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Chapter 5Chapter 5Electronic mail

security

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OutlineOutline

• Pretty good privacy• S/MIME• Recommended web sites

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Secure e-mailSecure e-mail

Alice: generates random symmetric private key, KS. encrypts message with KS (for efficiency) also encrypts KS with Bob’s public key. sends both KS(m) and KB(KS) to Bob.

Alice wants to send confidential e-mail, m, to Bob.

KS( ).

KB( ).+

+ -

KS(m

)

KB(KS )+

m

KS

KS

KB+

Internet

KS( ).

KB( ).-

KB-

KS

mKS(m

)

KB(KS )+

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Secure e-mailSecure e-mail

Bob: uses his private key to decrypt and recover KS

uses KS to decrypt KS(m) to recover m

Alice wants to send confidential e-mail, m, to Bob.

KS( ).

KB( ).+

+ -

KS(m

)

KB(KS )+

m

KS

KS

KB+

Internet

KS( ).

KB( ).-

KB-

KS

mKS(m

)

KB(KS )+

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Secure e-mail (continued)Secure e-mail (continued)• Alice wants to provide sender authentication message integrity.

• Alice digitally signs message.• sends both message (in the clear) and digital signature.

H( ). KA( ).-

+ -

H(m )KA(H(m))-

m

KA-

Internet

m

KA( ).+

KA+

KA(H(m))-

mH( ). H(m )

compare

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Secure e-mail (continued)Secure e-mail (continued)• Alice wants to provide secrecy, sender authentication, message integrity.

Alice uses three keys: her private key, Bob’s public key, newly created symmetric key

H( ). KA( ).-

+

KA(H(m))-

m

KA-

m

KS( ).

KB( ).+

+

KB(KS )+

KS

KB+

Internet

KS

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Pretty good privacy (PGP)Pretty good privacy (PGP)

• Internet e-mail encryption scheme, de-facto standard.

• uses symmetric key cryptography, public key cryptography, hash function, and digital signature as described.

• provides secrecy, sender authentication, integrity.

---BEGIN PGP SIGNED MESSAGE---Hash: SHA1

Bob:My husband is out of town tonight.Passionately yours, Alice

---BEGIN PGP SIGNATURE---Version: PGP 5.0Charset: noconvyhHJRHhGJGhgg/

12EpJ+lo8gE4vB3mqJhFEvZP9t6n7G6m5Gw2

---END PGP SIGNATURE---

A PGP signed message:

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Pretty Good PrivacyPretty Good Privacy

• Philip R. Zimmerman, the creator of PGP, was target of 3-year federal investigation

• 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?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

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Operational DescriptionOperational Description

• Consist of five services:– Authentication– Confidentiality– Compression– E-mail compatibility– Segmentation

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CompressionCompression

• 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 CompatibilityE-mail Compatibility• The 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 Segmentation and ReassemblyReassembly

• 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 too large.

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

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Summary of PGP Summary of PGP Services Services

Function Algorithm Used Digital Signature DSS/ SHA or

RSA/ SHA Message Encryption

CAST or I DEA or three-key triple DES with Diffi e-Hellman or RSA

Compression ZI P E-mail Compatibility

Radix-64 conversion

Segmentation -

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Format of PGP MessageFormat of PGP Message

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The Use of TrustThe Use of Trust

• Key legitimacy field• Signature trust field• Owner trust field

See Table 5.2 See Table 5.2 (W. Stallings)(W. Stallings)

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Revoking Public Revoking Public KeysKeys

• 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/MIMES/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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Electronic MailElectronic MailThree major components: • user agents • mail servers • simple mail transfer

protocol: SMTP

User Agent• a.k.a. “mail reader”• composing, editing,

reading mail messages• e.g., Eudora, • outgoing, incoming

messages stored on server

user mailbox

outgoing message queue

mailserver

useragent

useragent

useragent

mailserver

useragent

useragent

mailserver

useragent

SMTP

SMTP

SMTP

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Electronic Mail: mail Electronic Mail: mail serversservers

Mail Servers • mailbox contains incoming

messages for user• message queue of

outgoing (to be sent) mail messages

• SMTP protocol between mail servers to send email messages– client: sending mail

server– “server”: receiving mail

server

mailserver

useragent

useragent

useragent

mailserver

useragent

useragent

mailserver

useragent

SMTP

SMTP

SMTP

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SMTP SMTP [RFC 2821][RFC 2821]

• uses TCP to reliably transfer email message from client to server, port 25

• direct transfer: sending server to receiving server• three phases of transfer

– handshaking (greeting)– transfer of messages– closure

• command/response interaction– commands: ASCII text– response: status code and phrase

