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Electronic mail security

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Electronic mail security * Current email services are roughly like – PowerPoint PPT presentation

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Title: Electronic mail security


1
Electronic mail security
2
Outline
  • Pretty good privacy
  • S/MIME

3
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

4
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

5
Pretty Good Privacy
  • Philip R. Zimmerman is the creator of PGP.
  • PGP provides a confidentiality and authentication
    service that can be used for electronic mail and
    file storage applications.

6
Pretty Good Privacy-PGP
  • PGP is a remarkable phenomenon that provides
    confidentiality, authentication, and compression
    for email and data storage.
  • Its building blocks are made of the best
    available cryptographic algorithms RSA, DSS,
    Diffie-Hellman.
  • It is independent of operating system and
    processor.
  • It has a small set of easy-to-use commands

7
  • available on Unix, Windows, Macintosh and Amiga
    systems
  • originally free, now have commercial versions
    available also

8
Why Is PGP Popular?
  • It is availiable free on a variety of platforms.
  • Based on well known algorithms.
  • Wide range of applicability
  • Not developed or controlled by governmental or
    standards organizations

9
Operational Description
  • Consist of five services
  • Authentication
  • Confidentiality
  • Compression
  • E-mail compatibility
  • Segmentation

10
Notations
  • Ks session key used in Symmetric encryption
    scheme
  • KRa private key of User A (public key
    encryption)
  • KUa public key of User A (public key
    encryption)
  • EP public key encryption
  • DP public key decryption
  • EC symmetric encryption
  • DC symmetric decryption
  • H hash function
  • concatenation
  • Z compression
  • R64 conversion to radix 64 ASCII format

11
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12
PGP Operation 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

13
PGP Operation Confidentiality
  • sender generates message and random 128-bit
    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

14
PGP Operation Confidentiality Authentication
  • uses both services on same message
  • create signature attach to message
  • encrypt both message signature with session key
  • attach RSA encrypted session key

15
PGP Operation Compression
  • by default PGP compresses message after signing
    but before encrypting
  • The placement of the compression algorithm is
    critical
  • Signature is generated before compression
  • so can store uncompressed message signature for
    later verification
  • because compression is non deterministic
  • Msg encryption done after, to strengthen security
  • Uses ZIP compression algorithm

16
PGP Operation 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
  • The scheme uses radix-64 algorithm
  • maps 3 bytes to 4 printable chars
  • also appends a CRC

17
E-mail Compatibility
  • The use of radix-64 expands the message by 33.

18
Segmentation and Reassembly
  • E-mail facilities are 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 to
    large.
  • The receiver strip of all e-mail headers and
    reassemble the block.

19
Summary of PGP Services
20
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21
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
  • Assume 128 bit random no. (CAST-128)
  • generated using ANSI X12.17 mode
  • uses random inputs taken from previous uses and
    from keystroke timing of user

22
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
  • Key ID of KUa public key is KUa mod 264
  • will very likely be unique
  • also use key ID in signatures

23
Format of PGP Message
24
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

25
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26
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27
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28
S/MIME
  • Secure/Multipurpose Internet Mail Extension
  • S/MIME will probably emerge as the industry
    standard.
  • PGP for personal e-mail security

29
Simple Mail Transfer Protocol (SMTP, RFC 822)
  • SMTP Limitations - Can not transmit, or has a
    problem with
  • executable files, or other binary files (jpeg
    image)
  • national language characters (non-ASCII)
  • messages over a certain size
  • ASCII to EBCDIC translation problems
  • lines longer than a certain length (76 characters)

30
Header fields in MIME
  • MIME-Version Must be 1.0 -gt RFC 2045, RFC
    2046
  • Content-Type Describes the data contained in
    the body . More types being added by developers
    (application/word)
  • Content-Transfer-Encoding How message has been
    encoded (radix-64)
  • Content-ID Unique identifying character string.
  • Content Description Text description of the
    object with the body. Needed when content is not
    readable text (e.g.,audio data)

31
S/MIME Functions
  • Enveloped Data Encrypted content and encrypted
    session keys for recipients.
  • Signed Data Message Digest encrypted with
    private key of signer..Content signature is
    then encoded using base64.
  • Clear-Signed Data Only Digital Signature is
    encoded. Receiver can view msg content.
  • Signed and Enveloped Data Various orderings for
    encrypting and signing.

32
Algorithms Used
  • Message Digesting SHA-1 and MD5
  • Digital Signatures DSS RSA
  • Secret-Key Encryption(For message encryption)
    Triple-DES, RC2/40 (exportable)
  • Public-Private Key Encryption(For Session keys
    encryption) RSA with key sizes of 512 and 1024
    bits, and Diffie-Hellman.
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