Lecture 8 mail security

Outline

• Pretty Good Privacy (PGP)

• S/MIME

• Recommended web sites

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Security facilities in the TCP/IP protocol stack

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Pretty Good Privacy

• Philip R. Zimmerman is the creator of PGP (1992).

• PGP provides
 confidentiality
 and authentication
service that can be used for electronic mail and file storage
applications.

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Why Is PGP Popular?

• It is available free on a variety of platforms.

• Wide range of applicability

• Based on well known algorithms. (Why ?, Is it Secure ?)

• Not developed or controlled by governmental or standards
organizations (Is-it trust worthy)

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Operational Description

•Notations Z = Compression using ZIP
Ks = Session key used in Algorithm
symmetric encryption scheme R64 = Conversion to Radix 64
PRa = Private key of user A, ASCII format
used in public-key encryption EP = Public key encryption
scheme DP = Public key decryption
PUa = Public key of user A, EC = Symmetric Encryption
used in public-key encryption DC = Symmetric Decryption
scheme H = Hash Function (SHA-1
Used, 160 bit hash)
• Consist of five services:
| | : Concatenation
 Authentication
 Confidentiality
 Compression
 E-mail compatibility
 Segmentation
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Authentication

• The sender creates a message

• SHA-1 is used to generate a 160-bit hash code of the message

• The hash code is encrypted with RSA using the sender’s private
key, and the result is prepended to the message

• The reciever uses RSA with sender’s public key to decrypt and
recover the hash code

• The reciever generates a new hash code for the mesage and
compares it with the decryupted hash code.

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Confidentiality

• The sender generates a message and a random 128-bit number
to be used as a session key for this message only

• The message is encrypted using CAST -128 / IDEA / #DES with
the session key.

• The session key is encrypted with RSA using recipients public key
and is prepended to the message

• The reciever uses RSA with its private key to decrypt and recover
the session key.

• The session key is used to decrypt the message

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PGP Cryptographic Function

E[PUb, Ks]

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PGP Cryptographic Function

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Compression

• PGP compresses the message after applying the signature but
before encryption

• The placement of the compression algorithm is critical.

• The compression algorithm used is ZIP (described in appendix G or
search internet)

• Message encryption is applied after compression to strengthen
cryptographic security.

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E-mail Compatibility

• The scheme used is radix-64 conversion (see appendix or online).

• The use of radix-64 expands the message by 33%.

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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 to large.

• The receiver strip of all e-mail headers and reassemble the block.

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Transmission and Reception of PGP Messages assembly

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Format of PGP Message

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General Structure of Private and Public Key Rings
• Keys need to be stored and organized in a systematic way for
efficient and effective use by all parties

• Scheme used in PGP providesa pair of data structure at each node
 To store public / private key pairs owned by that node (Private Key
Ring)
 To store public keys of other users known at this node (Public Key Ring)

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General Structure of Private and Public Key
Rings

Least significant 64 bits

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PGP Message Generation

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PGP Message Reception

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The Use of Trust

• No specification for establishing certifying authorities or for
establishing trust
• Provides means of
 Using trust
 Associating trust with public keys
 Exploiting trust information.
• Basic Structure
 Key legitimacy field : indicates the extent to which PGP will trust
See Table 7.2

public key for user
(W. Stallings)

 Signature trust field : Indicates the degree to PGP user trusts the
signer to certify public keys
 Owner trust field : Indicates degree to which public key is trusted to
sign other public-key certificates; assigned by user

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PGP Trust Model (Example)

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(Reading Assignment)

Revoking Public Keys

• The owner issue a key revocation certificate.

• Normal signature certificate with a revoke indicator.

• Corresponding private key is used to sign the certificate.

