Applications in Plan 9 that use public keys for authentication, for example by calling tlsClient and okThumbprint (see pushtls (2)), check the remote side's public key by comparing against thumbprints from a trusted list. The list is maintained by people who set local policies about which servers can be trusted for which applications, thereby playing the role taken by certificate authorities in PKI-based systems. By convention, these lists are stored as files in /sys/lib/tls/ and protected by normal file system permissions.

Such a thumbprint file comprises lines made up of attribute/value pairs of the form attr = value or attr . The first attribute must be x509 and the second must be sha1= {hex checksum of binary certificate}. All other attributes are treated as comments. The file may also contain lines of the form #include file

For example, a web server might have thumbprint .EX x509 sha1=8fe472d31b360a8303cd29f92bd734813cbd923c cn=*.cs.bell-labs.com