat) 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, 2015-01-16 Quickstart ---------- Use an up-to-date release of OpenSSL 1.0.1 or newer, so as to get TLSv1.2 support. 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" 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 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". 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). 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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