Chapter 5OutlinePretty Good PrivacyWhy Is PGP Popular?Operational DescriptionPowerPoint PresentationCompressionE-mail CompatibilitySegmentation and ReassemblySummary of PGP ServicesSlide 11Format of PGP MessageSlide 13Slide 14Slide 15The Use of TrustSlide 17Revoking Public KeysS/MIMERFC 822, 2822Limitations of Simple Mail Transfer Protocols (e.g., SMTP, RFC 822)Header fields in MIMES/MIME FunctionsAlgorithms Used in S/MIMENew content types in S/MIMEUser Agent RoleSlide 27Recommended Web Sites1Chapter 5Chapter 5Electronic mail securityHenric JohnsonBlekinge Institute of Technology, Swedenhttp://www.its.bth.se/staff/hjo/[email protected] by Andrew Yang2OutlineOutline•Pretty good privacy•S/MIME•Recommended web sites3Pretty Good PrivacyPretty Good Privacy•Philip R. Zimmerman is the creator of PGP.•PGP provides a confidentiality and authentication service that can be used for electronic mail and file storage applications.4Why Is PGP Popular?Why Is PGP Popular?•It is availiable free on a variety of platforms.•Based on well known algorithms.•Wide range of applicability•Not developed or controlled by governmental or standards organizations5Operational DescriptionOperational Description•Consist of five services:–Authentication–Confidentiality–Compression–E-mail compatibility–Segmentation67CompressionCompression•PGP compresses the message after applying the signature but before encryption•The placement of the compression algorithm is critical.•The compression algorithm used is ZIP (described in appendix 5A)8E-mail CompatibilityE-mail Compatibility•The scheme used is radix-64 conversion (see appendix 5B).•The use of radix-64 expands the message by 33%.9Segmentation and Segmentation and ReassemblyReassembly•Often restricted to a maximum message length of 50,000 octets.•Longer messages must be broken up into segments.•PGP automatically subdivides a message that is too large.•The receiver strip off all e-mail headers and reassemble the block.10Summary of PGP Summary of PGP Services Services Function Algorithm Used Digital Signature DSS/ SHA or RSA/ SHA Message Encryption CAST or I DEA or three-key triple DES with Diffi e-Hellman or RSA Compression ZI P E-mail Compatibility Radix-64 conversion Segmentation -1112Format of PGP MessageFormat of PGP Message13141516The Use of TrustThe Use of Trust•Key legitimacy field•Signature trust field•Owner trust fieldSee Table 5.2 See Table 5.2 (W. Stallings)(W. Stallings)1718Revoking Public Revoking Public KeysKeys•The owner issue a key revocation certificate.•Normal signature certificate with a revote indicator.•Corresponding private key is used to sign the certificate.19S/MIMES/MIME•Secure/Multipurpose Internet Mail Extension•S/MIME will probably emerge as the industry standard.•PGP for personal e-mail security20RFC 822, 2822RFC 822, 2822•RFC 822/ 2822:RFC 822: Standard for the format of ARPA Internet text messages. D. Crocker . Aug-13-1982 (obsoleted by RFC 2822)RFC2822: Internet Message Format. P. Resnick, Ed. April 2001.• In comparison:RFC 821: Simple Mail Transfer Protocol. J. Postel. Aug-01-1982. (obsoleted by RFC 2821)RFC2821: Simple Mail Transfer Protocol. J. Klensin, Ed. April 2001.21Limitations of Simple Mail Limitations of Simple Mail Transfer Protocols (e.g., SMTP, Transfer Protocols (e.g., SMTP, RFC 822)RFC 822)•SMTP/822 Limitations - Can not transmit, or has a problem with:– executable files, or other binary files (jpeg image)– “national language” characters (non-ASCII)– messages over a certain size– ASCII to EBCDIC translation problems–lines longer than a certain length (72 to 254 characters)•MIME: 5 parts (RFCs 2045 through 2049)22Header fields in MIMEHeader fields in MIME•MIME-Version: Must be “1.0” -> RFC 2045, RFC 2046• Content-Type: More types being added by developers (application/word) See Table 5.3•Content-Transfer-Encoding: How message has been encoded (radix-64) See Table 5.4•Content-ID: (optional) Unique identifying character string.•Content Description: (optional) Needed when content is not readable text (e.g.,mpeg)•Example MIME message structure: Figure 5.823S/MIME FunctionsS/MIME Functions•Enveloped Data: Encrypted content and encrypted session keys for recipients.•Signed Data: Message Digest encrypted with private key of a “signer.”•Clear-Signed Data: Signed but not encrypted.•Signed and Enveloped Data: Various orderings for encrypting and signing.24Algorithms Used in Algorithms Used in S/MIMES/MIME•Message Digesting: SHA-1 and MDS•Digital Signatures: DSS•Secret-Key Encryption: Triple-DES, RC2/40 (exportable)•Public-Private Key Encryption: RSA with key sizes of 512 and 1024 bits, and Diffie-Hellman (for session keys).25New content types in New content types in S/MIMES/MIME•S/MIME secures a MIME entity with a signature, encryption, or both.•New types were added for this purpose: SeeTable 5.7•All of the new application types use the designation PKCS (public key cryptography specifications)26User Agent RoleUser Agent Role•S/MIME uses Public-Key Certificates - X.509 version 3 signed by Certification Authority•Functions:–Key Generation - Diffie-Hellman, DSS, and RSA key-pairs.–Registration - Public keys must be registered with X.509 CA.–Certificate Storage - Local (as in browser application) for different services.–Signed and Enveloped Data - Various orderings for encrypting and signing.27User Agent RoleUser Agent Role•Example: Verisign (www.verisign.com) See Table 5.8–Class-1: Buyer’s email address confirmed by emailing vital info.–Class-2: Postal address is confirmed as well, and data checked against directories.–Class-3: Buyer must appear in person, or send notarized documents.28Recommended Web Recommended Web SitesSites•PGP home page: www.pgp.com•MIT distribution site for PGP•S/MIME Charter•S/MIME Central: RSA Inc.’s Web
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