With the advent of public key cryptography (PKI), it is now possible to communicate securely with untrusted parties over the Internet without prior arrangement. One of the necessities arising from such communication is the ability to accurately verify someone's identity (i.e. whether the person you are communicating with is indeed the person who he/she claims to be). In order to be able to perform identity check for a given entity, there should be a fool-proof method of “binding” the entity's public key to its unique domain name (DN).
A X.509 digital certificate issued by a well known certificate authority (CA), like Verisign, Entrust, Thawte, etc., provides a way of positively identifying the entity by placing trust on the CA to have performed the necessary verifications. A X.509 certificate is a cryptographically sealed data object that contains the entity's unique DN, public key, serial number, validity period, and possibly other extensions.
The Windows Operating System offers a Certificate Viewer utility which allows you to double-click on any certificate and review its attributes in a human-readable format. For instance, the "General" tab in the Certificate Viewer Window (see below) shows who the certificate was issued to as well as the certificate's issuer, validation period and usage functions.
Certification Path graphic
The “Certification Path” tab contains the hierarchy for the chain of certificates. It allows you to select the certificate issuer or a subordinate certificate and then click on “View Certificate” to open the certificate in the Certificate Viewer.
Each end-user certificate is signed by its issuer, a trusted CA, by taking a hash value (MD5 or SHA-1) of ASN.1 DER (Distinguished Encoding Rule) encoded object and then encrypting the resulting hash with the issuer’s private key (CA's Private Key) which is a digital signature. The encrypted data is stored in the “signatureValue” attribute of the entity’s (CA) public certificate.
Once the certificate is signed by the issuer, a party who wishes to communicate with this entity can then take the entity’s public certificate and find out who the issuer of the certificate is. Once the issuer’s of the certificate (CA) is identified, it would be possible to decrypt the value of the “signatureValue” attribute in the entity's certificate using the issuer’s public key to retrieve the hash value. This hash value will be compared with the independently calculated hash on the entity's certificate. If the two hash values match, then the information contained within the certificate must not have been altered and, therefore, one must trust that the CA has done enough background check to ensure that all details in the entity’s certificate are accurate.
The process of cryptographically checking the signatures of all certificates in the certificate chain is called “key chaining”. An additional check that is essential to key chaining is verifying that the value of the "subjectKeyIdentifier” extension in one certificate matches the same in the subsequent certificate.
Similarly, the process of comparing the subject field of the issuer certificate to the issuer field of the subordinate certificate is called “name chaining”. In this process, these values must match for each pair of adjacent certificates in the certification path in order to guarantee that the path represents unbroken chain of entities relating directly to one another and that it has no missing links.
The two steps above are the steps to validate the Certification Path by ensuring the validity of all certificates of the certificate chain to the root certificate as described in the two paragraphs above.
Reference(s) used for this question:
FORD, Warwick & BAUM, Michael S., Secure Electronic Commerce: Building the Infrastructure for Digital Signatures and Encryption (2nd Edition), 2000, Prentice Hall PTR, Page 262.
and
https://www.tibcommunity.com/docs/DOC-2197
