Notes

Outline

Practical Aspects of Modern Cryptography Josh Benaloh & Brian LaMacchia

Lecture 5: SSL/TLS in depth, Certificates & Trust

SSL/PCT/TLS History 1994:  Secure Sockets Layer (SSL) V2.0 1995:  Private Communication Technology (PCT) V1.0 1996:  Secure Sockets Layer (SSL) V3.0 1997:  Private Communication Technology (PCT) V4.0 1999:  Transport Layer Security (TLS) V1.0

SSL/TLS You (client) Merchant (server)

SSL/TLS You (client) Merchant (server)

SSL/TLS You (client) Merchant (server)

SSL/TLS You (client) Merchant (server)

SSL/TLS You (client) Merchant (server)

SSL/TLS All subsequent secure messages are sent using the symmetric key and a keyed hash for message authentication.

The five phases of SSL/TLS Negotiate the ciphersuite to be used Establish the shared session key Client authenticates the server Optional, but almost always done Server authenticates the client Optional, and almost never done Authenticate previously exchanged data

Phase 1: Ciphersuite Negotiation Client hello (clientèserver) “Hi! I speak these n ciphersuites, and here’s a 28-byte random number (nonce) I just picked” Server hello (clientçserver) “Hello. We’re going to use this particular ciphersuite, and here’s a 28-byte nonce I just picked.” Other info can be passed along (we’ll see why a little later...)

TLS ciphersuites TLS_NULL_WITH_NULL_NULL TLS_RSA_WITH_NULL_MD5 TLS_RSA_WITH_NULL_SHA TLS_RSA_EXPORT_WITH_RC4_40_MD5 TLS_RSA_WITH_RC4_128_MD5 TLS_RSA_WITH_RC4_128_SHA TLS_RSA_EXPORT_WITH_RC2_CBC_40_MD5 TLS_RSA_WITH_IDEA_CBC_SHA TLS_RSA_EXPORT_WITH_DES40_CBC_SHA TLS_RSA_WITH_DES_CBC_SHA TLS_RSA_WITH_3DES_EDE_CBC_SHA TLS_DH_DSS_EXPORT_WITH_DES40_CBC_SHA TLS_DH_DSS_WITH_DES_CBC_SHA TLS_DH_DSS_WITH_3DES_EDE_CBC_SHA TLS_DH_RSA_EXPORT_WITH_DES40_CBC_SHA TLS_DH_RSA_WITH_DES_CBC_SHA TLS_DH_RSA_WITH_3DES_EDE_CBC_SHA TLS_DHE_DSS_EXPORT_WITH_DES40_CBC_SHA TLS_DHE_DSS_WITH_DES_CBC_SHA TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA TLS_DHE_RSA_EXPORT_WITH_DES40_CBC_SHA TLS_DHE_RSA_WITH_DES_CBC_SHA TLS_DHE_RSA_WITH_3DES_EDE_CBC_SHA TLS_DH_anon_EXPORT_WITH_RC4_40_MD5 TLS_DH_anon_WITH_RC4_128_MD5 TLS_DH_anon_EXPORT_WITH_DES40_CBC_SHA TLS_DH_anon_WITH_DES_CBC_SHA TLS_DH_anon_WITH_3DES_EDE_CBC_SHA

Phase 2: Establish the shared session key Client key exchange Client chooses a 48-byte “pre-master secret” Client encrypts the pre-master secret with the server’s RSA public key Clientèserver encrypted pre-master secret Client and server both compute PRF (pre-master secret, “master secret”, client nonce + server nonce) PRF is a pseudo-random function First 48 bytes output from PRF form master secret

TLS’s PRF PRF(secret, label, seed) = P_MD5(S1, label + seed) XOR P_SHA-1(S2, label + seed); where S1, S2 are the two halves of the secret P_hash(secret, seed) = HMAC_hash(secret, A(1) + seed) + HMAC_hash(secret, A(2) + seed) + HMAC_hash(secret, A(3) + seed) + ... A(0) = seed A(i) = HMAC_hash(secret, A(i-1))

Phases 3 & 4: Authentication More on this in a little bit...

Phase 5: Authenticate previously exchanged data “Change ciphersuites” message Time to start sending data for real... “Finished” handshake message First protected message, verifies algorithm parameters for the encrypted channel 12 bytes from: PRF(master_secret, “client finished”, MD5(handshake_messages) + SHA-1(handshake_messages))

Why do I trust the server key? How do I know I’m really talking to Amazon.com? What defeats a man-in-the-middle attack?

Why do I trust the server key? How do I know I’m really talking to Amazon.com? What defeats a man-in-the-middle attack?

