Crypto meets Web Security: Certificates and SSL/TLS
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1 CSE 484 / CSE M 584: Computer Security and Privacy Crypto meets Web Security: Certificates and SSL/TLS Spring 2016 Franziska (Franzi) Roesner franzi@cs.washington.edu Thanks to Dan Boneh, Dieter Gollmann, Dan Halperin, Yoshi Kohno, John Manferdelli, John Mitchell, Vitaly Shmatikov, Bennet Yee, and many others for sample slides and materials...
2 Authenticity of Public Keys? private key Alice Bob public key Problem: How does Alice know that the public key she received is really Bob s public key? 4/27/16 CSE 484 / CSE M Spring
3 Threat: Man-In-The-Middle (MITM) Google.com 4/27/16 CSE 484 / CSE M Spring
4 Distribution of Public Keys Public announcement or public directory Risks: forgery and tampering Public-key certificate Signed statement specifying the key and identity sig CA ( Bob, PK B ) Common approach: certificate authority (CA) Single agency responsible for certifying public keys After generating a private/public key pair, user proves his identity and knowledge of the private key to obtain CA s certificate for the public key (offline) Every computer is pre-configured with CA s public key 4/27/16 CSE 484 / CSE M Spring
5 Trusted Certificate Authorities 4/27/16 CSE 484 / CSE M Spring
6 Hierarchical Approach Single CA certifying every public key is impractical Instead, use a trusted root authority For example, Verisign Everybody must know the public key for verifying root authority s signatures Root authority signs certificates for lower-level authorities, lower-level authorities sign certificates for individual networks, and so on Instead of a single certificate, use a certificate chain sig Verisign ( AnotherCA, PK AnotherCA ), sig AnotherCA ( Alice, PK A ) What happens if root authority is ever compromised? 4/27/16 CSE 484 / CSE M Spring
7 You encounter this every day SSL/TLS: Encryption & authentication for connections (More on this later!) 4/27/16 CSE 484 / CSE M Spring
8 Example of a Certificate 4/27/16 CSE 484 / CSE M Spring
9 X.509 Certificate 4/27/16 CSE 484 / CSE M Spring
10 Hash collisions Many Challenges [more examples in section] Weak security at CAs Allows attackers to issue rogue certificates Users don t notice when attacks happen We ll talk more about this later Etc 4/27/16 CSE 484 / CSE M Spring
11 [Sotirov et al. Rogue Certificates ] Colliding Certificates set by the CA serial number validity period real cert domain name chosen prefix (difference) serial number validity period rogue cert domain name real cert RSA key Valid for both certificates! Hash to the same MD5 value! collision bits (computed)??? X.509 extensions signature identical bytes (copied from real cert) X.509 extensions signature 4/27/16 CSE 484 / CSE M Spring
12 Attacking CAs Security of DigiNotar servers: All core certificate servers controlled by a single admin password (Pr0d@dm1n) Software on publicfacing servers out of date, unpatched No anti-virus (could have detected attack) 4/27/16 CSE 484 / CSE M Spring
13 Consequences Attacker needs to first divert users to an attackercontrolled site instead of Google, Yahoo, Skype, but then For example, use DNS to poison the mapping of mail.yahoo.com to an IP address authenticate as the real site decrypt all data sent by users , phone conversations, Web browsing 4/27/16 CSE 484 / CSE M Spring
14 More Rogue Certs In Jan 2013, a rogue *.google.com certificate was issued by an intermediate CA that gained its authority from the Turkish root CA TurkTrust TurkTrust accidentally issued intermediate CA certs to customers who requested regular certificates Ankara transit authority used its certificate to issue a fake *.google.com certificate in order to filter SSL traffic from its network This rogue *.google.com certificate was trusted by every browser in the world 4/27/16 CSE 484 / CSE M Spring
15 Certificate Revocation Revocation is very important Many valid reasons to revoke a certificate Private key corresponding to the certified public key has been compromised User stopped paying his certification fee to this CA and CA no longer wishes to certify him CA s private key has been compromised! Expiration is a form of revocation, too Many deployed systems don t bother with revocation Re-issuance of certificates is a big revenue source for certificate authorities 4/27/16 CSE 484 / CSE M Spring
16 Certificate Revocation Mechanisms Certificate revocation list (CRL) CA periodically issues a signed list of revoked certificates Credit card companies used to issue thick books of canceled credit card numbers Can issue a delta CRL containing only updates Online revocation service When a certificate is presented, recipient goes to a special online service to verify whether it is still valid Like a merchant dialing up the credit card processor 4/27/16 CSE 484 / CSE M Spring
