
How is Asymmetric Cryptography Used? Each entity that needs to encrypt messages is assigned a key pair – one public key and one private key. As noted above, when using asymmetric cryptography, a message can be encrypted with either key, and once encrypted; it can only be decrypted by the OTHER key, not the original key that was used to encrypt it. Which of the two keys you choose to encrypt a message with, is determined by what you’re trying to achieve with the encryption. For example, encrypting the message with the private key known only to you means that it can be decrypted by anyone with the public key (translation: anyone). This doesn’t do much to maintain message confidentiality, but it does provide strong proof that the message is from you, since the message couldn’t be decrypted with your public key if it wasn’t from you. On the other hand, if you have someone’s public key, you can encrypt a message to him or her with his or her public key, so that only that person can decrypt it (with their private key). This means that once the message has been encrypted, it will remain confidential except to its intended recipient. However, since anyone with the public key can encrypt the message, this use of asymmetric cryptography doesn’t provide proof of the sender’s identity. More will be said about this later in the chapter, when we discuss applications of public key cryptography. In addition to its use in public/private key systems, asymmetric cryptography is also used for the exchange of secret symmetric keys used for encrypting files, so that the symmetric key information can be communicated securely between authorized parties. This gets around one of the problems with symmetric key systems – how to securely exchange key information. One asymmetric algorithm used for the secure exchange of a secret key is the DiffieHellman Key Agreement protocol. Why not send the entire file encrypted with asymmetric cryptography? One answer is that symmetric algorithms take less (often much less) CPU time/power to encrypt and decrypt. Because of the use of different keys to encrypt and decrypt, asymmetric cryptography has interesting uses in applications such as digital signatures, which will be discussed later.
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