Matakuliah
Tahun
Versi
: H0242 / Keamanan Jaringan
: 2006
:1
Pertemuan 12
E-mail Security
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Learning Outcomes
Pada akhir pertemuan ini, diharapkan
mahasiswa akan mampu :
– Mahasiswa dapat menggunakan e-mail
security
2
Outline Materi
•
•
•
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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
4
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 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
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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
CAST or IDEA or
Encryption
t hree - key triple DES
with Diffie
- Hellman
or RSA
Compression
ZIP
E- mail
Radix - 64 conversion
Compatibility
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
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