E-MAIL SECURITY

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E-MAIL SECURITY Chapter 15

• .for authentication and confidentiality
• PGP
• Uses best algorithms as building blocks
• General purpose
• Package/source code free
• Low-cost commercial version
• No government

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PGP CRYPTOGRAPHIC FUNCTIONS
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PGP for.
Authentication Confidentiality
Compression e-mail Segmentation
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DIGITAL SIGNATURES (fig 15.1a)
SHA-1 with RSA ? Signature (RSA,
KUa) ? KRa (H, KRa)
? Signed (alternative
DSS/SHA-1)
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DETACHED SIGNATURES
instead of.. Attached
Signatures use.. Detached
Signatures - Separate Transmission
- separate log detect
virus many signatures one doc
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CONFIDENTIALITY (fig 15.1b)
CAST or IDEA or 3DES CFB 64 Key
Distribution
RSA/Diffie-Hellman/El Gamal Symmetric Key used
once/message Random ? 128-bit key, Ks
key sent with message

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SYMMETRIC/PUBLIC COMBINATION

• Faster than just PUBLIC
• PUBLIC solves key distribution
• No protocol one-time message
• No handshaking
• One-time keys strengthen security
• (weakest link is public)

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CONFIDENTIALITY and AUTHENTICATION (fig 15.c)
Authentication - plaintext mess. stored
third-party can verify signature without
needing to know secret key Compression
Confidentiality
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COMPRESSION - why?

• Benefit - efficiency
• Why,
• Signature then Compression then
  Confidentiality ?
• Sign Uncompressed Message
• - off-line storage
• No need for single compression algorithm
• Encryption after compression is stronger
  

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E-Mail COMPATIBILITY
e-mail uses ASCII PGP(8-bit) ?
ASCII Base-64 3x8 ? 4 x ASCII CRC 33
Expansion !! (fig 15.2)
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RADIX-64 FORMAT
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Tx and Rx of PGP Messages
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SEGMENTATION / REASSEMBLY
Max length restriction e.g. internet 50,000
x 8-bits PGP Segments automatically but, One
session key,signature/message
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PGP KEYS

• one-time session
• use random number gen.
• 2. public
• 3. private
• 4. passphrase-based

key id file of key pairs for all users
multiple pairs
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SESSION-KEY GENERATION
CAST / IDEA / 3DES in CFB mode
plaintext - user key strokes
64
64
K
K user key strokes and old session key
128
64
64

New Session Key
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KEY IDENTIFIERS
Which public key? each public key has
key ID (least
64 bits) With high prob., no key ID collision
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MESSAGE FORMAT (fig 15.3)
Message,m data, filename, timestamp signature
(optional) includes digest
hash(m(data)T) therefore signature is T,
EKRa(digest),2x8(digest), KeyID session key
(optional) key, IDKUb
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MESSAGE FORMAT
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KEY RINGS (fig 15.4)
Private Key Ring store public/private pairs
of node A Public Key Ring store public keys
of all other nodes
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KEY RINGS
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ENCRYPTED PRIVATE KEYS on PRIVATE KEY-RING

• User passphrase
• System asks user for passphrase
• Passphrase ? 160-bit hash
• Ehash(private key)
• subsequent access requires passphrase

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PGP MESSAGE GENERATION
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PGP MESSAGE RECEPTION
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PUBLIC KEY MANAGEMENT
Problem need tamper-resistant public-keys
(e.g. in case A thinks KUc is KUb) Two
threats C ? A (forge Bs signature)
A ? B (decrypt by C) solution
Key-Revoking
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PGP TRUST MODEL EXAMPLE
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ZIP
freeware (c) UNIX, PKZIP Windows LZ77
(Ziv,Lempel) Repetitions ? short code (on
the fly) codes re-used algorithm
MUST be reversible
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ZIP (example)
(Fig 15.9) char ? 9 bits 1 bit 8-bit
ascii look for repeated sequences continue
until repetition ends e.g. the brown fox
? 8-bit pointer, 4-bit length, 00 ?
12-bit pointer, 6-bit length, 01 then jump ?
ptr length, ind compressed to 35x9-bit
two codes 343 bits Compression Ratio
424/343 1.24
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ZIP (example)
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COMPRESSION ALGORITHM

• Sliding History Buffer last N chars
• Look-Ahead Buffer next N chars
• Algorithm tries to match chars from 2. to 1.
• if no match,
• 9 bits LAB ? 9 bits SHB
• else if match found output
• indicator for length K string, ptr, length
• K bits LAB ? K bits SHB

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COMPRESSION ALGORITHM

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PGP RANDOM NUMBER GENERATION
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S/MIME

(Secure/Multipurpose Mail Extension) S/MIME -
commercial PGP - private S/MIME - based
on MIME (designed for
RFC822) RFC822 - traditional text-mail
internet standard Envelope
Contents
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CRYPTO ALGORITHMS USED in S/MIME

(Table 15.6) Sender/Recipients must agree on
common encryption algorithm S/MIME secures MIME
entity with signature and/or encryption MIME
entity entire message subpart of
message
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SECURING a MIME ENTITY

security data
MIME ENTITY
WRAPPED in MIME
MIME PREPARE
PKCS OBJECT
S/MIME
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S/MIME CERTIFICATE PROCESSING
Hybrid of X.509 certification authority
and PGPs web of trust Configure each client
? Trusted Keys Certification
Revocation List