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CRYPTOGRAPHY AND INFORMATION SECURITY
 Lecturer: Dr. Nguyen Nam Hong  Tel.: 048781437.  Mob.: 0912312816.  Email:  [email protected]  Website:  www.freewebs.com/namhongthanhloc

Chapter 16. Electronic Mail Security

Chapter 16. Electronic Mail Security (1/3)
16.01. Email Security 16.02. Email Security Enhancement 16.03. The Secure Email 16.04. Private Enhanced Mail (PEM) 16.05. PEM Implementation 16.06. PEM Security Services 16.07. PEM Format and Implementation 16.08. Pretty Good Privacy (PGP) 16.09. PGP Origins
Dr. Nguyen Nam Hong, Le Quy Don Technical University Slide 2 / 32

Chapter 16. Electronic Mail Security (2/3)
16.10. PGP Versions 16.11. Some PGP Versions in Windows 16.12. PGP Authentication 16.13. PGP Confidentials 16.14. PGP Uses 16.15. PGP Compression 16.16. PGP Email Compatibility 16.17. PGP Diagrams 16.18. PGP Session Key
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Chapter 16. Electronic Mail Security (3/3)
16.19. PGP Public and Private Key 16.20. PGP Key Rings 16.21. PGP Key Management 16.22. S/MIME 16.23. S/MIME Functions 16.24. S/MIME Cryptographic Algorithms 16.25. S/MIME Certificate Processing 16.16. Certificate Authorities 16.27. Summary
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16.01. 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

Dr. Nguyen Nam Hong, Le Quy Don Technical University

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16.02. 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
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16.03. The secure e-mail
• On open systems as in the case of the Internet, the secure email is achieved through the plataform S/MIME which means Secure Multipurpose Internet Mail Extensions. • In the beginning of the 90´s two systems or applications of secure email appear:  PEM: Private Enhanced Mail  PGP: Pretty Good Privacy • From both, PGP has been the one that became an standard for secure e-mail clients on closed environments. • Therefore we'll see only some of the generic aspects of PEM and we will analyze PGP in deeply.
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16.04. Private Enhanced Mail (PEM)
 It is a proposal of the IETF Internet Engineering


    

Task Force in 1985. The technic document is published in 1993. The technic specifications are in RFCs Request For Comments numbers 1421, 1422, 1423 and 1424. It used to link to protocol SMTP Simple Mail Internet Protocol. Encryption of the information: DES in CBC mode. Generation and key management: RSA from 508 to 1024 bits. Structure of certificates as per the X.509 rule. Session key: DES in CBC mode, TripleDES-EDE. Digital signature: RSA, MD2, MD5.
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Dr. Nguyen Nam Hong, Le Quy Don Technical University

16.05. PEM Implementation
• It is compatible with other engineering models like, for example, X.400. • PEM is implemented at the application level: • It is independent from the protocols of the OSI levels or from lower TCP/IP levels. • It is independent from the operative systems or from the computer. • It can be implemented as an independent module that works with the common email client for the user.
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16.06. PEM Security Services
 Security services contemplated:
 Source authentication.  Confidentiality.  Integrity of the message.  Non repudiation of the source when key

management with asymmetric key algorithm is used.  Security services not contemplated:  Access control.  Confidentiality in the traffic of the messages.  Non repudiation of the message by the receiver.
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16.07. PEM Format and Implementation
Header of the E-mail Service (Headers of RFC822)

Encapsuled header
Fields related to authentication, integrity and confidentiality

Blank line Encapsuled text
User's message with some optional fields

TIS/PEM UNIX Plataforms UNIX. Trusted Information System. Source code available for citizens or companies from USA or Canada. It uses a hierarchy of multilple certification. RIPEM It implements part of the PEM protocols without certificates for keys authentication. Free for non commercial applications. Export forbidden out of the United States. There are versions utilized all over the world.
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Dr. Nguyen Nam Hong, Le Quy Don Technical University

16.08. Pretty Good Privacy (PGP)
 widely used de facto secure email
 developed by Phil Zimmermann  selected best available crypto algs to use

 integrated into a single program
 available on Unix, PC, Macintosh and Amiga

systems  originally free, now have commercial versions available also
Dr. Nguyen Nam Hong, Le Quy Don Technical University Slide 12 / 32

16.09. PGP Origins
 Philip Zimmermann publishes the version 1.0 of


  

PGP in 1991 with minimum requirements of hardware and software. In 1992 appears version 2.0 on which programmers from all over the world participate. Its code it's written out of USA to avoid the restrictive laws regarding cryptographyc software and its legal problems. In 1993 version 2.3a appears that is very popular on FTP sites and valid for several platforms of operative systems. In 1994 the Massachusetts Institute of Technology MIT participates on the project and versions 2.4, 2.5 and 2.6 appear. Version 2.6.3i is popularized globally.
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Dr. Nguyen Nam Hong, Le Quy Don Technical University

