Demystifying%20PKI:

1
Demystifying PKI Introduction to The
Cryptography Behind Public Key Infrastructure
2
Security Services

• Data Integrity
• Verification that the data has not been modified
• Authentication
• e.g., your personal signature
• Non-Repudiation
• e.g., Sender/Receiver in a financial transaction
• Confidentiality
• i.e., scrambled text

3
Data Integrity

• The Assurance That the Data Has Arrived Intact,
  With No Tampering or Corruption of the Bits.
• Data Integrity Is Achieved Electronically Through
  the Use of Cryptographic Checksums (One-way
  Hashes) Over the Data.

4
Data Integrity Hash Functions

• Hash Functions are Complex Mathematical Functions
  Which Generate a Unique Fingerprint of the
  Data. Each String of Data is Mathematically
  Reduced to a Fixed-Size Output Block, Regardless
  of the Amount of Input Data
• The Same Output is Always Produced From The Same
  Input

User Data
Hash Function
Two Examples Secure Hash Algorithm (SHA) Message
Digest 5 RSA (MD-5)
The Result Produced By a Hashing Function is
Called a Message Digest
5
Authentication

• The Binding of the Senders (or Issuers)
  Credentials to the Data. This Process Can Be
  Likened to Your Personal Signature
• It Is Unique to You and Can Be Recognized
  (Verified) Later by All Parties Involved

6
Non-Repudiation

• The Fact That a Third Party Can Verify Your
  Authentication (e.g., Your Signature) on a
  Transaction Means That You Cannot Deny
  Participation in the Transaction

7
Confidentiality/Privacy

• Encryption (scrambling) of the data to prevent
  unauthorized disclosure.

8
Mechanics of Security

• Cryptographic algorithms (mathematical processes)
  used to implement security
• Symmetric vs. Asymmetric
• Key Generation
• Digital Signatures
• Encryption
• Public Key Infrastructure

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Symmetric Cryptography
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Encryption Algorithms

• Encryption Has Historically Been Used in Military
  Applications to Secure Tactical or Intelligence
  Related Information During Wartime.
• For This Reason, Encryption Is Classified As a
  Munition or Instrument of War by Most Countries.
  The Improper Use of Encryption Is Often
  Considered a Terrorist Act.
• Many Countries Place Restrictions on the Import
  and Export of Encryption, as Well as the Use of
  Encryption Within the Country.

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Encryption Algorithms
The problem of good cipher design is essentially
one of finding difficult problems..... we may
construct our cipher in such a way that breaking
it is equivalent to... the solution of some
problem known to be laborious. - Claude
Shannon (1949)
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Conventional Algorithms

• Also Called Secret-Key Algorithms
• Symmetric - Use The Same Key For Encryption and
  Decryption
• Security Depends on Keeping the Session Key Secret

Encryption
Decryption
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Symmetric Encryption/Decryption
Key Generation
Secret Key
Secret Key
Same Key
12A7BC544109FD00A6293FECC7293B9BCAA12020384AC6F4D9
3B8
DES
This is plain text. It can be a document, image,
or any other data file
DES
This is plain text. It can be a document, image,
or any other data file
SENDER
RECEIVER

• Secret Key used to encrypt data
• Sender and receiver must have same key
• Key distribution and compromise recovery are
  difficult

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Conventional Algorithms

• Stream Ciphers
• Perform a Mathematical Transformation Using One
  Bit From the Key String and One Bit From the Data
  Stream.

• The Classic Stream Cipher Is Called a Vernam
  Cipher
• It is Based on the Exclusive OR Function

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Stream Ciphers Vernam Cipher
Repeating Key Stream

101001011
101001011 101001011 101001011 ...
.XOR.

Message

Debit 500
101101011101101011101101011 ...
CryptoText
Ef2Uz7_at_W
16
Block Ciphers

• Perform a Mathematical Transformation On Data In
  Fixed-Size Blocks, One At a Time.
• The Cipher Mode Determines How The Algorithm Is
  Applied To Data Streams, Block-By-Block
• Block Ciphers are Fairly Similar From a
  Functional Point-of-View
• Well Now Look at an Example of One Well-Known
  Block Cipher in Detail...

