1. A-to-Z of Public Key Infrastructure (PKI)
Alberto Pace
An adaptation from Rafal Lukawiecki presentation SEC390 at Tech’Ed 2003

2. Objectives
Explain the basics of PKI without concentrating on any particular product
Introduce commonly used terminology
Point out those aspects of PKI that require careful planning and implementation
Outline some social issues associated with PKI

3. Agenda A Briefest Summary of Cryptography Fundamentals of PKI
Recommendations on PKI Deployment

4. PKI
“Public Key Infrastructure provides the components and services that enable practical deployment and operation of a system that uses certificates.” A. Nash, “PKI”, RSA Press
PKI is a group of solutions for key distribution problems and other issues:
Key generation
Certificate generation, revocation, validation
Managing trust
I consider Web-of-Trust systems (e.g. PGP) as a perfectly alternative and compatible implementation of PKI

5. A Summary of Cryptography

6. Encryption
Typically things sent over TCP/IP are not encrypted in any way. Anyone on the same network can listen in, grab the packets and read the contents
This is a Bad Thing
Would lower confidence in sending things like credit card numbers to ecommerce companies
Confidential
Remote logins

7. What Does Cryptography Solve?
Confidentiality
◄ Your data/service provides no useful information to unauthorised people
Integrity
◄ If anyone tampers with your asset it will be immediately evident
Authenticity
◄ We can verify that asset is attributable to its authors or caretakers
Non-repudiation
◄ The author or owner or caretaker of asset cannot deny that they are associated with it

8. Symmetric Key Cryptography
Plain-text input
Cipher-text
Plain-text output
“The quick brown fox jumps over the lazy dog”
“The quick brown fox jumps over the lazy dog”
Encryption
Decryption
Same key (shared secret)

9. Symmetric Encryption
With a big organization you can physically distribute keys to users by some secure channel (courier, most likely)
But this is vulnerable to compromise if your courier is compromised
Bigger problem: how do you communicate with someone you’ve never met before from another organization?

10. Asymmetric (Public Key) Encryption
In this approach, there are two completely separate keys: one to encrypt the message, and one to decrypt it.
This turns out to be very powerful, and is the basis for much of the crypto infrastructure on the net today
It solves the key-distribution problem. How?

11. Public Key Crypto
You publish, for all the world to see, your public key.
If someone wants to send you , they look up your public key. They encrypt the message using the public key and send it to you.
You decrypt the message using your private key.

13. Public Key Encryption Recipient’s private key Recipient’s public key
Clear-text Input
Cipher-text
Clear-text Output
“The quick brown fox jumps over the lazy dog”
“The quick brown fox jumps over the lazy dog”
Encryption
Decryption
public
private
Different keys
Recipient’s public key
Recipient’s private key

14. Public Key Crypto
In practice, public key crypto systems are slow compared to symmetric key crypto systems, by an order of about 1,000.
This leads to hybrid systems—a public key system is used to distribute a “session key”, a secret symmetric encryption key. They symmetric key is used for the bulk of communications. The public key crypto is used to solve the key distribution problem

15. Hybrid Encryption (Real World)
Launch key
for nuclear
missile
“RedHeat”
is...
RNG
Randomly-
Generated symmetric “session” key
Symmetric encryption
(e.g. DES)
*#$fjda^j
u539!3t
t389E
5e%32\^kd
Symmetric key encrypted asymmetrically (e.g., RSA)
Digital Envelope
User’s
public key
(in certificate)
As above, repeated for other recipients or recovery agents
Digital Envelope
Other recipient’s or agent’s public key (in certificate)
in recovery policy

16. Hybrid Decryption
*#$fjda^j
u539!3t
t389E
5e%32\^kd
Launch key
for nuclear
missile
“RedHeat”
is...
Symmetric decryption (e.g. DES)
Digital Envelope
Asymmetric decryption of “session” key (e.g. RSA)
Symmetric “session” key
Session key must be decrypted using the recipient’s private key
Digital envelope contains “session” key encrypted using recipient’s public key
Recipient’s private key

17. Public Key Distribution Problem
We just solved the problem of symmetric key distribution by using public/private keys
But…
Scott creates a keypair (private/public) and quickly tells the world that the public key he published belongs to Bill
People send confidential stuff to Bill
Bill does not have the private key to read them…
Scott reads Bill’s messages 

18. Digital Signature and Verification
Digital signature is a mechanism by which a message is authenticated i.e. proving that a message is effectively coming from a given sender, much like a signature on a paper document.
For instance, suppose that Alice wants to digitally sign a message to Bob. To do so, she uses her private-key to encrypt the message; she then sends the message along with her public-key
(typically, the public key is attached to the signed message). Since Alice’s public-key is the only key that can decrypt that message, a successful decryption constitutes a Digital Signature
Verification, meaning that there is no doubt that it is Alice’s private key that encrypted the message.

