question: public static String verifyCertificate ( X509Certificate cert , Collection < CRL > crls , Calendar calendar ) { if ( SignUtils . hasUnsupportedCriticalExtension ( cert ) ) return "Has unsupported critical extension" ; try { cert . checkValidity ( calendar . getTime ( ) ) ; } catch ( Exception e ) { return e . getMessage ( ) ; } if ( crls != null ) { for ( CRL crl : crls ) { if ( crl . isRevoked ( cert ) ) return "Certificate revoked" ; } } return null ; }
target:  Verifies a single certificate.