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DES Algorithm
Message
Encrypted Message
Encrypt
DES Key
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Anatomy of DES
Original Message Stream is Broken Into 64-Bit
Blocks (8 Ascii Characters)
Each Block is Separately Fed Into The DES
Algorithm (Hence the Term Block Cipher)
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Anatomy of DES
64-Bit Block of Original Text
The Original DES Key is 64 Bits
The Keysize is Reduced to 56 Bits During The
Initial Permutation
The Reduced 56-Bit Key Becomes The Working DES
Session Key
Initial Permutation
Bits 8,16,24,32,40,48,56,64
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Anatomy of DES
64-Bit Block of Original Text
56-Bit Key
Old Right Half
Blocking
Expansion
The 32-Bit Right Half of The Input Block is
Copied Into the Left Half of The Output Block
The 32-Bit Right Half of The Input Block is Then
Expanded to 48-Bits
Original Right Half is Copied to New Left Half
Old Right Half
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Anatomy of DES
Blocking
Permutation
Expansion
The 56-Bit Session Key is Further Reduced to a
48-Bit SubKey
Original Right Half is Copied to New Left Half
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Anatomy of DES
The Input is Shifted Into the S-Registers in
6-bit groups.
S5 S6 S7 S8
S1 S2 S3 S4
Inside Each Register, 2-bits are Used as Control
Bits, and 4-bits as Data
A Substitution Table is Used Inside Each Register
to Calculate Its Output
The S-Registers Perform Substitution and
Compaction, Converting the 48-Bit Block to 32-Bits
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Anatomy of DES
For Each of the Four Choices of the Two Control
Bits , the S-register Performs a Different
Substitution on the Half-byte Values of the Four
Input Bits
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Anatomy of DES
Blocking
Permutation
Expansion
Substitution and Compaction
Permutation
Original Right Half is Copied to New Left Half
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Anatomy of DES
New 64-Bit Block
64-Bit Block of Original Text
56-Bit Key
Blocking
32-Bit Right Half
Permutation
Expansion
48-Bit ExpandedRight Half
48-Bit SubKey
Substitution and Compaction
Permutation
Original Right Half is Copied to New Left Half
S5 S6 S7 S8
S1 S2 S3 S4
New Right Half
New 64-Bit Block (To Next Round)
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Triple Des Algorithm (TDES)
CryptoText
Message
Ef2Uz7_at_W
DES Decrypt
DES Encrypt
DES Encrypt
Debit 500

• Implements 3 Successive Iterations of DES

• Uses Two or Three 56-Bit Keys (112-bit or
  168-bit)

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Encryption Algorithms

• Strengthening Encryption Algorithms
• Strength of an Algorithm Measures How Long It
  Would Take an Adversary to Deduce the Key

• The More Difficult the Mathematics, the Stronger
  the Algorithm
• The Longer the Key, the Stronger the Algorithm
• The More Often the Key Is Changed, the Stronger
  the Security
• The Stronger the Algorithm, the Slower it Usually
  is Due to the Mathematical Overhead Required

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Asymmetric Cryptography
Most commonly known as Public Key Cryptography
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1. Key Generation
Key Pair Generation
Certification Authority
User Name Organization Location
Digital Certificate

• Key pair is use in public key cryptography
• Key generation provides the basis for trust
• Private key protected and never shared
• Public key bound in certificate and shared

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2. Digital Signature
A Digital Signature Is a Special Block That is
Appended to an Electronic Message.
Allows for Verification of the AUTHENTICATION of
the Sender and of the INTEGRITY of the content of
an Electronic Message. Only Public-key
Techniques Can Provide This.
Stock Purchase Order
Signature Block
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2. Digital Signature
How Alice Creates A Digital Signature
Secure Hash Algorithm
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2. Digital Signature
Dear Sir, Please Send Me The Widget. Please
Charge VISA Card 4123...
How Alice Creates A Digital Signature
Secure Hash Algorithm
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2. Digital Signature
Senders Certificate
Senders Token
Senders Private Key
Senders Public Key
Digitally Signed
VERIFIED
Sign
Verify
VERIFIED
SENDER
RECEIVER

• Sender uses private key to sign
• Receiver uses senders public key to verify
• Result is Pass or Fail

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Algorithms for Digital Signature

• Digital Signature Algorithm (DSA)
• Federal Standard (FIPS 186)
• Secure Hash Algorithm (SHA-1)
• Rivest Shamir Adleman (RSA)
• Message Digest 5 (MD5)
• Elliptic Curve Digital Signature Algorithm
  (ECDSA)

cme mod n mcd mod n
r(gk mod p) mod q s(k-1(H(m)xr)) mod q
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Digitized vs. Digital Signature
A Digitized signature is a scanned image that can
be pasted on any document A Digital Signature is
a numeric value that is created by performing a
cryptographic transformation of the data using
the signers private key
1A56B29FF6310CD3926109F200D5EF71 9A274C66821B09AC3
857FD62301AA27 00AB3758B6FE93DD
Digital Signature
Digitized Signature
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Digital Certificates

• Analogous to a Drivers License or Employee Badge
• Issued By Some Authority That Members Have in
  Common
• Issued Under Some Set of Rules (Policies)
• Document Issued Contains Public Information
• Not Sensitive
• Not Compromising
• Provides Trust to Peers, Identification to Others

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1. Message Encryption
Message Key
Use RNG to GENERATE
Senders Token
12A7BC544109FD00A6293FECC7293B9BCAA12020384AC6F4D9
3B8
This is plain text. It can be a document, image,
or any other data file
DES
SENDER