SSL/TLS You (client) Merchant (server)

What’s the “some other stuff” How can we convince Alice that some key belongs to Bob? Alice and Bob could have met previously & exchanged keys directly. Jeff Bezos isn’t going to shake hands with everyone he’d like to sell to... Someone Alice trusts could vouch to her for Bob and Bob’s key A third party can certify Bob’s key in a way that convinces Alice.

What is a certificate? A certificate is a digitally-signed statement that binds a public key to some identifying information. The signer of the certificate is called its issuer. The entity talked about in the certificate is the subject of the certificate. That’s all a certificate is, at the 30,000’ level.

Certificates are Like Marriage By the power vested in me I now declare this text and this bit string “name” and “key.” What RSA has joined, let no man put asunder. --Bob Blakley

Certs in the “real world” A driver’s license is like a certificate It is a “signed” document (sealed, tamper-resistant) It is created and signed by an “issuing authority” (the WA Dept. of Licensing) It binds together various pieces of identifying information Name License number Driving restrictions (must wear glasses, etc.)

More certs in the real world Many physical objects are like certificates: Any type of license – vehicle tabs, restaurant liquor license, amateur radio license, etc. Government-issued IDs (passports, green cards) Membership cards (e.g. Costco, discount cards) All of these examples bind an identity and certain rights, privileges or other identifiers “BAL ==N1TJT” signed FCC

Why do we believe what certs say? In the physical world, why do we trust the statements contained on a physical cert? We believe it’s hard to forge the cert We trust the entity that “signed” the cert In the digital world we need those same two properties We need to believe it’s hard to forge the digital signature on a signed document We need to trust the issuer/signer not to lie to us

Defeating Mallet Bob can convince Alice that his key really does belong to him if he can also send along a digital certificate Alice will believe & trust

Demo – SSL/TLS in a web browser

Getting a certificate How does Bob get a certificate for his key? He goes to a Certificate Authority (CA) that issues certificates and asks for one... Bob engages in a “certificate enrollment protocol” with the CA.

Certificate Authorities A certificate authority (CA) guarantees the connection between a key and an “end entity.” An end entity is: A person A role (“VP of sales”) An organization A pseudonym A piece of hardware or software An account Some CA’s only allow a subset of these types.

Certificate Enrollment Enrollment is the process of obtaining a certificate from a CA. Alice generates a key pair, creates a message containing a copy of the public key and her identifying information, and signs the message with the private key. Signing the message provided “proof-of-possession” (POP) of the private key as well as message integrity CA verifies Alice’s signature on the message

Certificate Enrollment (2) (Optional) CA verifies Alice’s ID through out-of-band means. CA creates a certificate containing the ID and public key, and signs it with the CA’s own key CA has certified the binding between key and ID Alice verifies the key, ID & CA signature Alice and/or the CA publish the certificate

Certificate Enrollment Flow

Using Certificates Now that Bob has a certificate, is it useful? Alice will believe Bob’s key belongs to Bob if Alice believes the certificate Bob gives her for his key. Alice will believe Bob’s key belongs to Bob if Alice trusts the issuer of Bob’s certificate to make key-name binding statements Have we made the situation any better?

Defeating Mallet Bob can convince Alice that his key really does belong to him if he can also send along a digital certificate Alice will believe & trust

Does Alice Trust Bob’s CA? How can we convince Alice to trust Bob’s CA? Alice and Bob’s CA could have met previously & exchanged keys directly. Bob’s CA isn’t going to shake hands with everyone he’s certified, let alone everyone whom Bob wants to talk to. Someone Alice trusts could vouch to her for Bob’s CA and Bob’s CA’s key Infinite Loop: See Loop, Infinite. Actually, it’s just a bounded recursion...

What’s Alice’s Trust Model Alice has to implicitly trust some set of keys Once she does that, those keys can introduce others to her. In the model used by SSL/TLS, CAs are arranged in a hierarchy Alice, and everyone else, trusts one or more “root CA” that live at the top of the tree Other models work differently (next class)

CA Hierarchies CAs can certify other CAs or “end entities” Certificates are links in a tree of EEs & CAs

BAL’s No-Frills Certs Certificates can contain all sorts of information inside them In abstract, they’re just statements by an issuer about a subject

Does Alice trust Bob’s Key? Alice trusts Bob’s key if there is a chain of certificates from Bob’s key to a root CA that Alice implicitly trusts

Chain Building & Validation “Given an end-entity certificate, does there exist a cryptographically valid chain of certificates linking it to a trusted root certificate?”