16.10. PGP versions
• Though there is more than a software offer for secure mail besides the PGP program, this became an standard in fact. • Although the last versions of the program oriented to Windows environments present high capability, the basic operations continue being the same as in knowing version 2.6.3i. • The new PGP versions on the Windows environment change very fast therefore it is very difficult to have up-to-date notes permanently.
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16.11. Some PGP versions in Windows
From version 5.0 to the current ones (versions 8.0 and next) the local encryption schemes, asymmetric cipher and digital signature have changed very little though they present bigger capabilities. Nevertheless, remember that some capabilities will only be activated on commercial versions.

PGP 7.0.3 PGP 6.5.1 PGP 8.0 We will see some operations from these three versions on detail. Remember, besides, that version 7.0.3 has not published its code.

Dr. Nguyen Nam Hong, Le Quy Don Technical University

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16.12. PGP Authentication
1. sender creates a message 2. SHA-1 used to generate 160-bit hash code of

message 3. hash code is encrypted with RSA using the sender's private key, and result is attached to message 4. receiver uses RSA or DSS with sender's public key to decrypt and recover hash code 5. receiver generates new hash code for message and compares with decrypted hash code, if match, message is accepted as authentic
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16.13. PGP Confidentiality
1. sender generates message and random 128-bit

2. 3. 4. 5.

number to be used as session key for this message only message is encrypted, using CAST-128 / IDEA/3DES with session key session key is encrypted using RSA with recipient's public key, then attached to message receiver uses RSA with its private key to decrypt and recover session key session key is used to decrypt message
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Dr. Nguyen Nam Hong, Le Quy Don Technical University

16.14. PGP Uses
 uses both services on same message
 create signature & attach to message  encrypt both message & signature

 attach RSA encrypted session key

Dr. Nguyen Nam Hong, Le Quy Don Technical University

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16.15. PGP 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

Dr. Nguyen Nam Hong, Le Quy Don Technical University

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16.16. PGP Email Compatibility
 when using PGP will have binary data 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
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16.17. PGP Diagrams

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16.18. 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 previous uses and from keystroke timing of user

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16.19. 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
Dr. Nguyen Nam Hong, Le Quy Don Technical University Slide 23 / 32

16.20. PGP Key Rings
 each PGP user has a pair of key rings:
 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

Dr. Nguyen Nam Hong, Le Quy Don Technical University

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16.21. 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
Dr. Nguyen Nam Hong, Le Quy Don Technical University Slide 25 / 32

16.22. S/MIME (1/2)
 S/MIME: Secure Multipurpose Internet Mail Extensions  Unlike the standard PGP, that is based on the trust among users, S/MIME uses digital certificates X.509 brought by a Certification Authority that the e-mail clients must recognized as such.  It will add encryption and signature services on the e-mail clients (Outlook Express, Netscape Messenger, ...) in MIME format.  It creates such an envelope on which the data are embedded encrypted and/or signed.
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16.22. S/MIME (2/2)
 It uses platforms of standards PKCS (Public-

Key Cryptography Standards).  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 various modern mail agents: MS Outlook, Netscape etc
Dr. Nguyen Nam Hong, Le Quy Don Technical University Slide 27 / 32

16.23. S/MIME Functions
 enveloped data
 encrypted content and associated keys  encoded signed data

 message + signed digest

 clear-signed data
 cleartext message + encoded signed digest

 signed & enveloped data
 nesting of signed & encrypted entities
Dr. Nguyen Nam Hong, Le Quy Don Technical University Slide 28 / 32

16.24. S/MIME Cryptographic Algorithms
 hash functions: SHA-1 & MD5
 digital signatures: DSS & RSA  session key encryption: ElGamal & RSA

 message encryption: Triple-DES, RC2/40 and

others  have a procedure to decide which algorithms to use

Dr. Nguyen Nam Hong, Le Quy Don Technical University

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16.25. 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

Dr. Nguyen Nam Hong, Le Quy Don Technical University

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16.26. Certificate Authorities
 have several well-known CA’s
 Verisign one of most widely used  Verisign issues several types of Digital IDs

 with increasing levels of checks & hence trust

Class 1 2+ 3+

Identity Checks Usage name/email check web browsing/email enroll/addr check email, subs, s/w valid. ID documents e-banking/service access
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Dr. Nguyen Nam Hong, Le Quy Don Technical University

16.27. Summary
 have considered:
 secure email  PEM

 PGP
 S/MIME

Dr. Nguyen Nam Hong, Le Quy Don Technical University

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