• Use token to generate a random message key
• Encrypt message with symmetric algorithm (DES)

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2. Key Transport (Wrap)
Senders Private Key
Recipients Public Key
Recipients Certificate
Senders Token
Message Key
RSA
(From previous step)
SENDER

• Encrypt message key with senders private key and
  recipients public key and a public key algorithm
  (RSA)

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3. Compose Message
12A7BC544109FD00A6293FECC7293B9BCAA12020384AC6F4D9
3B8
Encrypted Message
Senders Certificate
Wrapped Message Key
SENDER

• Send wrapped message key, encrypted message, and
  (optionally) senders certificate to recipient

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4. Key Transport (Unwrap)
RSA
RECIPIENT

• Use the senders public key and the recipients
  private key to unwrap the message key with public
  key algorithm (RSA)

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Alternative Key Exchange MethodThe
Diffie-Hellman Public Key System

• Diffie-Hellman Works Because of a One-Way
  Function
• The Function Is Easy to Compute but the Inverse
  Is Hard to Compute.
• Specifically D-H Uses Discrete Exponents and
  Discrete Logs.

(easy)
(hard)
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Diffie-Hellman

• Return to the 3rd Grade...
• Multiplication Is Easy and Division Is Hard
• Diffie-Hellman Is Based on X and /
• Bob and Alice Share a Generator (a) Value 10

Igor knows 10, 80 110 Division Required!
Alice
Bob
80
110
Secret 11
Secret 8
Public 11 x 10 110
Public 8 x 10 80
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5. Message Decryption
Message Key
12A7BC544109FD00A6293FECC7293B9BCAA12020384AC6F4D9
3B8
DES
This is plain text. It can be a document, image,
or any other data file
RECIPIENT

• Use unwrapped (RSA) or computed shared (D-H)
  message key to decrypt the data using a symmetric
  algorithm (e.g., DES)

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Key Recovery

• Key Recovery is a Mechanism By Which Law
  Enforcement Officials With Appropriate Authority
  Can Recover the Session Key Used To Encrypt a
  Given Data Stream
• In Typical Key Recovery Mechanisms, The Session
  Key is Encrypted Using the Public Key of a Key
  Recovery or Escrow Agent and Sent Along With The
  Data Stream in a Special Packet
• The Private Key of the Key Recovery Agent Can Be
  Used to Decrypt the Encrypted Session Key, Thus
  Making It Possible to Decrypt the Original Session

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Basic Key Recovery
This is a critical note on our 1999 revenue ...
)lt((_at_?lt)(_at_(_at_
Encrypted Data
KRF
Key Recovery Field
Message Key
Public Key
KEY RECOVERY AGENT
Private Key
KRF
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Using Security Services
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Using Security Services

• Digital Signatures

• Client Authentication on a Web Server
• Netscape, Microsoft
• Compared to Access Control List on Server
• Server Authentication on a Web Client
• Netscape, Microsoft
• Stops Man-in-the-middle Attack
• Message Authentication
• S/MIME E-mail Message
• Netscape, MS Outlook Express 98
• Audit
• Authentication of User Provides Non-repudiation
  of Client Access
• May Provide Legal Proof for Later Arbitration

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Using Security Services

• Confidentiality
• Link Encryption
• IPSec (Layers 2/3)
• Secure tunnel between VPN boxes
• SSL (Layers 4/5)
• Secure tunnel to web server
• Netscape, Microsoft
• FTP (Layers 6/7)
• Secure file transfer

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PKIPublic Key Infrastructure
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Digital Certificates
Certification Authority
Digital Certificate

• Certification Authority acts as a trusted third
  party
• Binds user information to public key.
• Issues an unforgeable certificate.
• Digital certificate can be published in a public
  directory/repository.
• Digital certificate can be used to provide the
  required security services integrity,
  confidentiality, authentication, authorization,
  and non-repudiation.
• ITU Recommendation X.509 is the accepted standard
  for digital certificates in Government and
  industry.

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X.509 Certificates (cont.)

• X.509 Version 3 certificates
• Defined extensions that can be added to the base
  certificate
• public key information
• policy information
• additional subject attribute information
• constraint information
• CRL information
• Widely accepted in Govt and industry.
• Commercial and Govt implementations.

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Public Key Infrastructure
Public Key Infrastructure
Public Key Certificates
Certification Authorities
PKI Services
Registration Management
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Risk Reduction and PKI
PKI
Business Requirements
Legal Requirements
Technology Requirements
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Summary

• Cryptographic Security Solutions
• Provide Security Assurances
• Privacy/Confidentiality
• Data Integrity
• Source and Destination (Client/Server/User)
  Authentication
• Access Control
• Non-Repudiation
• Support The Emerging PKI Marketplace
• PKI Security Solutions
• Enable Enterprise E-Commerce
• Issue, Manage, Revoke Certificates
• Apply Enterprise Certificate Policies and
  Procedures