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In cryptography, public-key cryptosystems are convenient in that they do not require the sender and receiver to share a common secret in order to communicate securely (among other useful properties). However, they often rely on complicated mathematical computations and are thus generally much more inefficient than comparable symmetric-key cryptosystems. In many applications, the high cost of encrypting long messages in a public-key cryptosystem can be prohibitive. A hybrid cryptosystem is one which combines the convenience of a public-key cryptosystem with the efficiency of a symmetric-key cryptosystem. A hybrid cryptosystem can be constructed using any two separate cryptosystems: * a key encapsulation scheme, which is a public-key cryptosystem, and * a data encapsulation scheme, which is a symmetric-key cryptosystem. The hybrid cryptosystem is itself a public-key system, whose public and private keys are the same as in the key encapsulation scheme. Note that for very long messages the bulk of the work in encryption/decryption is done by the more efficient symmetric-key scheme, while the inefficient public-key scheme is used only to encrypt/decrypt a short key value. Perhaps the most commonly used hybrid cryptosystems are the OpenPGP (RFC 4880) file format and the PKCS #7 (RFC 2315) file format, both used by many different systems. == Example == To encrypt a message addressed to Alice in a hybrid cryptosystem, Bob does the following: # Obtains Alice's public key. # Generates a fresh symmetric key for the data encapsulation scheme. # Encrypts the message under the data encapsulation scheme, using the symmetric key just generated. # Encrypt the symmetric key under the key encapsulation scheme, using Alice's public key. # Send both of these encryptions to Alice. To decrypt this hybrid ciphertext, Alice does the following: # uses her private key to decrypt the symmetric key contained in the key encapsulation segment. # uses this symmetric key to decrypt the message contained in the data encapsulation segment. 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「Hybrid cryptosystem」の詳細全文を読む スポンサード リンク
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