On Mon, Jun 29, 2026 at 03:53:41AM +0000, Peter Gutmann wrote:
> It's an FYI, not a you-must-do-this-or-else, so everyone's free to
> ignore it at their leisure.
Yes, entirely optional, but not universally ignored, for example:
$ printf "QUIT\r\n" |
openssl s_client -brief -ignore_unexpected_eof \
-starttls smtp -CAfile /etc/pki/tls/cert.pem \
-connect smtp.gmail.com:587 -groups mlkem1024
Connecting to 2404:6800:4003:c06::6d
CONNECTION ESTABLISHED
Protocol version: TLSv1.3
Ciphersuite: TLS_AES_256_GCM_SHA384
Requested Signature Algorithms:
ecdsa_secp256r1_sha256:rsa_pss_rsae_sha256:rsa_pkcs1_sha256:ecdsa_secp384r1_sha384:rsa_pss_rsae_sha384:rsa_pkcs1_sha384:rsa_pss_rsae_sha512:rsa_pkcs1_sha512:rsa_pkcs1_sha1
Peer certificate: CN=smtp.gmail.com
Hash used: SHA256
Signature type: ecdsa_secp256r1_sha256
Verification: OK
Negotiated TLS1.3 group: MLKEM1024
250 SMTPUTF8
DONE
Or:
$ posttls-finger -c -Lsummary -o tls_config_file=<(
printf "%s\n[c]\n%s\n[s]\n%s\n[d]\n%s\n" \
client=c ssl_conf=s system_default=d Groups=mlkem768
) dukhovni.org
posttls-finger: Verified TLS connection established
to mx1.imrryr.org[144.6.86.210]:25: TLSv1.3 with
cipher TLS_AES_256_GCM_SHA384 (256/256 bits)
key-exchange MLKEM768
server-signature ML-DSA-65 (raw public key)
The hybrid group is unsurprisingly the one selected by default (in
Postfix after HRR if mutually supported, with X25519 for the first
flight):
$ posttls-finger -c -Lsummary,trace dukhovni.org
posttls-finger: TLS protocol trace saved to
posttls-finger-20260629160007.334267-144.6.86.210-m802H7
posttls-finger: Verified TLS connection established
to mx1.imrryr.org[144.6.86.210]:25: TLSv1.3 with
cipher TLS_AES_256_GCM_SHA384 (256/256 bits)
key-exchange X25519MLKEM768
server-signature ML-DSA-65 (raw public key)
$ cat posttls-finger-20260629160007.334267-144.6.86.210-m802H7
Sent TLS Record
Header:
Version = TLS 1.0 (0x301)
Content Type = Handshake (22)
ClientHello, Length=508
...
extensions, length = 323
extension_type=supported_groups(10), length=18
X25519MLKEM768 (4588)
MLKEM768 (513)
ecdh_x25519 (29)
...
...
extension_type=key_share(51), length=38
NamedGroup: ecdh_x25519 (29)
key_exchange: (len=32): ...
extension_type=server_cert_type(20), length=3
rpk (2)
x509 (0)
extension_type=padding(21), length=147
Received TLS Record
Header:
(HRR)
...
extension_type=key_share(51), length=2
NamedGroup: X25519MLKEM768 (4588)
Sent TLS Record
Header:
Version = TLS 1.2 (0x303)
Content Type = Handshake (22)
...
ClientHello, Length=1541
...
extensions, length = 1356
extension_type=supported_groups(10), length=18
X25519MLKEM768 (4588)
MLKEM768 (513)
ecdh_x25519 (29)
...
...
extension_type=key_share(51), length=1222
NamedGroup: X25519MLKEM768 (4588)
key_exchange: (len=1216): ...
...
extension_type=server_cert_type(20), length=3
rpk (2)
x509 (0)
Received TLS Record
Header:
Version = TLS 1.2 (0x303)
Content Type = Handshake (22)
...
ServerHello, Length=1206
...
extensions, length = 1134
extension_type=key_share(51), length=1124
NamedGroup: X25519MLKEM768 (4588)
key_exchange: (len=1120): ...
Received TLS Record
Header:
Version = TLS 1.2 (0x303)
Content Type = ApplicationData (23)
Length = 28
Inner Content Type = Handshake (22)
EncryptedExtensions, Length=7
extensions, length = 5
extension_type=server_cert_type(20), length=1
rpk (2)
Received TLS Record
Header:
Version = TLS 1.2 (0x303)
Content Type = ApplicationData (23)
Length = 2004
Inner Content Type = Handshake (22)
Certificate, Length=1983
context (len=0):
raw_public_key, length=1974
ML-DSA-65 Public-Key:
...
No extensions
Received TLS Record
Header:
Version = TLS 1.2 (0x303)
Content Type = ApplicationData (23)
Length = 3334
Inner Content Type = Handshake (22)
CertificateVerify, Length=3313
Signature Algorithm: mldsa65 (0x0905)
Signature (len=3309): ...
Received TLS Record
Header:
Version = TLS 1.2 (0x303)
Content Type = ApplicationData (23)
Length = 69
Inner Content Type = Handshake (22)
Finished, Length=48
verify_data (len=48): ...
Sent TLS Record
Header:
Version = TLS 1.2 (0x303)
Content Type = ApplicationData (23)
Length = 69
Inner Content Type = Handshake (22)
Finished, Length=48
verify_data (len=48): ...
...
--
Viktor. 🇺🇦 Слава Україні!
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