External (other) Mail clients have started to deny/de-list anonymous ciphersuites. But the TLS support is often spotty. Worse, some clients (mainly Windows, which does not care one whit about your RFC specs or lists of common protocols and does its own thing) have started to refuse to send mail when they cannot get an encrypted connection, preferring instead to silently leave the emails in the mail queue.

If the server admin manually sends out the mailqueue, then these clients will do so, sending the messages in plaintext.

This creates problems where some subset of connections fail with Handshake Failure (40) -> cipher list mismatch , or: the server is not configured to support any of the ciphers in the list sent by the client in plaintext.

Using Wireshark to extract the actual protocol/messages, An example such list is:

0000   c0 2b c0 2f c0 23 c0 27 c0 13 00 9d 00 9c 00 3d
0010   00 3c 00 35 00 2f 00 0a

Which translates to;

Cipher Suites (12 suites)
Cipher Suite: TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 (0xc02b)
Cipher Suite: TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 (0xc02f)
Cipher Suite: TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256 (0xc023)
Cipher Suite: TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256 (0xc027)
Cipher Suite: TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA (0xc013)
Cipher Suite: TLS_RSA_WITH_AES_256_GCM_SHA384 (0x009d)
Cipher Suite: TLS_RSA_WITH_AES_128_GCM_SHA256 (0x009c)
Cipher Suite: TLS_RSA_WITH_AES_256_CBC_SHA256 (0x003d)
Cipher Suite: TLS_RSA_WITH_AES_128_CBC_SHA256 (0x003c)
Cipher Suite: TLS_RSA_WITH_AES_256_CBC_SHA (0x0035)
Cipher Suite: TLS_RSA_WITH_AES_128_CBC_SHA (0x002f)
Cipher Suite: TLS_RSA_WITH_3DES_EDE_CBC_SHA (0x000a)

For ECC cipher suites, the client also seems to send a 'supported groups' extension. Which I believe is interpreted by the server; it checks whether its curve is in this set. This client sent only 0x0017, which is

Supported Group: secp256r1 (0x0017)

But the mail server uses a secp384r1 key for its certificate (because that's the only ECC cert request our provider would sign successfully at the time). Is it true that this can lead to a handshake failure, even if the testssl output reports this?

Elliptic curves offered:     prime256v1 secp384r1 secp521r1 X25519 X448

There's also a 'Signature Hash Algorithms' section;

0000   04 01 05 01 02 01 04 03 05 03 02 03 02 02 06 01
0010   06 03

Translating to:

Signature Hash Algorithms (9 algorithms)
Signature Algorithm: rsa_pkcs1_sha256 (0x0401)
Signature Algorithm: rsa_pkcs1_sha384 (0x0501)
Signature Algorithm: rsa_pkcs1_sha1 (0x0201)
Signature Algorithm: ecdsa_secp256r1_sha256 (0x0403)
Signature Algorithm: ecdsa_secp384r1_sha384 (0x0503)
Signature Algorithm: ecdsa_sha1 (0x0203)
Signature Algorithm: SHA1 DSA (0x0202)
Signature Algorithm: rsa_pkcs1_sha512 (0x0601)
Signature Algorithm: ecdsa_secp521r1_sha512 (0x0603)

The relevant cert info, pulled with openssl x509 -text -in cert.cer is (omitting PII details);

Signature : ecdsa-with-SHA256
Signature Algorithm: ecdsa-with-SHA256
Public-Key: (384 bit)
ASN1 OID: secp384r1
X509v3 Key Usage: critical
    Digital Signature
X509v3 Basic Constraints: critical
X509v3 Extended Key Usage:
    TLS Web Server Authentication, TLS Web Client Authentication

One of these could also be the cause of the same problem.

