Electronic Mail Security - Network Security and Cryptography - Lecture Slides, Slides of Cryptography and System Security

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Cryptography and

Network Security

Chapter 15

Chapter 15 – Electronic Mail

Security

Despite the refusal of VADM Poindexter and LtCol North to appear, the Board's access to other sources of information filled much of this gap. The FBI provided documents taken from the files of the National Security Advisor and relevant NSC staff members, including messages from the PROF system between VADM Poindexter and LtCol North. The PROF messages were conversations by computer, written at the time events occurred and presumed by the writers to be protected from disclosure. In this sense, they provide a first-hand, contemporaneous account of events. —The Tower Commission Report to President Reagan on the Iran-Contra Affair, 1987

Email Security Enhancements

 confidentiality

 protection from disclosure

 authentication

 of sender of message

 message integrity

 protection from modification

 non-repudiation of origin

 protection from denial by sender

Pretty Good Privacy (PGP)

 widely used de facto secure email

 developed by Phil Zimmermann

 selected best available crypto algs to use

 integrated into a single program

 on Unix, PC, Macintosh and other systems

 originally free, now also have commercial versions available

PGP Operation – Confidentiality

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

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

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

PGP Operation – Summary

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

PGP Message Format

PGP Message Generation

PGP Message Reception

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 many mail agents

 eg MS Outlook, Mozilla, Mac Mail etc

S/MIME Functions

 enveloped data

 encrypted content and associated keys

 signed data

 encoded message + signed digest

 clear-signed data

 cleartext message + encoded signed digest

 signed & enveloped data

 nesting of signed & encrypted entities