19. Hashing
For Digital signature, another technique used is called hashing. Hashing produces a message digest that is a small and unique representation (a bit like a sophisticated checksum) of the complete message.
Hashing algorithms are a one-way encryption, i.e. it is impossible to derive the message from the digest.
The main reasons for producing a message digest are:
1. The message integrity being sent is preserved; any message alteration will immediately be detected;
2. The digital signature will be applied to the digest, which is usually considerably smaller than the message itself;
3. Hashing algorithms are much faster than any encryption algorithm (asymmetric
or symmetric).

22. Fundamentals of PKI

23. Is PKI relevant? Who uses all of that stuff?
Web’s HTTP and other protocols (SSL)
VPN (PPTP, IPSec, L2TP…)
(S/MIME, PGP, Exchange KMS)
Files (W2K EFS, PGP and many others)
Web Services (WS-Security)
Good ID Smartcards (Certificates and Challenge/Response)
Executables (.NET Assemblies, Drivers, Authenticode) …

24. Eureka!
We need PKI to solve that problem
And a few others…

25. .
2. What is a Certificate
A certificate is a piece of information that proves the identity of a public-key’s owner.
Like a passport, a certificate provides recognized proof of a person’s (or entity) identity.
Certificates are signed and delivered securely by a trusted third party entity called a Certificate Authority (CA).
As long as Bob and Alice trust this third party, the CA, they can be assured that the keys belong to the persons they claim to be.

26. . A certificate contains among other things: 1) The CA’s identity
2) The owner’s identity
3) The owner’s public-key
4) The certificate expiry date
5) The CA’s signature of that certificate
6) Other information that is beyond the scope of this article.
With a certificate instead of a public-key, a recipient can now verify a few things about the issuer
to make sure that the certificate is valid and belongs to the person claiming its ownership:

27. Certificates The simplest certificate just contains:
Information about the entity that is being certified to own a public key
That public key
And all of this is
Digitally signed by someone trusted (like your friend or a CA)

28. X.509 Certificate
OU=Project Botticelli…
The Key or Info About It

30. Authentication with Certificates
Melinda gets Bill’s certificate
She verifies its digital signature
She can trust that the public key really belongs to Bill
But is it Bill standing if front of her, or is that Scott?
Melinda challenges Bill to encrypt for her a phrase etc. she just made up (“I really need more shoes”)
Bill has, of course, the private key that matches the certificate, so he responds (“*&$^%£$&£fhsdf*&EHFDhd62^&£”)
Melinda decrypts this with the public key she has in the certificate (which she trusts) and if it matches the phrase she challenged Bill with then it must really be Bill himself! 
By the way, that’s the basic concept of how SSL works

31. What’s in the Store? Certificates are “safe”
No need to protect them too much, as they are digitally signed
Store anywhere, a file or a “dumb” memory-only smartcard
Private keys that match the public key are extremely vulnerable (key assets)
You must protect them well
Store in “Protected Storage” on your OS or a “smart” smartcard that will have crypto functionality on board

32. Key and certificate management
Key and certificate management is the set of operations required to create and maintain keys and certificates.
The following is the list of the major points being addressed in a managed PKI:
1. Key and certificate creation: How to generate key pairs? How to issue certificates to the users?
A PKI must offer software support for key pair generation as well as certificate requests.
In addition, procedures must be put in place to verify the user identity prior to allowing him to request a certificate.
Certificates are widely accessible because they are used for either encryption or signature
verification. Private-keys require some reasonable level of protection because they are used either for decryption or for digital signature. A strong password mechanism must be part of the features of an effective PKI.

33. 2. Private-key protection:
How will the user protect his private-key against misuse by other malicious users?
Certificates are widely accessible because they are used for either encryption or signature verification.
Private-keys require some reasonable level of protection because they are used either for decryption or for digital signature. A strong password mechanism must be part of the features of an effective PKI.
3. Certificate revocation: How to handle the situation where a user’s private-key has been compromised?
Similarly, how to handle the situation where an employee leaves the company? How to know whether or not a certificate has been revoked?
A PKI must provide a means by which a certificate can be revoked. Once revoked, this certificate must be included in a revocation list that is available to all users.
A mechanism must be provided to verify that revocation list and refuse to use a revoked certificate.

