How Asymmetric Key or Public Key Encryption Works

Plus Tips on Verifying Public Keys and using Digital Signatures When Encrypting Messages
Message encryption is the process of converting a message into an undecipherable form so that only the person who it is intended for can read it. In this age of hacking, high-tech snooping and intelligence gathering that is carried out even by governments of some countries, message encryption may be a reasonable option for many people.
Message encryption here will refer to methods of encoding or decoding messages that is used with desktop mail clients like Mozilla Thunderbird or Microsoft Outlook. This is in contrast to encrypted web-based mail like HushMail.
Encryption may be carried out using symmetric key encryption also known as single-key encryption or it can be done using asymmetric encryption popularly called public key encryption. This article will explain public key encryption.
Public key encryption is also known as public key cryptography and it involves the generation of two kinds of keys using an encryption protocol: a public key and a private key. Both keys are specific for a particular individual. A public key is used to encrypt messages and a private key is used to decrypt messages. The keys are generated in pairs: the messages encoded by a particular public key can be decoded only by a particular private key.
The public key is distributed by an individual so that people who want to send messages to him or her can make use of it to encrypt the message. This public key is distributed in such a way that people can verify that the public key belongs to a particular individual. Such distribution is done by the use of Certification Authorities and other methods which are explained below. When the encrypted message is received by the individual to whom it was sent, he or she can then use the private key to decrypt the message. The private key is known only to that particular individual. In contrast, symmetric key encryption involves the use of a single key which is known to both sender and receiver.
Public and private keys are generated using encryption protocols like PGP and S/MIME. PGP which stands for Pretty Good Privacy is the most popular encryption protocol but the S/MIME (Secure/Multipurpose Internet Mail Extensions) protocol is widely accepted as the standard for message security. These protocols can be downloaded as plug-ins and installed to work with your desktop mail client. Some mail clients already come preinstalled with encryption software.
Other Things You Should Know About Public Key Encryption
Verifying Public Keys
Since a public key is made publicly available, there is the possibility that problems may arise due to this. Person A could publish a key which purports to specify Person B. This will allow Person A to decrypt intercepted messages sent to Person B which have been encrypted with the false key. Person A may then re-encrypt the message with Person B's true public key before passing the message on. With this in mind, methods of distributing public keys have been created that ensure the validity of a public key. These include Certification Authorities and Web of Trust amongst others.
A certification authority (CA) such as Comodo or VeriSign publishes public keys for users. This implies that the keys have been authenticated by the CA and belong to those users. A web of trust (WOT) is a method of authenticating your public key with third party attestation to its veracity. In the WOT method, a user publishes his or her public key and includes certifying signatures from other trusted individuals to serve as authentication.
Using Digital Signatures with Message Encryption
Message encryption prevents your messages from being read but it may not prevent them from being altered. Applying a digital signature to an encrypted message allows the recipient to determine if the message has been altered. Public key encryption enables the use of digital signatures.
Note: Message encryption using the above protocols is not designed to be used with web mail such as Gmail but workarounds have been created for such use.

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