electronic mail security

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Electronic mail security

Ola FlygtVäxjö University, Sweden

http://w3.msi.vxu.se/users/ofl/[email protected]+46 470 70 86 49

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Outline

Pretty good privacy

S/MIME

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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.

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Why Is PGP Popular?

It is available free on a variety of platforms.

Based on well known algorithms.Wide range of applicabilityNot developed or controlled by

governmental or standards organizations

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Operational Description

Consist of five services:AuthenticationConfidentialityCompressionE-mail compatibilitySegmentation

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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 5A)

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E-mail CompatibilityThe scheme used is radix-64

conversion (see appendix 5B).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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Summary of PGP Services Function Algorithm Used Digital Signature DSS/ SHA or

RSA/ SHA Message Encryption

CAST or I DEA or three-key triple DES with Dif f ie-Hellman or RSA

Compression ZI P E-mail Compatibility

Radix-64 conversion

Segmentation -

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Format of PGP Message

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Obtaining trust

How can we obtain trust in the the other parties public key?CertificateGetting the key personally

PGP tries to give trust in a new wayWeb of trust

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The Use of TrustThe user maintains a data structure with certified keys together

with the following fieldsOwner trust field, assigned by userSignature trust field, cached copies of respective owner trust fields Key legitimacy field, calculated by PGP

Trust for each key us calculated as weighted sum of trust in signatures for this key

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Revoking Public Keys

The owner issue a key revocation certificate.

Normal signature certificate with a revote indicator.

Corresponding private key is used to sign the certificate.

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S/MIME

Secure/Multipurpose Internet Mail Extension

S/MIME will probably emerge as the industry standard.

PGP for personal e-mail security

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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 problemslines longer than a certain length (72 to 254

characters)

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Header fields in MIME

MIME-Version: Must be “1.0” -> RFC 2045, RFC 2046

Content-Type: More types being added by developers (application/word)

Content-Transfer-Encoding: How message has been encoded (radix-64)

Optional fields: Content-ID: Unique identifying character string. Content Description: Needed when content is

not readable text (e.g.,mpeg)

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Header fields in MIME, Content-Type

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Header fields in MIME, Content-Transfer-Encoding

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MIME ExampleReturn-Path: <[email protected]>Received: from webt.msi.vxu.se (localhost [127.0.0.1])

by babbage (Cyrus v2.1.16) with LMTP; Wed, 13 Feb 2008 09:22:44 +0100X-Sieve: CMU Sieve 2.2Received: from mailinone.vxu.se (mailinone.vxu.se [194.47.65.80])

by webt.msi.vxu.se (8.12.10/8.12.10) with ESMTP id m1D8MgvG008161for <[email protected]>; Wed, 13 Feb 2008 09:22:43 +0100 (MET)

Received: from [194.47.95.160] by mailinone.vxu.se (Sun Java System Messaging Server 6.2-8.01 (built Nov 27 2006)) with ESMTPSA id <[email protected]> for Ola.Flygt@tcp_msi-daemon (ORCPT [email protected]); Wed, 13 Feb 2008 09:22:41 +0100 (MET)Date: Wed, 13 Feb 2008 09:22:39 +0100From: Ola Flygt <[email protected]>Subject: A simple MIME exampleTo: Ola Flygt <[email protected]>Message-id: <[email protected]>MIME-version: 1.0 (Apple Message framework v753)X-Mailer: Apple Mail (2.753)Content-type: text/plain; charset=US-ASCII; format=flowedContent-transfer-encoding: 7bitX-Spam-Debug: msi.vxu.se 0X-Spam-Report: msi.vxu.se 0 () X-Spam-Score: 0X-Spam-Flag: NOX-Scanned-By: MIMEDefang 2.49 on 194.47.94.71

<x-flowed>This is a simple text message.

