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2. General functions OpenPGP provides data integrity services for messages and data files by using these core technologies: - digital signatures - encryption - compression - radix-64 conversion In addition, OpenPGP provides key management and certificate services, but many of these are beyond the scope of this document. 2.1. Confidentiality via Encryption OpenPGP uses two encryption methods to provide confidentiality: symmetric-key encryption and public key encryption. With public-key encryption, the object is encrypted using a symmetric encryption algorithm. Each symmetric key is used only once. A new "session key" is generated as a random number for each message. Since it is used only once, the session key is bound to the message and transmitted with it. To protect the key, it is encrypted with the receiver's public key. The sequence is as follows: 1. The sender creates a message. 2. The sending OpenPGP generates a random number to be used as a session key for this message only. 3. The session key is encrypted using each recipient's public key. These "encrypted session keys" start the message. 4. The sending OpenPGP encrypts the message using the session key, which forms the remainder of the message. Note that the message is also usually compressed. 5. The receiving OpenPGP decrypts the session key using the recipient's private key. 6. The receiving OpenPGP decrypts the message using the session key. If the message was compressed, it will be decompressed. With symmetric-key encryption, an object may be encrypted with a symmetric key derived from a passphrase (or other shared secret), or a two-stage mechanism similar to the public-key method described above in which a session key is itself encrypted with a symmetric algorithm keyed from a shared secret. Both digital signature and confidentiality services may be applied to the same message. First, a signature is generated for the message and attached to the message. Then, the message plus signature is encrypted using a symmetric session key. Finally, the session key is encrypted using public-key encryption and prefixed to the encrypted block. 2.2. Authentication via Digital signature The digital signature uses a hash code or message digest algorithm, and a public-key signature algorithm. The sequence is as follows: 1. The sender creates a message. 2. The sending software generates a hash code of the message. 3. The sending software generates a signature from the hash code using the sender's private key. 4. The binary signature is attached to the message. 5. The receiving software keeps a copy of the message signature. 6. The receiving software generates a new hash code for the received message and verifies it using the message's signature. If the verification is successful, the message is accepted as authentic. 2.3. Compression OpenPGP implementations MAY compress the message after applying the signature but before encryption. 2.4. Conversion to Radix-64 OpenPGP's underlying native representation for encrypted messages, signature certificates, and keys is a stream of arbitrary octets. Some systems only permit the use of blocks consisting of seven-bit, printable text. For transporting OpenPGP's native raw binary octets through channels that are not safe to raw binary data, a printable encoding of these binary octets is needed. OpenPGP provides the service of converting the raw 8-bit binary octet stream to a stream of printable ASCII characters, called Radix-64 encoding or ASCII Armor. Implementations SHOULD provide Radix-64 conversions. Note that many applications, particularly messaging applications, will want more advanced features as described in the OpenPGP-MIME document, RFC 2015. An application that implements OpenPGP for messaging SHOULD implement OpenPGP-MIME. 2.5. Signature-Only Applications OpenPGP is designed for applications that use both encryption and signatures, but there are a number of problems that are solved by a signature-only implementation. Although this specification requires both encryption and signatures, it is reasonable for there to be subset implementations that are non-comformant only in that they omit encryption.
Updated: 1999-09-30 wkoch