In a PureOS 9 Amber container I ran GnuTLS like this;
root@c025729fd1a8:/# gnutls-cli repo.puri.sm:443 Processed 0 CA certificate(s). Resolving 'repo.puri.sm:443'... Connecting to '138.201.228.45:443'... - Certificate type: X.509 - Got a certificate list of 3 certificates. - Certificate[0] info: - subject `CN=repo.pureos.net', issuer `CN=R3,O=Let's Encrypt,C=US', serial 0x0399fb524be1d68831ab006b7a29baaba15d, RSA key 2048 bits, signed using RSA-SHA256, activated `2021-08-28 15:21:11 UTC', expires `2021-11-26 15:21:10 UTC', pin-sha256="GmkUEOwRUB93Z3jBmt6Y24YxoQ4ldt7FSN5hvjJI/+E=" Public Key ID: sha1:b24b3ed9b73a339e94f397107d9b183afc3e620a sha256:1a691410ec11501f776778c19ade98db8631a10e2576dec548de61be3248ffe1 Public Key PIN: pin-sha256:GmkUEOwRUB93Z3jBmt6Y24YxoQ4ldt7FSN5hvjJI/+E= - Certificate[1] info: - subject `CN=R3,O=Let's Encrypt,C=US', issuer `CN=ISRG Root X1,O=Internet Security Research Group,C=US', serial 0x00912b084acf0c18a753f6d62e25a75f5a, RSA key 2048 bits, signed using RSA-SHA256, activated `2020-09-04 00:00:00 UTC', expires `2025-09-15 16:00:00 UTC', pin-sha256="jQJTbIh0grw0/1TkHSumWb+Fs0Ggogr621gT3PvPKG0=" - Certificate[2] info: - subject `CN=ISRG Root X1,O=Internet Security Research Group,C=US', issuer `CN=DST Root CA X3,O=Digital Signature Trust Co.', serial 0x4001772137d4e942b8ee76aa3c640ab7, RSA key 4096 bits, signed using RSA-SHA256, activated `2021-01-20 19:14:03 UTC', expires `2024-09-30 18:14:03 UTC', pin-sha256="C5+lpZ7tcVwmwQIMcRtPbsQtWLABXhQzejna0wHFr8M=" - Status: The certificate is NOT trusted. The certificate issuer is unknown. *** PKI verification of server certificate failed... *** Fatal error: Error in the certificate.
Possibly a duplicate of https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=961889