Cryptography and Network Security Chapter 15

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Cryptography and Cryptography and

Network Security Network Security

Chapter 15 Chapter 15

Fourth Edition Fourth Edition by William Stallings by William Stallings

Lecture slides by Lawrie Brown

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Chapter 15 –

Chapter 15 – Electronic Mail Electronic Mail Security

Security

Despite the refusal of VADM Poindexter and LtCol North to Despite the refusal of VADM Poindexter and LtCol North to

appear, the Board's access to other sources of appear, the Board's access to other sources of

information filled much of this gap. The FBI provided information filled much of this gap. The FBI provided

documents taken from the files of the National Security documents taken from the files of the National Security

Advisor and relevant NSC staff members, including Advisor and relevant NSC staff members, including

messages from the PROF system between VADM messages from the PROF system between VADM

Poindexter and LtCol North. The PROF messages were Poindexter and LtCol North. The PROF messages were

conversations by computer, written at the time events conversations by computer, written at the time events occurred and presumed by the writers to be protected occurred and presumed by the writers to be protected

from disclosure. In this sense, they provide a first-hand, from disclosure. In this sense, they provide a first-hand,

contemporaneous account of events.

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

on the Iran-Contra Affair, 1987

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Email Security Email Security

 email is one of the most widely used and email is one of the most widely used and regarded network services

regarded network services

 currently message contents are not secure currently message contents are not secure

 may be inspected either in transit may be inspected either in transit

 or by suitably privileged users on destination or by suitably privileged users on destination system

system

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Email Security Enhancements Email Security Enhancements

 confidentiality confidentiality

 protection from disclosure protection from disclosure

 authentication authentication

 of sender of message of sender of message

 message integrity message integrity

 protection from modification protection from modification

 non-repudiation of origin non-repudiation of origin

 protection from denial by sender protection from denial by sender

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Pretty Good Privacy (PGP) Pretty Good Privacy (PGP)

 widely used de facto secure email widely used de facto secure email

 developed by Phil Zimmermann developed by Phil Zimmermann

 selected best available crypto algs to use selected best available crypto algs to use

 integrated into a single program integrated into a single program

 on Unix, PC, Macintosh and other systems on Unix, PC, Macintosh and other systems

 originally free, now also have commercial originally free, now also have commercial versions available

versions available

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PGP Operation – PGP Operation –

Authentication Authentication

1. 1. sender creates message sender creates message

2. 2. use SHA-1 to generate 160-bit hash of use SHA-1 to generate 160-bit hash of message

message

3. 3. signed hash with RSA using sender's signed hash with RSA using sender's private key, and is attached to message private key, and is attached to message

4. 4. receiver uses RSA with sender's public receiver uses RSA with sender's public key to decrypt and recover hash code key to decrypt and recover hash code

5. 5. receiver verifies received message using receiver verifies received message using hash of it and compares with decrypted hash of it and compares with decrypted

hash code

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PGP Operation – PGP Operation –

Confidentiality Confidentiality

1. 1. sender generates message and 128-bit sender generates message and 128-bit random number as session key for it

random number as session key for it

2. 2. encrypt message using CAST-128 / IDEA / encrypt message using CAST-128 / IDEA / 3DES in CBC mode with session key

3DES in CBC mode with session key

3. 3. session key encrypted using RSA with session key encrypted using RSA with recipient's public key, & attached to msg recipient's public key, & attached to msg

4. 4. receiver uses RSA with private key to receiver uses RSA with private key to decrypt and recover session key

decrypt and recover session key

5. 5. session key is used to decrypt message session key is used to decrypt message

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PGP Operation – Confidentiality PGP Operation – Confidentiality

& Authentication

 can use both services on same message can use both services on same message

 create signature & attach to message create signature & attach to message

 encrypt both message & signature encrypt both message & signature

 attach RSA/ElGamal encrypted session key attach RSA/ElGamal encrypted session key

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PGP Operation – PGP Operation –

Compression Compression

 by default PGP compresses message by default PGP compresses message after signing but before encrypting

after signing but before encrypting

 so can store uncompressed message & so can store uncompressed message &

signature for later verification signature for later verification

 & because compression is non deterministic & because compression is non deterministic

 uses ZIP compression algorithm uses ZIP compression algorithm

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PGP Operation – Email PGP Operation – Email

Compatibility Compatibility

 when using PGP will have binary data to send when using PGP will have binary data to send (encrypted message etc)

(encrypted message etc)

 however email was designed only for text however email was designed only for text

 hence PGP must encode raw binary data into hence PGP must encode raw binary data into printable ASCII characters

printable ASCII characters

 uses radix-64 algorithm uses radix-64 algorithm

 maps 3 bytes to 4 printable chars maps 3 bytes to 4 printable chars

 also appends a CRC also appends a CRC

 PGP also segments messages if too big PGP also segments messages if too big

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PGP Operation – Summary

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PGP Session Keys PGP Session Keys

 need a session key for each message need a session key for each message

 of varying sizes: 56-bit DES, 128-bit CAST or of varying sizes: 56-bit DES, 128-bit CAST or IDEA, 168-bit Triple-DES

