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Public-key cryptography refers to a set of cryptographic algorithms that are based on mathematical problems that currently admit no efficient solution -- particularly those inherent in certain integer factorization, discrete logarithm, and elliptic curve relationships. It is computationally easy for a user to generate a public and private key-pair and to use it for encryption and decryption. The strength lies in the "impossibility" (computational impracticality) for a properly generated private key to be determined from its corresponding public key. Thus the public key may be published without compromising security. Security depends only on keeping the private key private. Public key algorithms, unlike symmetric key algorithms, do ''not'' require a secure channel for the initial exchange of one (or more) secret keys between the parties. Because of the computational complexity of asymmetric encryption, it is typically only used for short messages, typically the transfer of a symmetric encryption key. This symmetric key is then used to encrypt the rest of the potentially long & heavy conversation. The symmetric encryption/decryption is based on simpler algorithms and is much faster. Message authentication involves hashing the message to produce a "digest," and encrypting the digest with the private key to produce a digital signature. Thereafter anyone can verify this signature by (1) computing the hash of the message, (2) decrypting the signature with the signer's public key, and (3) comparing the computed digest with the decrypted digest. Equality between the digests confirms the message is unmodified since it was signed, and that the signer, and no one else, intentionally performed the signature operation — presuming the signer's private key has remained secret. The security of such procedure depends on a hash algorithm of such quality that it is computationally impossible to alter or find a substitute message that produces the same digest - but studies have shown that even with the MD5 and SHA-1 algorithms, producing an altered or substitute message is not impossible. The current hashing standard for encryption is SHA-2. The message itself can also be used in place of the digest. Public-key algorithms are fundamental security ingredients in cryptosystems, applications and protocols. They underpin various Internet standards, such as Transport Layer Security (TLS), S/MIME, PGP, and GPG. Some public key algorithms provide key distribution and secrecy (e.g., Diffie–Hellman key exchange), some provide digital signatures (e.g., Digital Signature Algorithm), and some provide both (e.g., RSA). Public-key cryptography finds application in, amongst others, the IT security discipline information security. Information security (IS) is concerned with all aspects of protecting electronic information assets against security threats.〔(【引用サイトリンク】publisher=SANS Institute )〕 Public-key cryptography is used as a method of assuring the confidentiality, authenticity and non-repudiability of electronic communications and data storage. ==Understanding== Public-key cryptography is often used to secure electronic communication over an open networked environment such as the Internet, without relying on a covert channel even for key exchange. Open networked environments are susceptible to a variety of communication security problems such as man-in-the-middle attacks and other security threats. Security properties required for communication typically include that the communication being sent must not be readable during transit (preserving confidentiality), the communication must not be modified during transit (preserving the integrity of the communication), the communication must originate from an identified party (sender authenticity) and to ensure non-repudiation or non-denial of the sending of the communication. Combining public-key cryptography with an Enveloped Public Key Encryption (EPKE)〔(【引用サイトリンク】work=RSA Laboratories )〕 method, allows for the secure sending of a communication over an open networked environment. The distinguishing technique used in public-key cryptography is the use of asymmetric key algorithms, where a key used by one party to perform either encryption or decryption is not the same as the key used by another in the counterpart operation. Each user has a pair of cryptographic keys – a public encryption key and a private decryption key. For example, a key pair used for digital signatures consists of a private signing key and a public verification key. The public key may be widely distributed, while the private key is known only to its proprietor. The keys are related mathematically, but the parameters are chosen so that calculating the private key from the public key is unfeasible. In contrast, symmetric-key algorithms – variations of which have been used for thousands of years – use a ''single'' secret key, which must be shared and kept private by both the sender and the receiver, for example in both encryption and decryption. To use a symmetric encryption scheme, the sender and receiver must securely share a key in advance. Because symmetric key algorithms are nearly always much less computationally intensive than asymmetric ones, it is common to exchange a key using a key-exchange algorithm, then transmit data using that key and a symmetric key algorithm. PGP and the SSL/TLS family of schemes use this procedure, and are thus called ''hybrid cryptosystems''. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Public-key cryptography」の詳細全文を読む スポンサード リンク
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