• messages must be in 7-bit ASCII

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Mail message formatMail message format

SMTP: protocol for exchanging email msgs

RFC 822: standard for text message format:

• header lines, e.g.,– To:– From:– Subject:different from SMTP

commands!• body

– the “message”, ASCII characters only

header

body

blankline

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SMTP, RFC 822SMTP, 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 (72 to 254 characters)

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Multimedia extensionsMultimedia extensions

• MIME: multimedia mail extension, RFC 2045, 2056

• additional lines in msg header declare MIME content type

From: alice@crepes.fr To: bob@hamburger.edu Subject: Picture of yummy crepe. MIME-Version: 1.0 Content-Transfer-Encoding: base64 Content-Type: image/jpeg

base64 encoded data ..... ......................... ......base64 encoded data

multimedia datatype, subtype,

parameter declaration

method usedto encode data

MIME version

encoded data

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MIME typesMIME typesContent-Type: type/subtype; Content-Type: type/subtype;

parametersparameters

Text• example subtypes: plain, html

Image• example subtypes: jpeg, gif

Audio• example subtypes: basic (8-bit mu-law encoded), 32kadpcm (32 kbps coding)

Video• example subtypes: mpeg,

quicktime

Application• other data that must be

processed by reader before “viewable”

• example subtypes: msword, octet-stream

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Multipart TypeMultipart TypeFrom: alice@crepes.fr To: bob@hamburger.edu Subject: Picture of yummy crepe. MIME-Version: 1.0 Content-Type: multipart/mixed; boundary=98766789 --98766789Content-Transfer-Encoding: quoted-printableContent-Type: text/plain

Dear Bob, Please find a picture of a crepe.--98766789Content-Transfer-Encoding: base64Content-Type: image/jpeg

base64 encoded data ..... ......................... ......base64 encoded data --98766789--

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Header fields in MIMEHeader 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)

• Content-ID: Unique identifying character string.

• Content Description: Needed when content is not readable text (e.g.,mpeg)

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S/MIME FunctionsS/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 UsedAlgorithms Used

• Message Digesting: SHA-1 and MD5

• Digital Signatures: DSS

• Secret-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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Negotiation between sending Negotiation between sending

and receiving agentsand receiving agents • both may announce their decrypting capabilities

• sending agent should select from the recipient capabilities list the highest capability of decryption

• if there is no such a list the sending agent should use the same algorithm which was used in messages received from the recipient

• if there are no messages from the recipient and the sender is willing to risk that the recipient will not be able to decrypt the message, it should use triple DES

• if the sender is not willing to risk, then it must use RC2/40

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Securing a MIME EntitySecuring a MIME Entity

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

• The MIME entity is prepared according to normal rules.

• Then the entity plus some security-related data are processed by S/MIME to produce PKCS object.

• The PKCS object is then treated as message content and wrapped in MIME.

• The message to be sent is converted to canonical form

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S/MIME MessagesS/MIME Messages

• EnvelopedData• SignedData• Clear Signing• Registration Request• Certification-Only messages

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S/MIME Certificate processingS/MIME Certificate processing• S/MIME uses Public-Key Certificates - X.509 version 3

mixed with a PGP kind of web of trust. The certificates are signed by CA

• User AgentFunctions:

– Key Generation - Diffie-Hellman, DSS, and RSA key-pairs.

– Registration - Public keys must be registered with X.509 CA.

– Certificate Storage and retrieval- Local (as in browser application) for different services.

– Signed and Enveloped Data - Various orderings for encrypting and signing.

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Verisign - Digital ID ContentsVerisign - Digital ID Contents

• Owner’s public key• Owner’s name or alias• Expiration date• serial number • name of the CA that issued Digital ID• digital signature of that CA

• sometimes :• user supplied address• e-mail address• basic registration info

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Classes of Verisign security for public-Classes of Verisign security for public-key certificateskey certificates

– 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.

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Future Enhanced Security ServicesFuture Enhanced Security Services

• Signed receipts - for proof of delivery• Security labels - to specify access restriction,

sensitivity of the message contents, • secure mailing lists - using the services of an

S/MIME Mail List Agent for encryption

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Recommended Web Recommended Web SitesSites

• PGP home page: www.pgp.com• International PGP: www.pgpi.org• MIT distribution site for PGP• Gnu Privacy Guard: www.gnupg.org• www.cosc.brocku.ca/Docs/GPG• S/MIME Central: RSA Inc.’s Web Site