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replaced http://tools.ietf.org/html/rfc with https://www.rfc-editor.org/rfc/rfc
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Community Bot
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openssl s_server -cert -dcert is only useful if the certs (and matching keys -key -dkey) are different algorithms. Then it will use e.g. the RSA key&cert for plain-RSA or DHE-RSA key exchange but the DSS key&cert for DHE-DSS. And similarly for ECDSA and ECDHE-ECDSA now that ECC is supported (since 1.0.0 except for RedHat).

Apache appears to be the same. http://httpd.apache.org/docs/2.4/mod/mod_ssl.html#sslcertificatefile says

This directive can be used multiple times (referencing different filenames) to support multiple algorithms for server authentication - typically RSA, DSA, and ECC. ...

and #sslcertificatekeyfile says

The directive can be used multiple times (referencing different filenames) to support multiple algorithms for server authentication. For each SSLCertificateKeyFile directive, there must be a matching SSLCertificateFile directive.

This would only help if you can get your clients to enable/disable different keyexchanges based on which CA they want you to use, which is probably harder and more confusing than using different hostnames.

There actually is a standard extension defined for TLS1.0 by 3546TLS1.0 by 3546 TLS1.1 by 4366TLS1.1 by 4366 TLS1.2 by 6066TLS1.2 by 6066 for the client to identify which CAs it trusts, and would like the server to use. But most extensions are optional and as far as I have seen no browser (or client at all) implements this one.

openssl s_server -cert -dcert is only useful if the certs (and matching keys -key -dkey) are different algorithms. Then it will use e.g. the RSA key&cert for plain-RSA or DHE-RSA key exchange but the DSS key&cert for DHE-DSS. And similarly for ECDSA and ECDHE-ECDSA now that ECC is supported (since 1.0.0 except for RedHat).

Apache appears to be the same. http://httpd.apache.org/docs/2.4/mod/mod_ssl.html#sslcertificatefile says

This directive can be used multiple times (referencing different filenames) to support multiple algorithms for server authentication - typically RSA, DSA, and ECC. ...

and #sslcertificatekeyfile says

The directive can be used multiple times (referencing different filenames) to support multiple algorithms for server authentication. For each SSLCertificateKeyFile directive, there must be a matching SSLCertificateFile directive.

This would only help if you can get your clients to enable/disable different keyexchanges based on which CA they want you to use, which is probably harder and more confusing than using different hostnames.

There actually is a standard extension defined for TLS1.0 by 3546 TLS1.1 by 4366 TLS1.2 by 6066 for the client to identify which CAs it trusts, and would like the server to use. But most extensions are optional and as far as I have seen no browser (or client at all) implements this one.

openssl s_server -cert -dcert is only useful if the certs (and matching keys -key -dkey) are different algorithms. Then it will use e.g. the RSA key&cert for plain-RSA or DHE-RSA key exchange but the DSS key&cert for DHE-DSS. And similarly for ECDSA and ECDHE-ECDSA now that ECC is supported (since 1.0.0 except for RedHat).

Apache appears to be the same. http://httpd.apache.org/docs/2.4/mod/mod_ssl.html#sslcertificatefile says

This directive can be used multiple times (referencing different filenames) to support multiple algorithms for server authentication - typically RSA, DSA, and ECC. ...

and #sslcertificatekeyfile says

The directive can be used multiple times (referencing different filenames) to support multiple algorithms for server authentication. For each SSLCertificateKeyFile directive, there must be a matching SSLCertificateFile directive.

This would only help if you can get your clients to enable/disable different keyexchanges based on which CA they want you to use, which is probably harder and more confusing than using different hostnames.

There actually is a standard extension defined for TLS1.0 by 3546 TLS1.1 by 4366 TLS1.2 by 6066 for the client to identify which CAs it trusts, and would like the server to use. But most extensions are optional and as far as I have seen no browser (or client at all) implements this one.

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dave_thompson_085
dave_thompson_085
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openssl s_server -cert -dcert is only useful if the certs (and matching keys -key -dkey) are different algorithms. Then it will use e.g. the RSA key&cert for plain-RSA or DHE-RSA key exchange but the DSS key&cert for DHE-DSS. And similarly for ECDSA and ECDHE-ECDSA now that ECC is supported (since 1.0.0 except for RedHat).

Apache appears to be the same. http://httpd.apache.org/docs/2.4/mod/mod_ssl.html#sslcertificatefile says

This directive can be used multiple times (referencing different filenames) to support multiple algorithms for server authentication - typically RSA, DSA, and ECC. ...

and #sslcertificatekeyfile says

The directive can be used multiple times (referencing different filenames) to support multiple algorithms for server authentication. For each SSLCertificateKeyFile directive, there must be a matching SSLCertificateFile directive.

This would only help if you can get your clients to enable/disable different keyexchanges based on which CA they want you to use, which is probably harder and more confusing than using different hostnames.

There actually is a standard extension defined for TLS1.0 by 3546 TLS1.1 by 4366 TLS1.2 by 6066 for the client to identify which CAs it trusts, and would like the server to use. But most extensions are optional and as far as I have seen no browser (or client at all) implements this one.