I use PyOpenSSL
But I was wondering if the function also checks the signatures along the certificate chain
Of course it does. Otherwise what is the purpose of chain verification? From the OpenSSL documentation (
man 1ssl verify on linux):
The final operation is to check the validity of the certificate chain. The validity period is checked against the current system time and the notBefore and notAfter dates in the certificate. The certificate signatures are also checked at this point.