The above approach should have worked if the server was configured with STARTTLS. The client would connect without encryption, typically but not necessarily on the default port 5222, and then proceed with establishing an encrypted connection. In my case, however, the server was not configured with STARTTLS, which is probably indicated by Secure Renegotiation IS NOT supported in the output. Thus, skipping -starttls worked:
$ openssl s_client -servername riabenko.com -connect xmpps-hosting.conversations.im:443
CONNECTED(00000003)
depth=1 C = DE, ST = Berlin, L = Berlin, O = Conversations, OU = Hosting, CN = Conversations CA
verify error:num=19:self-signed certificate in certificate chain
verify return:1
depth=1 C = DE, ST = Berlin, L = Berlin, O = Conversations, OU = Hosting, CN = Conversations CA
verify return:1
depth=0 CN = riabenko.com
verify return:1
---
Certificate chain
0 s:CN = riabenko.com
i:C = DE, ST = Berlin, L = Berlin, O = Conversations, OU = Hosting, CN = Conversations CA
a:PKEY: rsaEncryption, 4096 (bit); sigalg: RSA-SHA256
v:NotBefore: Aug 15 00:00:00 2021 GMT; NotAfter: Aug 15 00:00:00 2023 GMT
1 s:C = DE, ST = Berlin, L = Berlin, O = Conversations, OU = Hosting, CN = Conversations CA
i:C = DE, ST = Berlin, L = Berlin, O = Conversations, OU = Hosting, CN = Conversations CA
a:PKEY: rsaEncryption, 4096 (bit); sigalg: RSA-SHA256
v:NotBefore: Dec 13 11:54:28 2016 GMT; NotAfter: Dec 13 11:54:28 2026 GMT
---
Server certificate
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----
subject=CN = riabenko.com
issuer=C = DE, ST = Berlin, L = Berlin, O = Conversations, OU = Hosting, CN = Conversations CA
---
No client certificate CA names sent
Client Certificate Types: RSA sign, DSA sign, ECDSA sign
Requested Signature Algorithms: ECDSA+SHA256:ECDSA+SHA384:ECDSA+SHA512:Ed25519:Ed448:RSA-PSS+SHA256:RSA-PSS+SHA384:RSA-PSS+SHA512:RSA-PSS+SHA256:RSA-PSS+SHA384:RSA-PSS+SHA512:RSA+SHA256:RSA+SHA384:RSA+SHA512:ECDSA+SHA224:ECDSA+SHA1:RSA+SHA224:RSA+SHA1:DSA+SHA224:DSA+SHA1:DSA+SHA256:DSA+SHA384:DSA+SHA512
Shared Requested Signature Algorithms: ECDSA+SHA256:ECDSA+SHA384:ECDSA+SHA512:Ed25519:Ed448:RSA-PSS+SHA256:RSA-PSS+SHA384:RSA-PSS+SHA512:RSA-PSS+SHA256:RSA-PSS+SHA384:RSA-PSS+SHA512:RSA+SHA256:RSA+SHA384:RSA+SHA512:ECDSA+SHA224:RSA+SHA224
Peer signing digest: SHA256
Peer signature type: RSA-PSS
Server Temp Key: X25519, 253 bits
---
SSL handshake has read 3703 bytes and written 421 bytes
Verification error: self-signed certificate in certificate chain
---
New, TLSv1.2, Cipher is ECDHE-RSA-AES256-GCM-SHA384
Server public key is 4096 bit
Secure Renegotiation IS supported
Compression: NONE
Expansion: NONE
No ALPN negotiated
SSL-Session:
Protocol : TLSv1.2
Cipher : ECDHE-RSA-AES256-GCM-SHA384
Session-ID:
Session-ID-ctx:
Master-Key: CAA294AB238F342CAB537680C748895158A7D0ECC9292A467A816857F33FD454C35A4A2E374E3D0BDF29BACA0FE5DFCC
PSK identity: None
PSK identity hint: None
SRP username: None
Start Time: 1657319079
Timeout : 7200 (sec)
Verify return code: 19 (self-signed certificate in certificate chain)
Extended master secret: yes
---
^C
I terminated the connection with Ctrl+C, saved the certificate (begining with -----BEGIN CERTIFICATE----- and ending with -----END CERTIFICATE-----) to a file, and followed my distribution's instructions for adding it to the trust store, which for Fedora happen to be:
$ cat /etc/pki/ca-trust/source/README
This directory /etc/pki/ca-trust/source/ contains CA certificates and
trust settings in the PEM file format. The trust settings found here will be
interpreted with a high priority - higher than the ones found in
/usr/share/pki/ca-trust-source/.
=============================================================================
QUICK HELP: To add a certificate in the simple PEM or DER file formats to the
list of CAs trusted on the system:
Copy it to the
/etc/pki/ca-trust/source/anchors/
subdirectory, and run the
update-ca-trust
command.
If your certificate is in the extended BEGIN TRUSTED file format,
then place it into the main source/ directory instead.
=============================================================================
Please refer to the update-ca-trust(8) manual page for additional information.
