README.SSL 6.14 KB
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Fetchmail client-side SSL support
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Preface
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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 
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specific to fetchmail.

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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.
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	-- Matthias Andree, 2009-05-09


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Quickstart
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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
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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 "--"):
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    --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 
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practically impossible.

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This works as following: the server has a key pair (a secret and a public key), 
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and it sends the client its public key. Messages encrypted with the public key 
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can be decrypted using the private one and vice versa.

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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 
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using the usual POP/IMAP/whatever authentication mechanisms.

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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 
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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 
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authenticate against it by telling it your password).

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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 
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the server, and a digital signature and data about the signer.
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  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.
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You can specify this path using the "sslcertpath" option if it is 
different from the one OpenSSL uses by default.
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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 
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should be reasonably accurate when checking certificates.

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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 
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certificate was not granted to the rightful owner).

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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 
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case that it is not installed). Additionally, you might need to convert the 
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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 
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public key. If it does not, it might be an attack, but keep in mind that the 
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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.
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	- Thomas Moestl <tmoestl@gmx.net>
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	- Matthias Andree