1. ECE-8813 / CS 8803 http://www.csc.gatech.edu/~copeland/8813/ Prof. John A. Copeland john.copeland@ece.gatech.edu 404 894-5177 fax 404 894-0035 Office: GCATT Bldg 579 email or call for office visit, or call Kathy Cheek, 404 894-5696 Chapter 5b - Secure/Multipurpose Internet Mail Extensions S/MIME

2. MIME Headers Multipurpose Internet Mail Extensions (MIME) RFC 1341 and RFC 1521 MIME -Version:version number Content-Description: human-readable string Content-ID:unique identifier Content-Transfer-Encoding:body encoding >ASCII (Plain, quoted-printable, or Richtext) >Binary (base64) Content-Type:nature of the message >Image (gif, jpeg), Video (mpeg), >Application (Postscript, octet-stream > A.S.Tanenbaum, "Computer Networks," (3rd ed.) p.653 2

3. 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) MIME Defined Five New Headers 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) Carnonical Form: Standard format for use between systems ( not a “native” format - GIF). 3

4. Secure/MIME Can “sign” and/or encrypt messages 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. 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). 4

5. S/MIME - 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. 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 diectories. Class-3 Buyer must appear in person, or send notarized documents. 5

6. Data Compression (as in V.21bis modems) “the_thin_thinker” 6 t - 84h - 104 th - 256 e - 101_ - 32 he - 257e_ - 258_t - 259 “84, 104, 101, 32, 256, 105,110, 259, 104, 261...” thi - 260 i - 105 in - 261 n - 110 n_ - 262 _th - 263 hi- 264 ink - 265 Dictionary has 4096 entries (12-bit tokens). Entries 0 to 255 represent a single byte (permanent). Other entries are filled after a string match: = string plus first unmatched character. Message is encoded (compressed) by sending 12-bit tokens represent multiple bytes. Note that tokens 256, 259, and 261 below represent 2 bytes (16 bits) by a 12-bit token.