O X509v3 Authority Key Identifier
na saída openssl
da chave filha corresponderá à X509v3 Subject Key Identifier
da chave de assinatura.
Por exemplo, para o certificado SSL deste site e seu certificado pai:
# openssl x509 -text -noout -in subject.pem
...
Subject: C=US, ST=NY, L=New York, O=Stack Exchange, Inc., CN=*.stackexchange.com
...
X509v3 Authority Key Identifier:
keyid:51:68:FF:90:AF:02:07:75:3C:CC:D9:65:64:62:A2:12:B8:59:72:3B
X509v3 Subject Key Identifier:
5A:C1:42:63:C2:62:13:B3:9D:94:84:AA:32:1E:17:CB:6D:A3:86:7B
# openssl x509 -text -noout -in parent.pem
...
Subject: C=US, O=DigiCert Inc, OU=www.digicert.com, CN=DigiCert SHA2 High Assurance Server CA
...
X509v3 Subject Key Identifier:
51:68:FF:90:AF:02:07:75:3C:CC:D9:65:64:62:A2:12:B8:59:72:3B
X509v3 Authority Key Identifier:
keyid:B1:3E:C3:69:03:F8:BF:47:01:D4:98:26:1A:08:02:EF:63:64:2B:C3
51:68:FF:90:AF:02:07:75:3C:CC:D9:65:64:62:A2:12:B8:59:72:3B
é o que estabelece no certificado filho qual certificado o assinou, você deve ser capaz de usá-lo para localizar os certificados de autoridade corretos.