Certificate verification with cross signed CAs

[James Chapman]

I’ve been using openssl verify to check some certificate chains:

server -> ca -> roota
server -> alt_ca-> rootb

Certificates ca and alt_ca have the same subject and public key and different issuers.

openssl verify -trusted roota.pem -untrusted ca.pem server.pem 
openssl verify -trusted rootb.pem -untrusted alt_ca.pem server.pem 

Work fine, however if I include both the intermediate CAs then verify only succeeds when the correct untrusted CA is first in the list

openssl verify -trusted roota.pem -untrusted ca.pem -untrusted alt_ca.pem server.pem (OK)
openssl verify -trusted rootb.pem -untrusted ca.pem -untrusted alt_ca.pem server.pem (FAIL: error 20 at 1 depth lookup: unable to get local issuer certificate)

Is there a mechanism to support both ca.pem and alt_ca.pem without knowing which root the client has?

Many thanks
James