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Cryptography 101 Frank Hecker

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1 Cryptography 101 Frank Hecker hecker@netscape.com

2 Cryptography 101 Goal: provide a basic understanding of cryptography and related security technologies key to Netscape Topics: encryption algorithms, both symmetric and asymmetric (i.e., public key) hash functions, digital signatures, and X.509 certificates SSL S/MIME 2 2

3 You should be able to answer...
What are the real differences between the "40-bit" and "128-bit" versions? How are certificates used (or not used) in SSL connections? What are certificate authorities for, and what do they actually do? How does S/MIME differ from SSL? Where can SSL, S/MIME be best used? Other questions?

4 Why security technology?
Keep information secret confidentiality Protect information from tampering integrity Tell if person is who they say they are authentication Allow or deny access to data, etc. authorization (access control) Prove that person really did something nonrepudiation

5 Netscape Customer Example
Encryption Encryption provides confidentiality Data encrypted using encryption algorithm together with encryption key Use algorithm with decryption key to recover original data encrypt key Ke Intranets address three areas crucial to an organization’s information system: For information distribution, intranets enable more effective, timely communication among all employees, regardless of location; For client/server applications, intranets can be used for transactions and two-way publishing; For collaboration, they can make it easier to exchange and share information more quickly and effectively through technology like newsgroups, , and groupware. How do communication, applications, and collaboration interrelate? Netscape Customer Example Cadence Design Systems, a leading supplier of electronic design automation software, recently built a Web-based sales force automation system for: Communication—maps out each step of the sales cycle with links to sales support resources and provides an easy-to-find repository of sales tools and reference materials Client/Server Applications—uses online forms to facilitate communications with headquarters, and accesses and distributes a daily news feed on the industry Collaboration—allows global account teams to securely share information New sales reps can learn in hours what otherwise would have taken weeks to learn. The point-and-click Web interface requires no training to use, and information and documents are dynamically served from a database. Sales reps have more time to focus on their relationships with customers. The quick brown fox 4f60ce544b43c13f1d 4f60ce544b43c13f1d decrypt key Kd The quick brown fox 11 11 6 6 4

6 Secure Sockets Layer (SSL)
user requests URL HTTP user requests URL Communicator (Navigator) Enterprise Server GET def/xyz.html 48c00db62f7d userid/password 8f27a038ee3c <html><head>... 77d9421a

7 SSL-transmitted web page
Security indicator

8 Symmetric cryptography
In symmetric (or single key, or secret key) encryption algorithm decryption key is same as encryption key (or can be easily derived from it) Examples: RC4, DES, triple-DES encrypt key K The quick brown fox decrypt 4f60ce544b43c13f1d 8

9 Encryption strength For well-designed symmetric encryption algorithm, strength of algorithm is dependent on number of possible keys brute force attack: try all possible keys Adding one bit to key length makes algorithm twice as strong doubles number of possible keys For a given algorithm (e.g., RC4) 56-bit key is 216 stronger than 40-bit key 128-bit key is 288 stronger than 40-bit key

10 How SSL works (take 1) make TCP connection “I’d like to talk SSL” “OK, let’s talk SSL” have session key Ks transmit data over TCP encrypted using symmetric encryption algorithm with key Ks Problem: How do both sides agree on session key?

11 Public key cryptography
In asymmetric (or dual key, or public key) encryption algorithm decryption key is not same as encryption key (and cannot be easily derived from it) Examples: RSA, KEA encrypt key Kpublic Intranets address three areas crucial to an organization’s information system: For information distribution, intranets enable more effective, timely communication among all employees, regardless of location; For client/server applications, intranets can be used for transactions and two-way publishing; For collaboration, they can make it easier to exchange and share information more quickly and effectively through technology like newsgroups, , and groupware. How do communication, applications, and collaboration interrelate? Netscape Customer Example Cadence Design Systems, a leading supplier of electronic design automation software, recently built a Web-based sales force automation system for: Communication—maps out each step of the sales cycle with links to sales support resources and provides an easy-to-find repository of sales tools and reference materials Client/Server Applications—uses online forms to facilitate communications with headquarters, and accesses and distributes a daily news feed on the industry Collaboration—allows global account teams to securely share information New sales reps can learn in hours what otherwise would have taken weeks to learn. The point-and-click Web interface requires no training to use, and information and documents are dynamically served from a database. Sales reps have more time to focus on their relationships with customers. decrypt The quick brown fox 4f60ce544b43c13f1d decrypt key Kprivate encrypt 4f60ce544b43c13f1d The quick brown fox 6 11 11 6 4

