Introduction to Public Key Infrastructure (PKI)

1. Introduction to Public Key Infrastructure (PKI) Tammy C. Hensel The University of Texas-Houston Health Science Center

2. Topics • Goals of Secure Messaging • How Asymmetric Key Systems Meet These Goals • Attacks Against Asymmetric Key Systems • How PKI mitigates these attacks

3. Goals for Secure Messaging • Confidentiality • Integrity • Data Origin Authentication • Non-repudiation

4. Goals For Secure Messaging • Confidentiality: messages are kept private • Integrity: messages have not been altered in transit • Data origin authentication: recipient has assurance that the message really came from the ostensible author • Non-repudiation: author may not later claim that she did not write a certain message

5. How do we achieve these goals? • Confidentiality can be achieved via symmetric key systems or asymmetric key systems • Each has its benefits and drawbacks

6. Symmetric Key Systems • Same key is used for encryption and decryption • Alice generates a key and uses it to encrypt a message • Alice sends this key along with her message so that Bob can decrypt the message

7. Asymmetric Key Systems • Uses two separate keys: one for encryption and decryption • Private key – kept secret and never shared • Public key – advertised publicly as part of your certificate • Symmetric/shared secret/session key - This key is generated for one-time or one-session use, and then discarded.

8. THE KEYS Symmetric Key Shared Secret Key Session Key Private Key Public Key

9. Alice obtains Bob’s public key and encrypts the message using that key Only Bob’s private key can decrypt the message, which ensures that only Bob can read the message (Probably) Asymmetric Cryptography

10. Additional Benefits of Asymmetric Key Systems • Asymmetric key systems also provide integrity, data origin authentication, and non-repudiation • Alice can use her private key to “sign” a document • Bob knows that the message really came from Alice, and that the message has not been altered in transit (integrity)

11. Integrity with Digital Signatures • Digital signatures also provide integrity via a process called hashing • A hash also “encrypts” a message, but in this case, the goal is not confidentiality. • A hash is a “non-invertible” or one-way function, which means that once a hash is performed on a message, you cannot get the original message back

12. Hashing, continued • Hash algorithm defined: a one-way “encryption” algorithm that takes a message of any length and produces a smaller, unique output message • Analogy: Your fingerprint is a smaller version of you that uniquely identifies you, but you cannot be reconstructed from your fingerprint • Remember that hashing does not keep your data private!!!

13. How Hashing Creates a Signature • Alice hashes her message, then encrypts the hash with her private key • This process creates a “signature” that is appended to a plaintext message • Bob obtains Alice’s public key, decrypts the signature to uncover the plaintext hash, then runs the same hash function on the plaintext message.

14. Signing Message Message Hash Algorithm H Message Digest1 Signing Messages - Alice Encrypted Digest (Digital Signature) Encrypted Digest (Digital Signature) Alice’s private key

15. Receiving Message H Hash Algorithm Message Digest2 Receiving - Bob Match? Encrypted Digest • Message Digest1 (Digital Signature) Alice’s public key

16. Problems with Asymmetric Key Systems • Computational load for encryption • Man-in-the-middle attacks: public key substitution and signature forging

17. Computational Overhead • Asymmetric systems provide better security, but symmetric systems provide better performance • Solution: use the symmetric key to encrypt and decrypt the data; use public and private keys to encrypt and decrypt the symmetric key

18. Putting it All Together Message Encrypted Message Symmetric Key Digital Envelope Bob’s Public Key Encrypted Symmetric Key

19. Putting it All Together Encrypted Message Message Symmetric Key Digital Envelope Bob’s Private Key Encrypted Symmetric Key

20. Attacks Against Asymmetric Key Systems Public Key Substitution Risks

21. Public-Key Substitution Risk • Molly can remove Bob’s public key and replace it with her own. Then Alice encrypts using “Bob’s” public key. • Molly intercepts the message, decrypts it with her own private key, and modifies it. • Molly re-encrypts it with Bob’s real public key. Bob can decrypt it with his private key, so he never detects the attack.

22. Forging Signatures • Molly removes Alice’s public key and replaces it with her own. • Alice signs a message with her private key. Molly intercepts it, strips the signature, then modifies the message. • Molly creates a new signature for the message using her own private key.

23. Forging Signatures • Bob receives the signature and decrypts it with “Alice’s” public key. • Bob also runs the hash over Molly’s bogus message and verifies the signature.

24. The Problem • We need a way to tie a public/private key pair to a person • A digital signature only ties a message to a private key, not to a person!

25. The Solution • We need a trusted third party that can authoritatively bind a key pair to a person • This trusted third party is called a “certification authority” (CA) • The CA issues a digital certificate to each user, which contains the public key for that user

26. Certificates: Binds a Person to a Key Pair • The public key (embedded in a digital certificate) is in a public directory that is freely accessible • Now when you download someone’s public key, you know that it belongs to a specific person

27. How This Binding is Accomplished • The CA has a public and private key pair, just like people and devices • The CA uses its private key to sign the body of the certificate, just as people use personal private keys to sign messages • To verify, one must use the CA’s public key to decrypt the signature, just as one would verify a personal signature from another user!

28. How This Binding is Accomplished • If the CA is a widely recognized authority, its certificate (along with its public key) will already be embedded in browsers • Two matching hashes ensure that the contents of the certificate have not been tampered with – certificate integrity

29. X.509 Certificate Format Serial Number Issuer X.500 distinguished name Validity Period Subject X.500 distinguished name Public key Key/certificate usage Extensions CA Digital Signature

30. ReviewThis page will help you check your understanding of digital IDs • Define confidentiality, integrity, data origin authentication, and non-repudiation • What does it mean when I receive a message that is digitally signed? What does it mean when I receive a message that is encrypted? • From a technical standpoint, how do I send a message with a digital signature? How do I send an encrypted message? • What could happen if someone were to obtain my private key? What security goals does this weaken? What is the most secure way to maintain the private key?