IDEA, 168-bit Triple-DES

 generated using ANSI X12.17 mode generated using ANSI X12.17 mode

 uses random inputs taken from previous uses random inputs taken from previous

uses and from keystroke timing of user

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PGP Public & Private Keys PGP Public & Private Keys

 since many public/private keys may be in use, since many public/private keys may be in use, need to identify which is actually used to encrypt need to identify which is actually used to encrypt

session key in a message session key in a message

 could send full public-key with every message could send full public-key with every message

 but this is inefficient but this is inefficient

 rather use a key identifier based on key rather use a key identifier based on key

 is least significant 64-bits of the key is least significant 64-bits of the key

 will very likely be unique will very likely be unique

 also use key ID in signatures also use key ID in signatures

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PGP Message Format

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PGP Key Rings PGP Key Rings

 each PGP user has a pair of keyrings: each PGP user has a pair of keyrings:

 public-key ring contains all the public-keys of public-key ring contains all the public-keys of other PGP users known to this user, indexed other PGP users known to this user, indexed

by key ID by key ID

 private-key ring contains the public/private private-key ring contains the public/private key pair(s) for this user, indexed by key ID &

key pair(s) for this user, indexed by key ID &

encrypted keyed from a hashed passphrase encrypted keyed from a hashed passphrase

 security of private keys thus depends on security of private keys thus depends on the pass-phrase security

the pass-phrase security

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Key for the Next Slides Key for the Next Slides

Ks = session key used in symmetric encryption scheme Ks = session key used in symmetric encryption scheme

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

EP = public-key encryption DP = public-key decryption DP = public-key decryption EC = symmetric encryption EC = symmetric encryption DC DC = = symmetric decryption symmetric decryption H = hash function

H = hash function

|| = concatenation

Z = compression using ZIP algorithm Z = compression using ZIP algorithm

R64 = conversion to radix 64 ASCII format

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PGP Message Generation

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PGP Message Reception

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PGP Key Management PGP Key Management

 rather than relying on certificate authorities rather than relying on certificate authorities

 in PGP every user is own CA in PGP every user is own CA

 can sign keys for users they know directly can sign keys for users they know directly

 forms a “web of trust” forms a “web of trust”

 trust keys have signed trust keys have signed

 can trust keys others have signed if have a chain of can trust keys others have signed if have a chain of signatures to them

signatures to them

 key ring includes trust indicators key ring includes trust indicators

 users can also revoke their keys users can also revoke their keys

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S/MIME (Secure/Multipurpose S/MIME (Secure/Multipurpose

Internet Mail Extensions) Internet Mail Extensions)

 security enhancement to MIME email security enhancement to MIME email

 original Internet RFC822 email was text only original Internet RFC822 email was text only

 MIME provided support for varying content MIME provided support for varying content types and multi-part messages

types and multi-part messages

 with encoding of binary data to textual form with encoding of binary data to textual form

 S/MIME added security enhancements S/MIME added security enhancements

 have S/MIME support in many mail agents have S/MIME support in many mail agents

 eg MS Outlook, Mozilla, Mac Mail etc eg MS Outlook, Mozilla, Mac Mail etc

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S/MIME Functions S/MIME Functions

 enveloped data enveloped data

 encrypted content and associated keys encrypted content and associated keys

 signed data signed data

 encoded message + signed digest encoded message + signed digest

 clear-signed data clear-signed data

 cleartext message + encoded signed digest cleartext message + encoded signed digest

 signed & enveloped data signed & enveloped data

 nesting of signed & encrypted entities nesting of signed & encrypted entities

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S/MIME Cryptographic S/MIME Cryptographic

Algorithms Algorithms

 digital signatures: DSS & RSA digital signatures: DSS & RSA

 hash functions: SHA-1 & MD5 hash functions: SHA-1 & MD5

 session key encryption: ElGamal & RSA session key encryption: ElGamal & RSA

 message encryption: AES, Triple-DES, message encryption: AES, Triple-DES, RC2/40 and others

RC2/40 and others

 MAC: HMAC with SHA-1 MAC: HMAC with SHA-1

 have process to decide which algs to use have process to decide which algs to use

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S/MIME Messages S/MIME Messages

 S/MIME secures S/MIME secures a MIME entity with a a MIME entity with a signature, encryption, or both

signature, encryption, or both

 forming a MIME wrapped PKCS object forming a MIME wrapped PKCS object

 have a range of content-types: have a range of content-types:

 enveloped data enveloped data

 signed data signed data

 clear-signed data clear-signed data

 registration request registration request

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S/MIME Certificate Processing S/MIME Certificate Processing

 S/MIME uses X.509 v3 certificates S/MIME uses X.509 v3 certificates

 managed using a hybrid of a strict X.509 managed using a hybrid of a strict X.509 CA hierarchy & PGP’s web of trust

CA hierarchy & PGP’s web of trust

 each client has a list of trusted CA’s certs each client has a list of trusted CA’s certs

 and own public/private key pairs & certs and own public/private key pairs & certs

 certificates must be signed by trusted CA’s certificates must be signed by trusted CA’s

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Certificate Authorities Certificate Authorities

 have several well-known CA’s have several well-known CA’s

 Verisign one of most widely used Verisign one of most widely used

 Verisign issues several types of Digital IDs Verisign issues several types of Digital IDs

 increasing levels of checks & hence trust increasing levels of checks & hence trust

Class

Class Identity Checks Identity Checks Usage Usage

1 1 name/email check name/email check web browsing/email web browsing/email

2 2 + enroll/addr check + enroll/addr check email, subs, s/w validate email, subs, s/w validate

3 3 + ID documents + ID documents e-banking/service access e-banking/service access