Cryptography and Network Security Chapter 18

Fifth Edition by William Stallings Lecture slides by Lawrie Brown

Chapter 15 – Electronic Mail Security

Despite the refusal of VADM Poindexter and LtCol North to appear, the Board's access to other sources of information filled much of this gap. The FBI provided documents taken from the files of the National Security Advisor and relevant NSC staff members, including messages from the PROF system between VADM Poindexter and LtCol North. The PROF messages were conversations by computer, written at the time events occurred and presumed by the writers to be protected from disclosure. In this sense, they provide a first-hand, contemporaneous account of events.

—The Tower Commission Report to President Reagan on the Iran-Contra Affair, 1987

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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 3

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

Pretty Good Privacy (PGP)

 widely used de facto secure email program  developed by Phil Zimmermann  selected best available crypto algs to use  integrated into a single program  on Unix, PC, Macintosh and other systems  originally free, now also have commercial versions available 5

PGP Operation – Authentication

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sender creates message make SHA-1160-bit hash of message attached RSA signed hash to message receiver decrypts & recovers hash code receiver verifies received message hash 6

PGP Operation – Confidentiality

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sender forms 128-bit random session key encrypts message with session key attaches session key encrypted with RSA receiver decrypts & recovers session key session key is used to decrypt message 7

PGP Operation – Confidentiality & Authentication

 can use both services on same message  create signature & attach to message  encrypt both message & signature  attach RSA/ElGamal encrypted session key 8

PGP Operation – Compression

 by default PGP compresses message after signing but before encrypting  so can store uncompressed message & signature for later verification  & because compression is non deterministic  uses ZIP compression algorithm 9

PGP Operation – Email Compatibility

     when using PGP will have binary data to send (encrypted message etc) however email was designed only for ASCII 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(A cyclic redundancy check is an error-detecting code commonly used in digital networks and storage devices to detect accidental changes to raw data) PGP also segments messages if too big 10

PGP Operation – Summary

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PGP Session Keys

 PGP makes use of four types of keys one time session symmetric keys, public keys, private keys, and passphrase-based symmetric keys.

 need a session key for each message  of varying sizes: 56-bit DES, 128-bit CAST or IDEA, 168-bit Triple-DES  Random numbers generated using ANSI X12.17 mode  uses random inputs taken from previous uses and from keystroke timing of user 12

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 13

PGP Message Format

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 A message consists of three components:  the message component,  a signature (optional),  and a session key component (optional).  The message component includes the actual data to be stored or transmitted, as well as a filename and a timestamp that specifies the time of creation.

 The signature component includes a  timestamp,  encrypted SHA-1 message digest,   leading two digest octets for verification, and the Key ID of the sender’s public key.  The session key component includes the session key and the identifier of the recipient's public key that was used by the sender to encrypt the session key. The entire block is usually encoded with radix 64 encoding.

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 17

PGP Key Rings

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PGP Message Generation

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 The sending PGP entity performs the following steps:  1. Signing the message:  a. PGP retrieves the sender's private key from the private-key ring using your_userid as an index. If your_userid was not provided in the command, the first private key on the ring is retrieved.  b. PGP prompts the user for the passphrase to recover the unencrypted private key.  c. The signature component of the message is constructed.

 2. Encrypting the message:  a. PGP generates a session key and encrypts the message.  b. PGP retrieves the recipient's public key from the public-key ring using her_userid as an index.

 c. The session key component of the message is constructed.

PGP Message Reception

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 PGP crypto services (again ignoring compression and radix-64 conversion for simplicity). The receiving PGP entity performs the following steps:  1. Decrypting the message:  a. PGP retrieves the receiver's private key from the private-key ring, using the Key ID field in the session key component of the message as an index.

 b. PGP prompts the user for the passphrase to recover the unencrypted private key. 

 c. PGP then recovers the session key and decrypts the message.  2. Authenticating the message:  a. PGP retrieves the sender's public key from the public-key ring, using the Key ID field in the signature key component of the message as an index.  b. PGP recovers the transmitted message digest.  c. PGP computes the message digest for the received message and compares it to the transmitted message digest to authenticate.

PGP Key Management

 rather than relying on certificate authorities  in PGP every user is own CA  can sign keys for users they know directly  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 25

PGP Trust Model Example

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S/MIME (Secure/Multipurpose Internet Mail Extensions)

 security enhancement to MIME email  original Internet RFC822 email was text only  MIME provided support for varying content types and multi-part messages  with encoding of binary data to textual form  S/MIME added security enhancements  have S/MIME support in many mail agents  eg MS Outlook, Mozilla, Mac Mail etc 27

S/MIME Functions

 enveloped data  encrypted content and associated keys  signed data  encoded message + signed digest  clear-signed data  cleartext message + encoded signed digest  signed & enveloped data  nesting of signed & encrypted entities 28

S/MIME Cryptographic Algorithms

 digital signatures: DSS & RSA  hash functions: SHA-1 & MD5  session key encryption: ElGamal & RSA  message encryption: AES, Triple-DES, RC2/40 and others  MAC: HMAC with SHA-1  have process to decide which algs to use 29

S/MIME Messages

 S/MIME secures a MIME entity with a signature, encryption, or both  forming a MIME wrapped PKCS(public key cryptography specifications) object  have a range of content-types:      enveloped data signed data clear-signed data registration request certificate only message 30

S/MIME Certificate Processing

 S/MIME uses X.509 v3 certificates  managed using a hybrid of a strict X.509 CA hierarchy & PGP’s web of trust  each client has a list of trusted CA’s certs  and own public/private key pairs & certs  certificates must be signed by trusted CA’s 31

Certificate Authorities

 have several well known CA’s  Verisign one of most widely used  Verisign issues several types of Digital IDs  increasing levels of checks & hence trust

ClassIdentity ChecksUsage

1 name/email checkweb browsing/email 2 + enroll/addr checkemail, subs, s/w validate 3 + ID documentse-banking/service access 32

S/MIME Enhanced Security Services

 3 proposed enhanced security services:  signed receipts  security labels  secure mailing lists 33

Domain Keys Identified Mail

 a specification for cryptographically signing email messages  so signing domain claims responsibility  recipients / agents can verify signature  proposed Internet Standard RFC 4871  has been widely adopted 34

Internet Mail Architecture

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Email Threats

 see RFC 4684-

Analysis of Threats Motivating DomainKeys Identified Mail

 describes the problem space in terms of:  range: low end, spammers, fraudsters  capabilities in terms of where submitted, signed, volume, routing naming etc  outside located attackers 36

DKIM Strategy

 transparent to user  MSA sign  MDA verify  for pragmatic reasons 37

DCIM Functional Flow

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Summary

 have considered:  secure email  PGP  S/MIME  domain-keys identified email 39