12 Strength of RSA public key cryptography
Public key cryptography is based on existence of certain hard problems figuring out private key from public key requires solving the hard problem For RSA, public/private keys are created using product of two large prime numbers hard problem is factoring the product (modulus) to recover original primes RSA strength depends on modulus length 512-bit modulus (export) or 1024-bit (US)

13 How SSL works (take 2) make TCP connection “I’d like to talk SSL” “OK, here’s my public key” generate random session key Ks “Here’s session key, encrypted using your public key” decrypt session key using private key transmit data encrypted using session key Ks Problem: Must do (slow) public key operations for every SSL connection

14 How SSL works (take 3) make TCP connection “I’d like to talk SSL”
“OK, here’s my public key” generate random secret value “Here’s secret value, encrypted using your public key” decrypt secret value using private key use shared secret to make session keys K1 and K2 transmit data encrypted using session keys

15 What’s left to do? We seem to have basic problem of confidentiality solved, but… “Man in the middle” can corrupt encrypted data and mess up transaction MITM can breach confidentiality by substituting his public key for server’s Server doesn’t have strong authentication for client Time to talk about hash functions, digital signatures, and certificates!

16 Hash functions Takes original data (any length) and computes fixed-length hash code different data means different hash code can’t recover data from hash code Examples: MD5 (128-bit hash code), SHA-1 (160-bit hash code) The quick brown fox... hash function 85d013f4 The quick red fox... hash function ad917c7f

17 Message authentication codes
Essentially a secure checksum hash code computed from original data and shared secret value transmitted with data (like checksum) Used in SSL to protect integrity of data The quick brown fox jumps over... 2a487c81fe215c hash function f730d1f4 The quick brown fox jumps over... f730d1f4

18 Netscape Customer Example
Digital signatures Signer generates digital signature compute hash code from original data encrypt hash code using signer’s private key Others verify digital signature decrypt hash code using signer’s public key compute second copy of hash code from copy of original data two copies of hash code should match No match means data was altered or signer is imposter or using wrong public key Intranets address three areas crucial to an organization’s information system: For information distribution, intranets enable more effective, timely communication among all employees, regardless of location; For client/server applications, intranets can be used for transactions and two-way publishing; For collaboration, they can make it easier to exchange and share information more quickly and effectively through technology like newsgroups, , and groupware. How do communication, applications, and collaboration interrelate? Netscape Customer Example Cadence Design Systems, a leading supplier of electronic design automation software, recently built a Web-based sales force automation system for: Communication—maps out each step of the sales cycle with links to sales support resources and provides an easy-to-find repository of sales tools and reference materials Client/Server Applications—uses online forms to facilitate communications with headquarters, and accesses and distributes a daily news feed on the industry Collaboration—allows global account teams to securely share information New sales reps can learn in hours what otherwise would have taken weeks to learn. The point-and-click Web interface requires no training to use, and information and documents are dynamically served from a database. Sales reps have more time to focus on their relationships with customers. 11 11 4 6 6

19 Signing and verifying Bad! OK The quick brown fox... hash function
85d013f4 encrypt key Kprivate 85d013f4 a3ff369b The quick brown fox... a3ff369b The quick red fox... decrypt key Kpublic a3ff369b 85d013f4 Bad! OK The quick brown fox... The quick red fox... hash function 85d013f4 ad917c7f

20 Certificates A certificate consists of (at least)
public key identity associated with public key digital signature on certificate contents Certificate can be signed by owner of public key (self-signed) by trusted third party (certificate authority) Examples: X.509v3 certs, PGP certs John Doe d90e891a

21 How SSL works (take 4) make TCP connection “I’d like to talk SSL”
“OK, here’s my public key (in certificate)” generate random secret value “Here’s secret value, encrypted using your public key” decrypt secret value using private key use shared secret to make session keys K1 and K2 transmit data encrypted using session keys

