Most large email providers, such as Gmail and Yahoo, do not respect the privacy of your inbox. Unless you are using a secure email service that respects your privacy, the answer is probably no. Thus, the session key component and signature component appear only once, at the beginning of the first segment.At the receiving end, PGP must strip off all e-mail headers and reassemble the entire original block.Are your emails and attachments safe from prying eyes? The segmentation is done after all of the other processing, including the radix-64 conversion. To accommodate this restriction, PGP automatically subdivides a message that is too large into segments that are small enough to send via e-mail. Any message longer than that must be broken up into smaller segments, each of which is mailed separately. For example, many of the facilities accessible through the Internet impose a maximum length of 50,000 octets. Each group of three octets of binary data is mapped into four ASCII characters.Į-mail facilities often are restricted to a maximum message length. The scheme used for this purpose is radix-64 conversion. To accommodate this restriction, PGP provides the service of converting the raw 8-bit binary stream to a stream of printable ASCII characters. However, many electronic mail systems only permit the use of blocks consisting of ASCII text. Thus, part or all of the resulting block consists of a stream of arbitrary 8-bit octets.
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If the confidentiality service is used, the message plus signature are encrypted. If only the signature service is used, then the message digest is encrypted. When PGP is used, at least part of the block to be transmitted is encrypted. Because the compressed message has less redundancy than the original plaintext, cryptanalysis is more difficult. Message encryption is applied after compression to strengthen cryptographic security.
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If one signed a compressed document, then it would be necessary either to store a compressed version of the message for later verification or to recompress the message when verification is required.Ģ. It is preferable to sign an uncompressed message so that one can store only the uncompressed message together with the signature for future verification. The signature is generated before compression for two reasons:ġ. This has the benefit of saving space both for e-mail transmission and for file storage. PGP compresses the message after applying the signature but before encryption. The 64-bit cipher feedback (CFB) mode is used. In both cases, the symmetric encryption algorithm CAST-128 may be used. The session key is used to decrypt the message.Īnother basic service provided by PGP is confidentiality, which is provided by encrypting messages to be transmitted or to be stored locally as files. The receiver uses RSA with its private key to decrypt and recover the session key.ĥ. The session key is encrypted with RSA, using the recipient's public key, and is prepended to the message.Ĥ. The message is encrypted, using CAST-128 (or IDEA or 3DES) with the session key.ģ. The sender generates a message and a random 128-bit number to be used as a session key for this message only.Ģ. To protect the key, it is encrypted with the receiver's public key. Because it is to be used only once, the session key is bound to the message and transmitted with it. This is referred to in the documentation as a session key it is in reality a one-time key. That is, a new key is generated as a random 128-bit number for each message. In PGP, each symmetric key is used only once. Introduction: -PGPprovidesfive services: authentication, confidentiality, compression, e-mail compatibility, and segmentation.