34. 4. Key backup and recovery: What happens to encrypted files when a user loses his privatekey?
Without key backup, all messages and files that have been encrypted with his public-key can no longer be decrypted and are lost forever.
A PKI must offer private-key backup and a private-key recovery mechanism such that the user can get back his private-key to be able to get access to his files11.
5. Key and certificate update: What happens when a certificate reaches or is near its expiry date?
Keys and certificates have a finite lifetime. A PKI must offer a mechanism to at least update the expiry date for that certificate. Good practice though is to update the user’s keys and certificates.
The key and certificate update can be automatic in which case the end user gets notified that his keys have been updated, or can require that the user performs an action during or before his keys and certificates expire;

35. Certificate Revocation
Keys get compromised, as a fact of life
You or your CA issue a certificate revocation certificate
Must be signed by CA, of course
And you do everything you can to let the world know that you issued it
This is not easy
Certificate Revocation Lists (CRL) are used
They require that the process of cert validation actively checks the CRL and keep it up-to-date
There are some scalability issues
Many people disable this function
That is why short expiration policies are important

36. Storing Certificates and Keys
Certificates need to be stored so that interested users can obtain them
Keys need to be stored for data recovery purposes
This weakens the system, but is a necessity
This is a function of most certificate servers such as certificate services in Windows 2003 Server
Those servers are also responsible for issuing, revoking, signing etc. of certs

37. Developers: Which API?
CAPI (Crypto API, Cryptographic API) is the underlying API provided by the operating system
Mature
Not too easy to use
Good functionality
.NET Framework System.Security.Cryptography
Newer, but wraps some CAPI functions
Extremely easy to use
Not all needed functionality is present

38. .NET Framework API Comprehensive cryptographic library
Easy, unified, stream-based architecture
System.Security.Cryptography
Open & extensible model (for new algorithms)
Some implementations just CAPI wrappers, some completely managed by .NET
Configuration classes for control
Streaming model for block algorithms
Supporting CBC (Cipher Block Chaining)

39. Crypto Object Model (subsection)
Abstract
Base Classes
(only one shown)
Symmetric
Algorithm
Abstract
Algorithm
Classes
TripleDES
Rijndael
RC2
Algorithm Implementation Classes
(fully implemented)
TripleDESCrypto
ServiceProvider
(CryptoAPI)
Rijndael
Managed
(C#)
RC2Crypto
ServiceProvider

40. Recommendations on PKI Deployment

41. CA Services
If you decide against web-of-trust, you need to make an important decision:
Use a well known CA
Your certs will be universally recognised but you are dependent on the trustworthiness of the CA
Establish your own CA
No one except your explicitly nominated partners or clients will recognise your certs but you are in full control
In addition, you may want to outsource CA services altogether

42. Identity Management Process
Consider using Windows Server 2003 as it integrates active directory managament of users with PKI provisioning
Microsoft is investing heavily in identity management across directory boundaries
Between Active Directories
Between heterogenous systems

43. Social Problem Real-life certificates are well understood
What do you trust more: a passport or a driving license?
Digital certificates are a long way from public understanding
Is Verisign Class 1 better or worse than Class 5? What about BT Class 2 versus Thawte Class 3?
Easier if you just deploy internal PKI
Use real-life names, like “passport”, “company id” etc. if possible

44. Common Strength Recommendations (Jun 2003)
Minimum
Recommended
Symmetric Key
96 bits (avoid DES as it can do only 56, instead use AES-Rijndael or RC5)
256 bits (Rijndael, RC5 128bits, not DES)
Asymmetric Key
1024 (RSA)
4096 (RSA)
ECC Key
192 bits
256 bits
Hash: SHA/MD5
128 bits (absolutely not 64 bits)
256 bits or more
Common Cert Classes
Class 2
Class 3 at least

45. Word About Smartcards
Most smartcards are “dumb”, i.e. they are only a memory chip
This is OK for a certificate store, but not recommended for storing a private key used in a challenge test (verifying identity)
Anyway, they are still better than leaving keys on a floppy disk
Cryptographically-enabled smartcards are more expensive but they give much more security
Private key is secure and used as needed
Additional protection (password, biometrics) is possible
Hardware implements some algorithms
Self-destruct is possible

46. Certificate Revocations
It is a good idea to prepare one in advance if possible!
Keep it really safe
Particularly important in web-of-trust systems in case you lose access to your private key
Please, please enable checking and updating of CRL (revocation list) on all of your systems
Apply numerous security patches – this was a particularly “patchy” area recently

47. Summary
Asymmetric encryption solved the extremely difficult problem of key symmetric key exchange
It created a smaller, easier to solve problem of asymmetric key management…
Which is solved with PKI
Bringing additional benefits, such as trust and identity management

48. Resources and Reading Visit www.microsoft.com/security
Review session slides on crypto & security
For more detail, read:
PKI, A. Nash et al., RSA Press, ISBN
Applied Cryptography, B. Schneier, John Wiley & Sons, ISBN
Foundations of Cryptography, O. Goldereich, ECCC-Books/oded_book_readme.html
Handbook of Applied Cryptography, A.J. Menezes, CRC Press, ISBN
Cryptography in C and C++, M. Welschenbach, Apress, ISBN X (includes code samples CD)

49. Community Resources Community Resources
Most Valuable Professional (MVP)
Newsgroups
Converse online with Microsoft Newsgroups, including Worldwide
User Groups
Meet and learn with your peers