It appears that 'supported group' might not restrict the ECDSA cert. Yet, there's still the problem that this cert does not work. And it's my postfix server/openSSL deciding that it does not, so it's not the fault of some ill-behaved client. Meaning: there's some needle in the haystack here that I'm missing why these clients won't talk; there's some detail in the Client Hello message that causes a handshake failure of

 SSL routines:tls_post_process_client_hello:no shared cipher

Dealing with multiple such clients

There's more types/versions of mail clients talking to this mail server. The usual procedure would be to

Be conservative in what you send, be liberal in what you accept

J. Postel

so as to be able to receive encrypted mail from multiple clients, running software that can date from as far back as the early 2000s, supporting various versions of TLS and so on. When anonymous connections are tolerated this isn't a problem. OpenSSL has a huge list of certs it supports, and the server/client tend to auto-negotiate the strongest cipher using the Postfix cipherlist, which is optimized for both compatibility and security (use whatever is strongest that both accept), and can gracefully downgrade*.

But when a client wants one specific type and only supports 'not-anonymous' certs, I.e. ones that have been signed by a CA that it knows, then (for cost purposes) there's only one cert, not one cert per algorithm (since there's many algorithms).

Option A: multiple certificates

One way to solve it is to perhaps add another cert/key for a different algorithm. Postfix supports the smtpd_tls_eccert_file and smtpd_tls_eckey_file parameters, so you could add both an ECC and an RSA cert, by using the smtpd_tls_key_file and smtpd_tls_cert_file for RSA and the former two for ECDSA. For the sake of argument though, consider what happens when we have two clients that need to speak ECC, but both want a different curve.

I could also add them using the smtpd_tls_chain_files option, where you can supposedly use a list of keys/certs; [key1, cert1, chain1, key2, cert2, ..., keyN, certN, chainN].

Option B, try it anyway?

Interestingly, the clients in question do seem to reference secp384r1 in their list of 'Signature Hash Algorithms' in the 'signature_algorithms' extension. (see above)

Can I tell postfix to use /send the secp384 cert for this client anyway, instead of aborting when it thinks it can't match because the client sent a malformed request for it, i.e. to ignore supported_groups in favour of signature_algorithms if the group in question can be found in the latter?

Option C, modify the client

Can a Microsoft Exchange server be modified so that it will accept an anonymous certificate for outgoing mail, not incoming mail? There's lots of instructions for the former, but none for the latter.

Which option would you recommend / What's a good way to go about this? Needing more and more certificates to deal with all the clients can get cumbersome.

[*] To the extent this is possible; due to mitigations for various downgrade attacks, protocol exploits that could downgrade previous TLS versions.

  • "Mail clients have started to deny/de-list anonymous ciphersuites" - you are using the wrong terminology. Anonymous ciphers are the ones which include no authentication in the cipher which also means that no certificates are used. This is different from using self-signed certificates, which you talk about. Anonymous ciphers (no certificates) were never supported in the first place by common mail clients. Oct 11, 2021 at 14:30
  • "for ECC cipher suites, the client also seems to send a 'supported groups' extension. ... Supported Group: secp256r1 (0x0017) .... But the mail server uses a secp384r1 key for its certificate" - You are mixing things up here. The supported group extension is about the group used for key exchange (ECDHE) and not about the group used in the certificate (ECDSA). Oct 11, 2021 at 14:34
  • "Is it true that this can lead to a handshake failure, even if the testssl output reports this? ... The remainder of the question assumes that this can be a valid cause;" - since this assumption is wrong (extension is about group for key exchange, not certificate) the rest of the question is based on false assumptions. Therefore I propose to close it. Oct 11, 2021 at 14:37
  • This might all be true, but it's still true that whatever is reported in the packet; my postfix, which is 'the' industry standard mailserver, won't accept a connection from the windows exchange server using that ECDSA cert, while once I add an RSA cert it works fine. The rejection reason is that there is no supported cipher, hence, there is some wrong detail.
    – aphid
    Oct 12, 2021 at 7:38
  • 1
    @SteffenUllrich: that's incorrect for TLS1.2, which this Q is. For ECDHE-ECDSA, supported_groups (formerly supported_curves) restricts both the keyexchange and the cert/signature; see rfc4492 or rfc8422 5.1 (2nd para under 'actions of receiver') and 5.3. OP: using sighash values to specify the curve for ECDSA e.g. 0503 = ecdsa_secp384r1_sha384 is only in TLS1.3 which are you aren't using; in TLS1.2 05,03 = sha384+ecdsa with any curve allowed by supported_curves/groups (which in your case is NOT p-384), see rfc5246 Oct 13, 2021 at 2:08


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