"3-D Secure is a secure payment protocol on the Internet. It was developed by Visa to increase the level of transaction security, and it has been adapted by Master Card. It gives a better authentication of the holder of the payment card, during purchases made on websites. The basic concept of this (XML-based) protocol is to link the financial authorization process with an online authentication system. This authentication model comprise 3 domains (hence the name 3D) which are:
1. The Acquirer Domain
2. The Issuer Domain
3. The interoperability Domain

Phil Zimmermann developed PGP in 1991. It is a decentralized encryption program that provides cryptographic privacy and authentication for data communication. PGP encryption uses a serial combination of Hashing, data compresion, symmetric key cryptography and asymmetric key cryptography and works on the concept of "web of Trust" It is also called "Pretty Good Privacy".

(i) Information leakage
(ii) Tampering
(iii) Payment frauds
(iv) Malicious code threats
(v) Distributed denial of service attacks
(vi) Cyber squatting
(vii) Typopiracy
(viii) Hacking

(i) The certificate authority maintains a database of public keys called repository so that it can verify the user with digital signatures.
(ii) Expired certificates are usually deleted from the database by the certificate authority.