Lecture 9: Email Security via PGP
CS 436/636/736
Spring 2012
Nitesh Saxena
Course Admin
• HW3 Posted
– Due at 11am Apr 12 (next Thursday)
Lecture 9 - Email Security via PGP
Email Security
• Email is one of the most widely used and
regarded network services
• By default, email communication is NOT
“secure”
– may be inspected either in transit, or by suitably
privileged users on destination system
– may be impersonated/spoofed
Lecture 9 - Email Security via PGP
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
Lecture 9 - Email Security via PGP
Pretty Good Privacy (PGP)
• Open source, freely available software
package for secure e-mail
• De facto standard for secure email
• Developed by Phil Zimmermann
• Selected best available crypto algorithms to
use
• Runs on a variety of platforms like Unix, PC,
Macintosh and other systems
• Originally free (now also have commercial
versions available)
Lecture 9 - Email Security via PGP
PGP Operation – Authentication
Just use digital signatures:
1.
2.
3.
4.
Sender creates message
Generates a digital signature for the message
Use SHA-1 to generate 160-bit hash of message
Signed hash with RSA (or DSA) using sender's private key,
and is attached to message
5. Receiver uses RSA (or DSA) with sender's public key to
decrypt and recover hash code
6. Receiver verifies received message using hash of it and
compares with decrypted hash code
Lecture 9 - Email Security via PGP
PGP Operation – Confidentiality
1. Sender generates a message
2. Generates a128-bit random number as session key
3. Encrypts the message using CAST-128 / IDEA / 3DES
in CBC mode with session key
4. Session key encrypted using RSA with recipient's
public key and attached to the msg
1. Can also use El Gamal
5. Receiver uses RSA with private key to decrypt and
recover session key
6. Session key is used to decrypt message
Lecture 9 - Email Security via PGP
PGP Operation – Confidentiality &
Authentication
• Can use both services on the same message
– create signature & attach it to the message
– encrypt both message & signature
– attach RSA (or El Gamal) encrypted session key
This sequence is preferred because
--one can store the plaintext message/file and its signature
--no need to decrypt the message/file again and again
Lecture 9 - Email Security via PGP
PGP Operation – Compression
• PGP compresses messages to save space for e-mail
transmission and storage
• By default, PGP compresses message after signing
but before encrypting
– so can store uncompressed message & signature for later
verification
– Encryption after compression strengthens security
(because compression has less redundancy)
• uses ZIP compression algorithm
Lecture 9 - Email Security via PGP
PGP Operation – Email Compatibility
• When using PGP will have binary data (8-bit octets)
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
(maximum length 50,000 octets)
Lecture 9 - Email Security via PGP
PGP Operation – Summary
Lecture 9 - Email Security via PGP
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
• Uses random inputs taken from
– actual keys hit
– keystroke timing of a user
– mouse movement
Lecture 9 - Email Security via PGP
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
Lecture 9 - Email Security via PGP
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
Lecture 9 - Email Security via PGP
PGP Message Format
Lecture 9 - Email Security via PGP
PGP Message Generation
Lecture 9 - Email Security via PGP
PGP Message Reception
Lecture 9 - Email Security via PGP
PGP Key Distribution
• Public keys for encrypting session keys /
verifying signatures.
• Private keys for decrypting session keys /
creating signatures.
• Where do these keys come from and on what
basis can they be trusted?
Lecture 9 - Email Security via PGP
PGP Key Distribution
• PGP adopts a trust model called the web of trust.
• No centralized authority
• Individuals sign one another’s public keys, these
“certificates” are stored along with keys in key rings.
• PGP computes a trust level for each public key in key
ring.
• Users interpret trust level for themselves.
Lecture 9 - Email Security via PGP
PGP Key Distribution Issues
• Original intention was that all e-mail users would
contribute to web of trust.
• Reality is that this web is sparsely populated.
• How should security-unaware users assign and
interpret trust levels?
• Later versions of PGP support X.509 certs.
Lecture 9 - Email Security via PGP
PGP in Practice
• PGP plugins available
– Standalone Email clients
– Browser (e.g., FireGPG)
Lecture 9 - Email Security via PGP
Summary
• have considered:
– secure email
– PGP
Lecture 9 - Email Security via PGP
References
• Chapter 18 of Stallings
Lecture 9 - Email Security via PGP