Demo – Viewing cert chains

Expiration & Revocation Certificates (at least, all the ones we’re concerned with) contain explicit validity periods – “valid from” & “expires on” Expiration dates help bound the risk associated with issuing a certificate Sometimes, though, it becomes necessary to “undo” a certificate while it is still valid Key compromise Cert was issued under false pretenses This is called revoking a certificate

Certificate Revocation A CA revokes a certificate by placing the cert on its Certificate Revocation List (CRL) Every CA issues CRLs to cancel out issued certs A CRL is like anti-matter – when it comes into contact with a certificate it lists it cancels out the certificate Think “1970s-style credit-card blacklist” Relying parties are expected to check CRLs before they rely on a certificate “The cert is valid unless you hear something telling you otherwise”

The Problem with CRLs Blacklists have numerous problems Not issued frequently enough to be effective against a serious attack Expensive to distribute (size & bandwidth) Vulnerable to simple DOS attacks If you block on lack of CRL access, why have off-line support in the first place?

The Problem with CRLs (2) CRL design made it worse CLRs can contain retroactive invalidity dates A CRL issued today can say a cert was invalid as of last week. Checking that something was valid at time t wasn’t sufficient! Back-dated CRLs can appear at any time in the future If you rely on certs & CRLs you’re screwed because the CA can change the rules out from under you later.

The Problem with CRLs (3) Revoking a CA cert is more problematic than revoking an end-entity cert When you revoke a CA cert, you potentially take out the entire subordinate structure, depending on what chaining logic you use How do you revoke a self-signed cert? “The cert revokes itself.” Huh? Do I accept the CRL as valid & bounce the cert? Do I reject the CRL because the cert associated with the CRL signing key was revoked?

The Problem with CRLs (4) You can’t revoke a CRL Once you commit to a CRL, it’s a valid state for the entirety of its validity period What happens if you have to update the CRL while the CRL you just issued is still valid? You can update it, but clients aren’t required to fetch it since the one they have is still valid! Bottom line: yikes! We need something else

Online Status Checking OCSP: Online Certificate Status Protocol A way to ask “is this certificate good right now? Get back a signed response from the OCSP server saying, “Yes, cert C is good at time t” Response is like a “freshness certificate” OCSP response is like a selective CRL Client indicates the certs for which he wants status information OCSP responder dynamically creates a lightweight CRL-like response for those certs

Final thoughts on Revocation From a financial standpoint, it’s the revocation data that is valuable, not the issued certificate itself For high-valued financial transactions, seller wants to know your cert is good right now Same situation as with credit cards, where the merchant wants the card authorized right now at the point-of-sale Card authorizations transfer risk from merchant to bank – thus they’re worth $$$ Same with cert status checks

Chaining Certificates In theory, building chains of certificates should be easy “Just link them together like dominos” In practice, it’s a lot more complicated...

Chain Building Details (1)

Chain Building Details (2)

Chain Building Details (3)

Chaining Certificates How do we determine whether two certificates chain together? You’d think this was an easy problem... But it’s actually a question with religious significance in the security community “Are you a believer in names, or in keys?” In order to understand the schism, we need to digress for a bit and talk about names and some history

The X.500 Directory Model The model SSL/TLS uses, the X.509 certificate model, is based on names Names as principles Specifically, X.509 is based on the X.500 directory model X.500 defined a global, all-encompassing directory, to be run by the telcos One directory to rule them all, one directory to define them...

X.500 Distinguished Names In the X.500 model, everything has a single, unique, global, assigned name There is a worldwide hierarchy, and you’re in it!

X.500 DNs Typical X.500 DN C=US/ L=Area 51/ O=Hanger 18/ OU=X.500 credential acquisition for extra-terrestrial visitors/ CN=John Whorfin When the X.500 revolution comes, your DN will be lined up against the wall and shot

Problems with X.500 DNs No one ever figured out how to make them work No clear plan on how to organize the one global hierarchy People couldn’t even agree on the meaning of “localities” Hierarchical naming model fits the military & governments real well, but doesn’t work well for businesses & individuals

Problems with X.500 DNs (2) Consider the following simple cases Communal living (jails, college dormitories) Nomadic peoples Merchant ships Quasi-permanent non-continental structures Oil drilling platforms US APO addresses

Problems with X.500 DNs (3) What is C, SP, L for a corporation? Location of headquarters? Location of office where the CA is located? Location of incorporation? What is C, SP, L for a person? Current residence? Place of birth? Place of work? Solution: Define in the certificate practice statement (CPS), incorporated by reference in the cert, which no one but lawyers ever reads.

DNs in Practice Name is unique within the scope of the CA’s name Public CAs (e.g. Verisign) typically set C = CA Country O = CA Name OU = Certificate type/class CN = User name E= email address

Private-label DNs If you own the CA, you get to decide what fields go in the DN Really varies on what the software supports Can get really strange as people try to guess values for fields that are required by software Software requires an OU, we don’t have OUs, so I better make something up!