</x-flowed>

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MIME Example 2Return-Path: <[email protected]>Received: from webt.msi.vxu.se (localhost [127.0.0.1])


by webt.msi.vxu.se (8.12.10/8.12.10) with ESMTP id m1D8O7vG009405for <[email protected]>; Wed, 13 Feb 2008 09:24:07 +0100 (MET)

Received: from [194.47.95.160] by mailinone.vxu.se (Sun Java System Messaging Server 6.2-8.01 (built Nov 27 2006)) with ESMTPSA id <[email protected]> for Ola.Flygt@tcp_msi-daemon (ORCPT [email protected]); Wed, 13 Feb 2008 09:24:07 +0100 (MET)Date: Wed, 13 Feb 2008 09:24:05 +0100From: Ola Flygt <[email protected]>Subject: A HTML exampleTo: Ola Flygt <[email protected]>Message-id: <[email protected]>MIME-version: 1.0X-Mailer: Apple Mail (2.753)Content-type: multipart/alternative; boundary=Apple-Mail-46--901936981X-Spam-Debug: msi.vxu.se 0X-Spam-Report: msi.vxu.se 0 () X-Spam-Score: 0X-Spam-Flag: NOX-Scanned-By: MIMEDefang 2.49 on 194.47.94.71

<x-html><!x-stuff-for-pete base="" src="" id="0" charset=""><html><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; ">This is a mail using HTML encoding of the text.</body></html></x-html>

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MIME Example 3Return-Path: <[email protected]>Received: from webt.msi.vxu.se (localhost [127.0.0.1])


by webt.msi.vxu.se (8.12.10/8.12.10) with ESMTP id m1D8UFvG013858for <[email protected]>; Wed, 13 Feb 2008 09:30:16 +0100 (MET)

Received: from [194.47.95.160] by mailinone.vxu.se (Sun Java System Messaging Server 6.2-8.01 (built Nov 27 2006)) with ESMTPSA id <[email protected]> for Ola.Flygt@tcp_msi-daemon (ORCPT [email protected]); Wed, 13 Feb 2008 09:30:15 +0100 (MET)Date: Wed, 13 Feb 2008 09:30:14 +0100From: Ola Flygt <[email protected]>Subject: An image exampleTo: Ola Flygt <[email protected]>Message-id: <[email protected]>MIME-version: 1.0 (Apple Message framework v753)X-Mailer: Apple Mail (2.753)Content-type: multipart/mixed; boundary=Apple-Mail-47--901568932X-Spam-Debug: msi.vxu.se 0X-Spam-Report: msi.vxu.se 0 () X-Spam-Score: 0X-Spam-Flag: NOX-Scanned-By: MIMEDefang 2.49 on 194.47.94.71

<x-flowed>This is an example including a small image.

</x-flowed>Attachment converted: HD:pastedGraphic.tiff (TIFF/«IC») (00280A8B)

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S/MIME Functions

Enveloped Data: Encrypted content and encrypted session keys for recipients.

Signed Data: Message Digest encrypted with private key of “signer.”

Clear-Signed Data: Signed but not encrypted.

Signed and Enveloped Data: Various orderings for encrypting and signing.

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Algorithms Used

Message Digesting: SHA-1 and MD5Digital Signatures: DSSSecret-Key Encryption: Triple-DES,

RC2/40 (exportable)Public-Private Key Encryption: RSA

with key sizes of 512 and 1024 bits, and Diffie-Hellman (for session keys).

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User Agent Role

S/MIME uses Public-Key Certificates - X.509 version 3 signed by Certification Authority

Functions: Key Generation - Diffie-Hellman, DSS, and RSA key-

pairs. Registration - Public keys must be registered with

X.509 CA. Certificate Storage - Local (as in browser

application) for different services. Signed and Enveloped Data - Various orderings

for encrypting and signing.

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User Agent Role

Example: Verisign (www.verisign.com)

Class-1: Buyer’s email address confirmed by emailing vital info.

Class-2: Postal address is confirmed as well, and data checked against directories.

Class-3: Buyer must appear in person, or send notarized documents.

electronic mail security

Documents

key personallypgp

key revocation certificate

header fields

usersignature trust

parties public key

content description

key triple des

corresponding private