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Pertemuan 12 E-mail Security

Matakuliah : H0242 / Keamanan Jaringan

Tahun : 2006

Versi : 1

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Learning Outcomes

Pada akhir pertemuan ini, diharapkan mahasiswa akan mampu :

– Mahasiswa dapat menggunakan e-mail security

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Outline Materi

• Konsep kerja e-mail• Layanan keamanan pada e-mail• PGP• S / MIME

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Email Security

• E-mail is one of the most widely used and regarded network services

• Currently message contents are not secure, missing the following security characteristics

– 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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Pretty Good Privacy (PGP)

• Widely used de facto secure email• Selected best available crypto algorithms to

use, integrated into a single program originally free, now have commercial versions available

• Wide range of applicability• Not developed or controlled by governmental

or standards organizations

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PGP Operational Description

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

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Confidentiality & Authentication

Uses both services on same message– Create signature & attach to

message– Encrypt both message & signature– Attach RSA encrypted session key

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Authentication

1. Sender creates a message2. SHA-1 used to generate 160-bit hash code of

message3. 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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Confidentiality

1. Sender generates message and random 128-bit number to be used as session key for this message only

2. Message is encrypted, using CAST-128 / IDEA/3DES with session key

3. Session key is encrypted using RSA with recipient's public key, then attached to message

4. Receiver uses RSA with its private key to decrypt and recover session key

5. Session key is used to decrypt message

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Compression

• By default PGP compresses message after signing but before encrypting– Store uncompressed message &

signature for later verification because compression is non deterministic

• Uses ZIP compression algorithm

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Email Compatibility

• When using PGP will have binary data to send (encrypted message, etc);however, email was designed only for text

• 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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E-mail Compatibility

• The scheme used is radix-64 conversion • The use of radix-64 expands the message by 33%.

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Segmentation & 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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Sumary of PGP Services

Function Algorithm Used

Digital Signature DSS/SHA or

RSA/SHA

Message

Encryption

CAST or IDEA or

t hree -key triple DES

with Diffie -Hellman

or RSA

Compression ZIP

E-mail

Compatibility

Radix -64 conversion

Segmentation -

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SMTP

SMTP (RFC 822) Limitations – Can not transmit, or has a problem with:

• Executable files, or other binary files (JPG)• 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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S/MIME

• Secure/Multipurpose Internet Mail Extensions• Security enhancement to MIME email

– MIME provided support for varying content types and multi-part messages with encoding of binary data to textual form

– S/MIME added security enhancements• Support in various modern mail agents: MS Outlook,

Netscape, etc• Will probably emerge as the industry standard,

whereas PGP for personal e-mail security

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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

• 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

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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 MDS

• Digital Signatures:– DSS

• Secret-Key Encryption:– Triple-DES– RC2/40 (exportable)

• Public-Private Key Encryption:– RSA with key sizes of 512 and 1024 bits– 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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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

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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 certificates

and own public/private key pairs & certificates• Certificates must be signed by trusted CA’s