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Chapter 5

Chapter 5. Electronic mail security . Outline. Pretty good privacy S/MIME Recommended web sites. Pretty 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.

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Chapter 5

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  1. Chapter 5 Electronic mail security

  2. Outline • Pretty good privacy • S/MIME • Recommended web sites

  3. Pretty 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.

  4. 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 organizations

  5. Operational Description • Consist of five services: • Authentication • Confidentiality • Compression • E-mail compatibility • Segmentation

  6. PGP Operation – Authentication • sender creates message • use SHA-1 to generate 160-bit hash of message • signed hash with RSA using sender's private key, and is attached to message • receiver uses RSA with sender's public key to decrypt and recover hash code • receiver verifies received message using hash of it and compares with decrypted hash code

  7. PGP Operation – Confidentiality • sender generates message and 128-bit random number as session key for it • encrypt message using CAST-128 / IDEA / 3DES in CBC mode with session key • session key encrypted using RSA with recipient's public key, & attached to msg • receiver uses RSA with private key to decrypt and recover session key • session key is used to decrypt message

  8. PGP Operation – Confidentiality & Authentication • can use both services on same message • create signature & attach to message • encrypt both message & signature • attach RSA/ElGamal encrypted session key

  9. PGP Operation – Compression • by default PGP compresses message after signing but before encrypting • so can store uncompressed message & signature for later verification • & because compression is non deterministic • uses ZIP compression algorithm

  10. PGP Operation – Email Compatibility • when using PGP will have binary data to send (encrypted message etc) • however email was designed only for text • hence PGP must encode raw binary data into printable ASCII characters • uses radix-64 algorithm • maps 3 bytes to 4 printable chars • also appends a CRC • PGP also segments messages if too big

  11. PGP Operation – Summary

  12. Compression • 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)

  13. E-mail Compatibility • The scheme used is radix-64 conversion (see appendix 5B). • The use of radix-64 expands the message by 33%.

  14. Segmentation and Reassembly • 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 to large. • The receiver strip of all e-mail headers and reassemble the block.

  15. Sumary of PGP Services

  16. PGP Session Keys • need a session key for each message • of varying sizes: 56-bit DES, 128-bit CAST or IDEA, 168-bit Triple-DES • generated using ANSI X12.17 mode • uses random inputs taken from previous uses and from keystroke timing of user

  17. PGP Public & Private Keys • since many public/private keys may be in use, need to identify which is actually used to encrypt session key in a message • could send full public-key with every message • but this is inefficient • rather use a key identifier based on key • is least significant 64-bits of the key • will very likely be unique • also use key ID in signatures

  18. Format of PGP Message

  19. PGP Key Rings • each PGP user has a pair of keyrings: • public-key ring contains all the public-keys of other PGP users known to this user, indexed by key ID • private-key ring contains the public/private key pair(s) for this user, indexed by key ID & encrypted keyed from a hashed passphrase • security of private keys thus depends on the pass-phrase security

  20. PGP Key Management • rather than relying on certificate authorities • in PGP every user is own CA • can sign keys for users they know directly • forms a “web of trust” • trust keys have signed • can trust keys others have signed if have a chain of signatures to them • key ring includes trust indicators • users can also revoke their keys

  21. The Use of Trust • Key legitimacy field • Signature trust field • Owner trust field See Table 5.2 (W. Stallings)

  22. Revoking Public Keys • The owner issue a key revocation certificate. • Normal signature certificate with a revote indicator. • Corresponding private key is used to sign the certificate.

  23. S/MIME • Secure/Multipurpose Internet Mail Extension • S/MIME will probably emerge as the industry standard. • PGP for personal e-mail security

  24. Simple Mail Transfer Protocol (SMTP, RFC 822) • SMTP 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)

  25. Header fields in MIME • MIME-Version: Must be “1.0” -> RFC 2045, RFC 2046 • Content-Type: More types being added by developers (application/word) • Content-Transfer-Encoding: How message has been encoded (radix-64) • Content-ID: Unique identifying character string. • Content Description: Needed when content is not readable text (e.g.,mpeg)

  26. S/MIME (Secure/Multipurpose Internet Mail Extensions) • security enhancement to MIME email • original Internet RFC822 email was text only • MIME provided support for varying content types and multi-part messages • with encoding of binary data to textual form • S/MIME added security enhancements • have S/MIME support in various modern mail agents: MS Outlook, Netscape etc

  27. S/MIME Functions • Enveloped Data: Encrypted content and encrypted session keys for recipients. • Signed Data: Message Digest encrypted with private key of “signer.” • Clear-Signed Data: Signed but not encrypted. • Signed and Enveloped Data: Various orderings for encrypting and signing.

  28. Algorithms Used • 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).

  29. User 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.

  30. User Agent Role • Example: Verisign (www.verisign.com) • 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.

  31. Certificate Authorities • Verisign issues several types of Digital IDs • increasing levels of checks & hence trust Class Identity Checks Usage 1 name/email check web browsing/email 2 + enroll/addr check email, subs, s/w validate 3 + ID documents e-banking/service access

  32. Recommended Web Sites • PGP home page: www.pgp.com • MIT distribution site for PGP • S/MIME Charter • S/MIME Central: RSA Inc.’s Web Site

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