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S/MIME

• Secure/Multipurpose Internet Mail Extension (RFC5751)

• S/MIME on the IETF standard track
 Will be the commercial standard for secure e-mails

• Uses X.509 certificates (Public-Key Cryptography Standards (PKCS) #7) to
sign/encrypt messages
 PKCS # 7: An updated Cryptographic Message Syntax (CMS)
– CMS is the IETF's standard for cryptographically protected messages which is
used to digitally sign, digest, authenticate or encrypt digital data.
• Provides same features as PGP
 authentication, message integrity and non-repudiation of origin
– provided by use of digital signatures
 privacy, data security
– provided by use of encryption

• PGP for personal e-mail security, S/MIME for professional e-mail security 23
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S/MIME Fucntion

• Enveloped Data
 Consists of encrypted content of any type and encrypteed-content
encryption key
• Signed Data
 Digital signature is formed by taking the message digest and then
encrypted with public key
 Contents + Signature are encoded using base64 encoding
 Can only viewed by recipeint with S/MIME capabilities.
• Clear-Signed Data
 Digital signature are formed and encoded using base64
 All can see message but can not verify signature.
• Singed and Enveloped Data
 Encrypted data may be signed
 Signed data or clear-signed data may be encrypted

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Plain Mail (just MIME)
Content-Type: multipart/mixed; boundary=bar

--bar
Content-Type: text/plain; charset=iso-8859-1
Content-Transfer-Encoding: quoted-printable

=A1Hola Michael!

How do you like the new S/MIME specification?

It's generally a good idea to encode lines that begin with
From=20because some mail transport agents will insert a greater-
than (>) sign, thus invalidating the signature.
Also, in some cases it might be desirable to encode any =20
trailing whitespace that occurs on lines in order to ensure =20
that the message signature is not invalidated when passing =20
a gateway that modifies such whitespace (like BITNET). =20

--bar
Content-Type: image/jpeg
Content-Transfer-Encoding: base64

iQCVAwUBMJrRF2N9oWBghPDJAQE9UQQAtl7LuRVndBjrk4EqYBIb3h5QXIX/LC//
jJV5bNvkZIGPIcEmI5iFd9boEgvpirHtIREEqLQRkYNoBActFBZmh9GC3C041WGq
uMbrbxc+nIs1TIKlA08rVi9ig/2Yh7LFrK5Ein57U/W72vgSxLhe/zhdfolT9Brn
HOxEa44b+EI=

--bar-- 25
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S/MIME filenames

Media Type File
Extension
application/pkcs7-mime (SignedData, .p7m
EnvelopedData)
application/pkcs7-mime (degenerate SignedData .p7c
certificate management message)

application/pkcs7-mime (CompressedData) .p7z

application/pkcs7-signature (SignedData) .p7s

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S/MIME singed message
Content-Type: multipart/signed;
protocol="application/pkcs7-signature";
micalg=sha1; boundary=boundary42

--boundary42
Content-Type: text/plain

This is a clear-signed message.

--boundary42
Content-Type: application/pkcs7-signature; name=smime.p7s
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename=smime.p7s

ghyHhHUujhJhjH77n8HHGTrfvbnj756tbB9HG4VQpfyF467GhIGfHfYT6
4VQpfyF467GhIGfHfYT6jH77n8HHGghyHhHUujhJh756tbB9HGTrfvbnj
n8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4
7GhIGfHfYT64VQbnj756

--boundary42--
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Algorithms Used in S/MIME
• Message Digesting
 MUST : Absolute Requirement
– SHA-1
 SHOULD : May be required in particular cases
– MD5 (Receiver)

• Digital Signatures
 MUST : DSS (Sender / Receiver)
 SHOULD : RSA (Key size of 512 – 1024 bits) (Sender / Receiver)

• Encryption with one time session key
 MUST
– Triple-DES (Sender / Receiver)
 SHOULD
– AES, RC2/40 (Sender)
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Algorithms Used in S/MIME

• Asymmetric encryption of the session key
 MUST
– RSA with key sizes of 512 to 1024 bits (Sender / Receiver)
 SHOULD
– Diffie-Hellman (for session keys). (Sender / Receiver)

• Creation of MAC
 MUST : HMAC with SHA-1 (Receiver)
 SHOULD : HMAC with SHA-1 (Sender)

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Recommended Web Sites

• PGP home page: www.pgp.com
• MIT distribution site for PGP
• GOOGLE -> PGP
• S/MIME Central: RSA Inc.’s Web Site

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Lecture 8 mail security

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