The two main uses for public-key cryptography are:
- Public-key encryption: a message encrypted with a recipient's public key cannot be decrypted by anyone except a possessor of the matching private key - it is presumed that this will be the owner of that key and the person associated with the public key used. This is used to attempt to ensure confidentiality.
- Digital signatures: a message signed with a sender's private key can be verified by anyone who has access to the sender's public key, thereby proving that the sender had access to the private key and, therefore, is likely to be the person associated with the public key used. This also ensures that the message has not been tampered with (on the question of authenticity, see also message digest).
An analogy to public-key encryption is that of a locked mail box with a mail slot. The mail slot is exposed and accessible to the public - its location (the street address) is, in essence, the public key. Anyone knowing the street address can go to the door and drop a written message through the slot. However, only the person who possesses the key can open the mailbox and read the message.
An analogy for digital signatures is the sealing of an envelope with a personal wax seal. The message can be opened by anyone, but the presence of the unique seal authenticates the sender.
A central problem with the use of public-key cryptography is confidence (ideally, proof) that a particular public key is correct, and belongs to the person or entity claimed (i.e. is "authentic"), and has not been tampered with, or replaced by, a malicious third party (a "man-in-the-middle"). The usual approach to this problem is to use a public-key infrastructure (PKI), in which one or more third parties - known as certificate authorities - certify ownership of key pairs. PGP, in addition to being a certificate authority structure, has used a scheme generally called the "web of trust", which decentralizes such authentication of public keys by a central mechanism, and substitutes individual endorsements of the link between user and public key. To date, no fully satisfactory solution to this "public key authentication problem" has been found.
Read more about this topic: Public-key Cryptography
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