William Stallings, Cryptography and Network Security 4/e
Chapter 15: Electronic Mail Security Fourth Edition by William Stallings Lecture slides by Lawrie Brown (Modified by Prof. M. Singhal, U of Kentucky) 1 Email Security email is one of the most widely used and
regarded network services currently message contents are not secure may be inspected either in transit or by suitably privileged users on destination system 2 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 3
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 algs to use Runs on a variety of platforms like Unix, PC, Macintosh and other systems originally free (now also have commercial versions available) 4
PGP Operation Authentication 1. sender creates message 2. Generates a digital signature for the message 3. use SHA-1 to generate 160-bit hash of message 4. signed hash with RSA using sender's private key, and is attached to message 5. receiver uses RSA 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 5 PGP Operation Confidentiality 1. sender generates a message and encrypts it. 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
5. receiver uses RSA with private key to decrypt and recover session key 6. session key is used to decrypt message 6 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 7 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 8 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)
9 PGP Operation Summary 10 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 11 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 12 PGP Message Format
13 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 14 PGP Message Generation 15 PGP Message Reception
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