17 Attempt to Fix CA Problems: Convergence Background observation: Attacker will have a hard time mounting man-in-themiddle attacks against all clients around the world Basic idea: Lots of nodes around the world obtaining SSL/TLS certificates from servers Check responses across servers, and also observe unexpected changes from existing certificates 4/27/16 CSE 484 / CSE M Spring
18 Keybase Basic idea: Rely on existing trust of a person s ownership of other accounts (e.g., Twitter, GitHub, website) Each user publishes signed proofs to their linked account 4/27/16 CSE 484 / CSE M Spring
19 SSL/TLS Secure Sockets Layer and Transport Layer Security protocols Same protocol design, different crypto algorithms De facto standard for Internet security The primary goal of the TLS protocol is to provide privacy and data integrity between two communicating applications Deployed in every Web browser; also VoIP, payment systems, distributed systems, etc. 4/27/16 CSE 484 / CSE M Spring
20 TLS Basics TLS consists of two protocols Familiar pattern for key exchange protocols Handshake protocol Use public-key cryptography to establish a shared secret key between the client and the server Record protocol Use the secret symmetric key established in the handshake protocol to protect communication between the client and the server 4/27/16 CSE 484 / CSE M Spring
21 Basic Handshake Protocol ClientHello C Client announces (in plaintext): Protocol version it is running Cryptographic algorithms it supports Fresh, random number S 4/27/16 CSE 484 / CSE M Spring
22 Basic Handshake Protocol C, version c, suites c, N c ServerHello C Server responds (in plaintext) with: Highest protocol version supported by both the client and the server Strongest cryptographic suite selected from those offered by the client Fresh, random number S 4/27/16 CSE 484 / CSE M Spring
23 Basic Handshake Protocol C, version c, suites c, N c version s, suite s, N s, ServerKeyExchange C Server sends his public-key certificate containing either his RSA, or his Diffie-Hellman public key (depending on chosen crypto suite) S 4/27/16 CSE 484 / CSE M Spring
24 Basic Handshake Protocol C, version c, suites c, N c version s, suite s, N s, certificate, ServerHelloDone C ClientKeyExchange The client generates secret key material and sends it to the server encrypted with the server s public key (if using RSA) S 4/27/16 CSE 484 / CSE M Spring
25 Basic Handshake Protocol C, version c, suites c, N c version s, suite s, N s, certificate, ServerHelloDone C {Secret c } PKs if using RSA C and S share secret key material (secret c ) at this point switch to keys derived from secret c, N c, N s switch to keys derived from secret c, N c, N s S Finished Finished Record of all sent and received handshake messages 4/27/16 CSE 484 / CSE M Spring
26 Core SSL 3.0 Handshake (Not TLS) C, version c =3.0, suites c, N c version s =3.0, suite s, N s, certificate, ServerHelloDone C {Secret c } PKs if using RSA C and S share secret key material (secret c ) at this point switch to keys derived from secret c, N c, N s switch to keys derived from secret c, N c, N s S Finished Finished 4/27/16 CSE 484 / CSE M Spring
27 Version Rollback Attack C, version c =2.0, suites c, N c Server is fooled into thinking he is communicating with a client who supports only SSL 2.0 Version s =2.0, suite s, N s, certificate, ServerHelloDone C {Secret c } PKs if using RSA S C and S end up communicating using SSL 2.0 (weaker earlier version of the protocol that does not include Finished messages) 4/27/16 CSE 484 / CSE M Spring
28 Chosen-Protocol Attacks Why do people release new versions of security protocols? Because the old version got broken! New version must be backward-compatible Not everybody upgrades right away Attacker can fool someone into using the old, broken version and exploit known vulnerability Similar: fool victim into using weak crypto algorithms Defense is hard: must authenticate version in early designs Many protocols had version rollback attacks SSL, SSH, GSM (cell phones) 4/27/16 CSE 484 / CSE M Spring
29 Version Check in SSL 3.0 C, version c =3.0, suites c, N c Embed version number into secret version s =3.0, suite s, N s, certificate for PK s, ServerHelloDone C {version c, secret c } PKs Check that received version is equal to the version in ClientHello S C and S share secret key material secret c at this point switch to key derived from secret c, N c, N s switch to key derived from secret c, N c, N s 4/27/16 CSE 484 / CSE M Spring
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