[lamps] Re: TLS Client Certificates; a survey

[Viktor Dukhovni <[e-mail]>]

On Tue, Mar 24, 2026 at 10:17:39AM +0900, Seo Suchan wrote:

> what's gmail currently doing? I don't run mail server so I don't know what
> cert it sends for smtp

I think we're veering off topic, but FWIW:

Gmail's serverAuth certificate is issuer by a WebPKI CA, and can be
(weakly) verified by MTAs that support MTA-STS (initial policy
bootstrapping is TOFU, as is policy refresh in various scenarios).

    $ curl -sLo - https://mta-sts.gmail.com/.well-known/mta-sts.txt
    version: STSv1
    mode: enforce
    mx: gmail-smtp-in.l.google.com
    mx: *.gmail-smtp-in.l.google.com
    max_age: 86400

    $ posttls-finger -c -lverify -F /etc/pki/tls/cert.pem gmail.com
    posttls-finger: gmail-smtp-in.l.google.com[...]:25: matched peername: gmail-smtp-in.l.google.com
    posttls-finger: gmail-smtp-in.l.google.com[...]:25: subject_CN=mx.google.com, issuer=WR2, cert fingerprint=..., pkey fingerprint=...
    posttls-finger: Verified TLS connection established to gmail-smtp-in.l.google.com[...]:25: TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519MLKEM768 server-signature ECDSA (prime256v1) server-digest SHA256

-- 
    Viktor.  🇺🇦 Слава Україні!