Public key cryptography uses a pair of keys, one public and one private, to encrypt and decrypt data. The public key can be shared, while the private key must be kept secret. The system is mathematically secure and commonly used in applications like email encryption.
Public key cryptography is a widely adopted cryptographic system used to encrypt data. Unlike symmetric cryptography, which uses a single key, this type of system is considered asymmetric because it is based on a pair of keys. Public key cryptography was originally introduced in the 1970s by cryptographers Whitfield Diffie and Martin Hellman. Such encryption systems are often referred to as Diffie-Hellman encryption as a way to pay homage to the inventors.
As mentioned, public key cryptography uses two keys: one public and one private. Both keys play a role in encrypting a message to protect the data it contains and decrypting the message so it can be read. In this system, the public key can be freely shared and distributed. The private key, however, must be kept secret and must be accessible only to the key owner. To provide an extra layer of security, the private key is protected by an encrypted passphrase, which is essentially a stronger version of a password created by the owner of the key.
Both public and private key are related by a mathematical aspect. Despite this relationship, it is mathematically impossible for a private key to derive from a public key. This is because they serve two very distinct purposes. The public key is designed to encrypt the initial message, while the purpose of the private key is to decrypt it. Any message encrypted with a public key can only be viewed after being decrypted by the corresponding private key.
How public key cryptography works can be examined using a communication between John and Jane as an example. John generates a key pair and sends the public key to Jane, who uses the key to encrypt the message so only John can read it. When he receives the message from Jane, John uses her secret key to decrypt and read it. Since John created the key pair, he is the owner of the private key, and therefore the only person who can decrypt and read the message.
Anyone wishing to use public key cryptography to secure communications can easily do so via email. For example, if John wants to keep his communication with Jane private, he could simply attach the public key he needs for encryption to the message. Since the key can be shared with anyone, sending it via email is not a security risk. A common example of applications that use this system is the popular encryption software known as Pretty Good Privacy® (PGP®).
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