22 Certificates and authentication
Goal: Prove entity is who they claim to be First prove that entity knows private key corresponding to a known public key entity can decrypt something encrypted with public key or entity can sign something with private key, signature verifiable using public key Then map from public key to an identity (i.e., identity included in certificate) Note: certificate by itself proves nothing

23 Basic SSL (final take) make TCP connection “I’d like to talk SSL”
“OK, here’s my public key (in certificate)” cert checked for validity, but not yet authenticated “Here’s secret value, encrypted using your public key” decrypt secret value using private key “Done with handshake, switching to encrypted mode” (sent encrypted using session keys generated from secret) if works, server now authenticated transmit encrypted application data

24 SSL with client authentication
make TCP connection “I’d like to talk SSL” “Here’s my certificate. What’s yours?” “Here’s my certificate too” cert checked for validity, but not yet authenticated “Here’s secret value, encrypted using your public key” “Here’s something signed using my private key” if verified, client now authenticated “Done with handshake, switching to encrypted mode” (sent encrypted using session keys generated from secret) transmit data encrypted using session keys

25 Certificates and trust
Binds a public key to an identity (person’s name, server hostname, etc.) but how much you trust that binding is a separate issue If self-signed, you must decide level of trust with each new certificate seen If signed by CA, can trust new certificates based on your trust in CA and its policies verifying certificates requires public key of CA (i.e., certificate for CA itself)

26 What SSL does/doesn’t do
SSL provides confidentiality of transmitted data from client to server and server to client authentication of server to client authentication of client to server (optional) integrity of transmitted data SSL does not provide confidentiality, etc., for data in a store and forward environment (e.g., ) 8

27 S/MIME Emerging standard for secure document transfer (e.g., in , etc.) works with standard Internet message types (RFC 822, MIME) Goals of S/MIME confidentiality of document contents integrity of document contents nonrepudiation: can prove sender wrote document S/MIME uses encryption and/or signing 7 10 8

28 S/MIME (signing only) SMTP (signature OK) The quick brown fox...
SMTP mail server SMTP The quick brown fox... a3ff369b mail server S/MIME mail client The quick brown fox... a3ff369b (signature OK) The quick brown fox...

29 How S/MIME signing works
Start with MIME-compliant message body (text and attachments) Sign content using sender’s private key Include copy of sender’s certificate On receipt, validate signature using public key from sender’s certificate Note: This assumes that sender’s certificate is valid and trusted for signing Get signer’s authenticated identity from certificate

30 S/MIME (encryption only)
SMTP SMTP mail server 4f60ce544b43c13f1d The quick brown fox... key Kpublic of recipient encrypt mail server S/MIME mail client 4f60ce544b43c13f1d The quick brown fox... decrypt key Kprivate of recipient

31 How S/MIME encryption works
Start with MIME-compliant message body (text and attachments) Pick random key and encrypt message using some symmetric algorithm Encrypt symmetric key using recipient’s public key (requires their certificate) On receipt, decrypt symmetric key using recipient’s private key Use symmetric key to decrypt message

32 S/MIME signing/encryption
Can combine signing and encryption in single S/MIME message Start with MIME-compliant body Sign content using sender’s private key Encrypt signed message using random symmetric key then encrypt symmetric key using recipient’s public key On receipt, reverse operations: use recipient’s private key in decrypting, then sender’s public key to verify signature

33 S/MIME-secured message
security indicator

34 Certificate creation, retrieval
create key pair at client send public key plus identity to CA CA verifies identity, signs key+indentity client gets certificate and installs Certificate retrieval not needed for SSL (exchanged in-band) not needed for S/MIME signed messages needed for S/MIME encryption if sender doesn’t have certificate for recipient

35 Summary of Crypto 101 Goal is to implement security services: confidentiality, integrity, etc. Services implemented using cryptography encryption provides confidentiality hash function plus shared secret provides integrity (MAC) public key encryption plus hash function provides integrity, nonrepudiation (digital signature) authentication is obtained as a by-product of key exchange or of signing

36 For more information SSL and cryptography tutorials
Chapter 1, Netscape Certificate Server Administrator’s Guide SSL FAQ (from ssl-talk mailing list) S/MIME For serious crypto enthusiasts news:mcom.crypto.interest Applied Cryptography (2nd. Ed.), Bruce Schneier

37 Cryptography 101 The End

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