Chapter 6

Electronic Mail Security

MSc. NGUYEN CAO DATDr. TRAN VAN HOAI

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Email Security

Email is one of the most widely used and regarded network services

SMTP (Simple Mail Transfer Protocol)▫25/TCP▫Commands, responses: ASCII ▫Separate headers from envelope▫Binary content, structure

MIME (multipurpose internet mail extensions)

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Email Security

Mail servers & mail agents use SMTP for exchange.

Email clients use SMTP typically for relaying only, preferring POP/IMAP for receiving

Currently message contents are not secure :▫may be inspected either in transit ▫or by suitably privileged users on destination

system

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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

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Email Security ApproachesPretty Good Privacy (PGP)▫Personal e-mail security for many users▫Basic security services

Confidentiality service Authentication service

S/MIME (Secure/Multipurpose Internet Mail Extension)▫A security enhancement to the MIME▫ Industry standard for commercial and organizational

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Pretty Good Privacy (PGP)

widely used confidentiality and authentication service for securing electronic mail and other file storage applications

developed by Phil Zimmermannselected best available crypto algorithms to useintegrated into a single programavailable on Unix, PC, Macintosh systems originally free, now have commercial versions

available also

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PGP Operational description

Consist of five services:

▫Authentication

▫Confidentiality

▫Compression

▫E-mail compatibility

▫Segmentation

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PGP Operation – Authentication

1. sender creates message2. Generates a digital signature for the message3. use SHA-1 to generate 160-bit hash of

message 4. signed hash with RSA using sender's private

key, and is attached to message5. receiver uses RSA with sender's public key to

decrypt and recover hash code6. receiver verifies received message using hash

of it and compares with decrypted hash code

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PGP Operation – Authentication

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PRa = private key of user A, used in public-key encryption scheme

PUa=public key of user A, used in public-key encryption schemeEP = public-key encryptionDP = public-key decryptionH = hash function || = concatenationZ= compression using ZIP algorithm

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PGP Operation – Confidentiality

1. sender generates a message and encrypts it.2. Generates a128-bit random number as session

key3. Encrypts the message using CAST-128 / IDEA /

3DES in CBC mode with session key4. session key encrypted using RSA with

recipient's public key and attached to the msg5. receiver uses RSA with private key to decrypt

and recover session key6. session key is used to decrypt message

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PGP Operation – Confidentiality

Ks

=session key used in symmetric encryption scheme

Ks=session key used in symmetric encryption schemeEC = symmetric encryptionDC = symmetric decryption

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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/ElGamal 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

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PGP Operation – Confidentiality & Authentication

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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

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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 texthence PGP must encode raw binary data into

printable ASCII charactersuses radix-64 algorithm▫maps 3 bytes to 4 printable chars▫also appends a CRC

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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 too large

The receiver strips off all e-mail headers and reassemble the block

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PGP Operation – Summary

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PGP Services – Summary

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Function Algorithm Used Digital Signature DSS/SHA or RSA/SHA Message Encryption CAST or IDEA or three-

key triple DES with Diffie-Hellman or RSA

Compression ZIP E-mail Compatibility Radix-64 conversion

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PGP Session Keys

Need a session key for each message▫of varying sizes: 56-bit DES, 128-bit CAST or

IDEA, 168-bit Triple-DESGenerated using ANSI X12.17 modeUses random inputs taken from -actual keys hit -keystroke timing of a user

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PGP Public & Private Keyssince 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

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PGP Message Format

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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

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PGP Message Generation

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PGP Message Reception

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The main issue: PGP key management

does not rely on certificate authoritiesin PGP every user is own CA▫can sign keys for users they know directly

(certificates are like X.509)forms a “web of trust”▫trust keys have signed▫can trust keys others have signed if have a chain

of signatures to themkey ring includes trust indicatorsusers can also revoke their keys

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Summary

have considered:▫Email security▫PGP

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