Mạng máy tính 1 - Chapter 6: Electronic mail security

Chapter 6 Electronic Mail Security MSc. NGUYEN CAO DAT Dr. TRAN VAN HOAI 1 BK TP.HCM 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) BK TP.HCM 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 3 BK TP.HCM 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 4 BK TP.HCM Email Security Approaches Pretty 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 5 BK TP.HCM Pretty Good Privacy (PGP) widely used confidentiality and authentication service for securing electronic mail and other file storage applications developed by Phil Zimmermann selected best available crypto algorithms to use integrated into a single program available on Unix, PC, Macintosh systems originally free, now have commercial versions available also 6 BK TP.HCM PGP Operational description Consist of five services: ▫ Authentication ▫ Confidentiality ▫ Compression ▫ E-mail compatibility ▫ Segmentation 7 BK TP.HCM 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 8 BK TP.HCM PGP Operation – Authentication 9 PRa = private key of user A, used in public-key encryption scheme PUa=public key of user A, used in public-key encryption scheme EP = public-key encryption DP = public-key decryption H = hash function  || = concatenation Z= compression using ZIP algorithm BK TP.HCM 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 10 BK TP.HCM PGP Operation – Confidentiality Ks =session key used in symmetric encryption scheme Ks=session key used in symmetric encryption scheme EC = symmetric encryption DC = symmetric decryption 11 BK TP.HCM 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 12 BK TP.HCM PGP Operation – Confidentiality & Authentication 13 BK TP.HCM 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 14 BK TP.HCM 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 15 BK TP.HCM 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 BK TP.HCM PGP Operation – Summary 17 BK TP.HCM PGP Services – Summary 18 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 BK TP.HCM 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 Generated using ANSI X12.17 mode Uses random inputs taken from -actual keys hit -keystroke timing of a user 19 BK TP.HCM 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 20 BK TP.HCM PGP Message Format 21 BK TP.HCM 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 22 BK TP.HCM PGP Message Generation 23 BK TP.HCM PGP Message Reception 24 BK TP.HCM The main issue: PGP key management does not rely on certificate authorities in 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 them key ring includes trust indicators users can also revoke their keys BK TP.HCM Summary have considered: ▫ Email security ▫ PGP 26