emailubuntussltls1.2dovecot

PHP imap_open(), OpenSSL, and no Cipher

I have moved a PHP script to another server, and now fail to login to an IMAP (TLS) postbox:

TLS/SSL failure for mail.servername.de: SSL negotiation failed

It seems that the problem is caused by OpenSSL, because when I try to connect to the Mailserver from both servers, I get a connection in one case (the mailserver asking for input), but none in the other (the connection is closed, I am back to bash):

 openssl s_client -crlf -connect mail.servername.de:993

The most obvious difference is here:

verify return:1
---

<snip>

-----END CERTIFICATE-----
subject=/CN=mail.servername.de
issuer=/C=US/O=Let's Encrypt/CN=Let's Encrypt Authority X3
---
No client certificate CA names sent
Peer signing digest: SHA512
Server Temp Key: DH, 1024 bits
---
SSL handshake has read 3398 bytes and written 483 bytes
Verification: OK
---
New, TLSv1.2, Cipher is DHE-RSA-AES256-GCM-SHA384
Server public key is 2048 bit

And on th other server (where no connection is made)

verify return:1
depth=0 CN = mail.servername.de
verify return:1
140410888582464:error:141A318A:SSL routines:tls_process_ske_dhe:dh key too small:../ssl/statem/statem_clnt.c:2149:
---

<snip>

-----END CERTIFICATE-----
subject=CN = mail.servername.de

issuer=C = US, O = Let's Encrypt, CN = Let's Encrypt Authority X3

---
No client certificate CA names sent
---
SSL handshake has read 3167 bytes and written 318 bytes
Verification: OK
---
New, (NONE), Cipher is (NONE)
Server public key is 2048 bit

On the mailserver dovecot is configured not to accept non-encrypted connections. But, I assume it already fails due to dh key too small, which seems to relate to cipher negotiation.

Now I simply fail to put the things together... Why does the SSL connection work from one server, but not from the other?

Solution

  • TL;DR: your new host has a newer version of OpenSSL probably with higher security settings which prohibit connecting to the host for reasons explained below.

    "dh key too small" comes from OpenSSL and because of too low security.

    Things changed, and for example in newest Debian versions and with OpenSSL 1.1.1 (and I guess it is similar for newer versions), the security was enhanced.

    The best and simplest explanation I have found is on Debian wiki at https://wiki.debian.org/ContinuousIntegration/TriagingTips/openssl-1.1.1 which says:

    In Debian the defaults are set to more secure values by default. This is done in the /etc/ssl/openssl.cnf config file. At the end of the file there is:

    [system_default_sect]
MinProtocol = TLSv1.2
CipherString = DEFAULT@SECLEVEL=2

    This can results in errors such as:

    dh key too small
ee key too small
ca md too weak

    Now the possible solutions in descending order of preference: