1 Fetchmail client-side SSL support
2 =================================
7 Note: there is a separate document "README.SSL-SERVER" describing the server-
8 side requirements for proper SSL support. It has checklist-style and is not
11 In case of troubles, mail the README.SSL-SERVER file to your ISP and
12 have them check their server configuration against it.
14 Unfortunately, fetchmail confuses SSL/TLS protocol levels with whether
15 a service needs to use in-band negotiation (STLS/STARTTLS for POP3/IMAP4) or is
16 totally SSL-wrapped on a separate port. For compatibility reasons, this cannot
17 be fixed in a bugfix release.
19 -- Matthias Andree, 2009-05-09
25 For use of SSL or TLS with in-band negotiation on the regular service's port,
26 i. e. with STLS or STARTTLS, use these command line options
28 --sslproto tls1 --sslcertck
30 or these options in the rcfile (after the respective "user"... options)
32 sslproto tls1 sslcertck
35 For use of SSL or TLS on a separate port, if the whole TCP connection is
36 SSL-encrypted from the very beginning, use these command line options (in the
37 rcfile, omit all leading "--"):
39 --ssl --sslproto ssl3 --sslcertck
41 or these options in the rcfile (after the respective "user"... options)
43 ssl sslproto ssl3 sslcertck
46 Background and use (long version :-))
49 Using fetchmail's "ssl" option, you can have the data transferred between you
50 and the server in an encrypted form, so that eavesdropping should become
51 practically impossible.
53 This works as follows: the server has a key pair (a secret and a public key),
54 and it sends the client its public key. Messages encrypted with the public key
55 can be decrypted using the private one and vice versa.
57 A symmetric session key (symmetric means that the same key is used for
58 encryption and decryption) can now be agreed upon by the two parties using the
59 secure channel the key pair builds. The session key is now used to encrypt the
62 In the fetchmail case, the client can now authenticate itself to the server by
63 using the usual POP/IMAP/whatever authentication mechanisms.
65 However, so called man-in-the-middle attacks are still possible: in such
66 a setting, an attacker pretends to be the server, and thus can e.g. get your
67 authentication information if you do not use a challenge based authentication
68 mechanism (because it is thought to be the real server, fetchmail will try to
69 authenticate against it by telling it your password).
71 So, not only do you need to prove your identity to the server, the
72 server likewise needs to prove (authenticate) its identity to you.
73 In the standard setting, the server has a certificate (the client can have
74 a certificate too to prove its identity, but this is not covered by this
75 document). This certificate contains the server's public key, some data about
76 the server, and a digital signature and data about the signer.
77 Digital signatures can also be made using a key pair as described earlier.
79 To check this certificate, you should use the new option "sslcertck". When it
80 is specified, the signature of the server certificate is checked against local
81 trusted certificates to see whether the owner of one of the certificates has
82 signed that server certificate, and if so, whether the signature is valid.
83 So, if the server certificate is signed by a Certification Authority (CA),
84 you put the CA's certificate into a directory where you keep trusted
85 certificates, and point fetchmail to it. Fetchmail will then accept
86 certificates signed by the owner of that certificate with the private key
87 belonging to the public key in the certificate.
88 You can specify this path using the "sslcertpath" option if it is
89 different from the one OpenSSL uses by default.
91 The idea is that the CA only gives certificates to entities whose identity it
92 has checked and verified (and in this case, that the server name you specify
93 does belong to it). So, if you trust the CA's verification process, you can be
94 reasonably sure that if a certificate is signed by the CA, it really belongs to
95 the server and owner that it claims to.
97 Certificates are only valid in a certain time window, so your system clock
98 should be reasonably accurate when checking certificates.
100 Additionally, CAs keep Certificate Revocation Lists (CRLs) in which they note
101 the certificates that are to be treated as invalid (e.g. because the server
102 name has changed, another certificate was granted, or even because the
103 certificate was not granted to the rightful owner).
105 The certificate directory must be hashed in a way OpenSSL expects it: each time
106 you modify a file in that directory or add a file to it, you need to use the
107 "c_rehash" perl script that comes with OpenSSL (in the tools/ subdirectory, in
108 case that it is not installed). Additionally, you might need to convert the
109 certificates to different formats (the PEM format is expected and usually is
110 available, DER is another one; you can convert between both using the
111 openssl(1) utility's x509 sub-mode).
113 The really paranoid (who chose to not trust a CA) can check the fingerprint of
114 the public key that is used by the server. The fingerprint is a hash of that
115 key that (hopefully) has few collisions and is hard to attack using a "birthday
116 attack", i.e. nobody can generate a second key that hashes to the same value of
117 the original key in reasonable time. So, if the fingerprint matches, you can be
118 reasonably sure that you are talking to the original server, because
119 only that server knows the secret key, and it is very hard to generate a
120 matching secret key from the public key. If the fingerprint does not
121 match, there might be an attack, but, before panicking, keep in mind
122 that the server key may also have changed legitimately.
124 Fetchmail will present the fingerprint to you in verbose mode. You can have
125 fetchmail check the fingerprint (using the "sslfingerprint" option, and giving
126 the desired fingerprint as an argument).
128 The fingerprints fetchmail uses are MD5 sums. You can generate them e.g. using
129 openssl(1)'s "x509 -fingerprint" command. The format is a hexadecimal string
130 with a ":" separating two bytes (i.e. a ":" between every two hex "digits"). The
131 match is case insensitive since release 6.3.10 (upper-case digits A to F were
132 required up to and including release 6.3.9).
134 *CAVEAT*: OpenSSL must be at least version 0.9.7 to be able to check CRLs.
136 - Thomas Moestl <tmoestl@gmx.net>