Introduction to Practical Cryptography

Protocols

Agenda

•

Authentication

• Security Handshakes – One-way – Two-way • Mediated Authentication • Kerberos

Authentication

Prove continuity in relationship – Basis of trust – Identification Who you are (biometrics) Physical authentication: where you are What you know Password: snoopy1 Mother’s maiden name: jones What you have (tokens) Pets name: snoopy

Network Authentication

• Password • One-time Passwords (ex. tokens) • Network address – Caller-id - credit card – IP address – MAC address – banks • Cryptographic protocols

Concerns

• Impersonation • Malicious insiders • Eavesdropping – Keyboard sniffers – Shoulder-surfing – Network sniffers – Trojan horses

2-Way Authentication

• Authentication often needed in both directions • Server trusting user is not only concern – User must trust server – Ex. User accessing online bank account • Variety of “solutions” in user applications

Password-based Authentication

• Proof by sharing • Doesn’t prevent insider attacks (system admin) • What is an appropriate password – length? snoopy, snoopy1, snoopy12 – reusable ? snoppy1, snoopy2, …. snoopy10, snoopy1 – timeframe? • How to do initial password distribution? lastname123, employee# • Simple approach, works with humans … until user has too many to remember – reuse across systems – Variations of something common: dog’s name – post-it on monitor – inconvenient to update, varying rules on what is appropriately complex, how often to change snoopy1, Snoopy1, snoopy-1

Storing Passwords

• Per-node • Central repository • Hashed passwords • Password encryption – Salted passwords

Password Guessing

• Online – Limited tries, exponential delays, alarm – But attacker can temporarily disable a user’s account – ex. 3 tries and account locked until user calls help desk • Offline: “dictionary” attack – Must capture password file – Try "obvious" passwords: snoopy, snoopy1, 1snoopy – Significant dates, easy patterns, personal information – While most systems disallow “dictionary words”, complexity rules still give information – know need a digit, punctuation character … Snoopy-12

Passwords as Keys

• Directly as the key • Convert to secret key – Transient, one-way transformation (hash) – Increase work of attacker • Seed to pseudo-random number generator

One-Time Passwords

• List of passwords used once only • Need to re-initialize periodically

OTPs – List Based

• Example: – Hash password 1000 times, store result on server – Client hashes 999 times, sends to server – Server verifies hash of received value matches stored – result – Store received hash • Must be re-initialized periodically - over secure channel

Lamport’s Hash

• One-time passwords • Server stores n, hash n (password) • User sends x = hash with n-1 , x n-1 (password) • Server computes hash(x), if = stored value, replaces stored value • Safe from eavesdropping, server compromise not a problem for user • No public key cryptography • Authentication not mutual • Add salt to password before hashing – In case Alice uses same password on multiple systems – Salt must be stored on Alice’s system – Server uses hash n (password | salt)

OTP Generators (Tokens)

• Examples – RSA – VASCO Digipass • Use a block cipher – Repeatedly encrypt – Continuously update every x seconds – Update each time user presses button • Some work in both directions – Customer enters OTP – Server returns OTP, customer (manually) compares it to value on token

Tokens - Issues

• Help desk required – Synchronization not perfect – Premature battery death • Cost – $15-$25 – banks with million customers • User still needs pin (something you know + something you have) • “Necklace of Tokens” issue – Only recently integrated with cell phones – Still rare to have multiple tokens on single device • Non-standard algorithms – OATH effort

Cryptographic Authentication

• Tokens, smart cards use crypto • Use a password (or key) in a cryptographic protocol – Prove possession of key – Mutual authentication • Usually coupled with encryption of data after authentication • Certificates – PKI covered in another lecture

Bob:xyz Alice:who Fey: bin Carol: 123 Emily: dog Dave: cat

Pairwise Keying

Bob:xyz Alice:abc Fey: ghj George: 123 Emily: mkl Dave: klj

Trusted Intermediaries (KDC)

Key Distribution Center George-Bob:xyz George-Alice:who George-Fey: bin … Bob:13 Carol: 31 Dave: 7 ….

KDC

• Can impersonate any node • Single point of failure • Potential bottleneck

Certificate Authority

• Central point for certificates • Signs cert for Alice containing her public key • Others need only CA’s public key • Revocation? – Real time with online KDC – Offline CA –expiration date, certificate revocation list

Agenda

• • Authentication

Security Handshakes

– One-way – Two-way • Mediated Authentication • Kerberos

Security Handshakes

• Considerations when creating protocols – Attacks/Information leakage – One-way – Mutual Authentication

Alice

One-way Challenge-Response

I’m Alice

Bob

challenge R f(K,R) K = shared key f: – encryption function – Bob just decrypts and verifies time in within allowed skew – hash – Bob needs to hash all times in allowable interval or Alice sends time Problems?

– Authentication not mutual – Connection hijacking after authentication – attacker spoofs Alice or Bob’s source address and send packets if conversation not encrypted – Off-line password/key attack – depends on length of K – Compromise of database/disk if K is stored, or temporary memory access

One-Way using Timestamp

I’m Alice, f(K,timestamp)

Bob Alice

• Problems?

– Impersonate Alice if intercept and send message – race condition – Keep list of used time stamps to prevent quick replay, but if use same K with multiple servers, could send message to another server and impersonate Alice – Clock skew/synchronization

Alice

One-way Using Public Key

Bob

I’m Alice R [R] Apriv Bob decrypts with Alice’s public key and verifies R was returned.

Alice

Alice proves to Bob she has her private key by returning R I’m Alice [R] Apub R

Bob

[R] Ax = R signed with Alice’s x key, where x is private (priv) or public (pub) key

One-way Problems

• First case: – Can send anything to Alice as R and get Alice to sign it • Second case: – Intercepted an encrypted message for Alice, send it and get Alice to decrypt it

Mutual Authentication

Mutual Authentication with Secret Key

Bob Alice

I’m Alice R1 f(K,R1) R2 f(K,R2)

Mutual Authentication with Secret Key

More efficient version:

Alice

I’m Alice, R2 R1, f(K,R2) f(K,R1)

Bob

Mutual Authentication with Secret Key

Reflection attack:

Trudy

I’m Alice, R2

Bob

Doesn’t know K so can’t send f(K,R1) R1, f(K,R2) I’m Alice, R1

Bob Trudy

Now use f(K,R1) in above attempt R3, f(K,R1) Solutions: •Separate keys for each direction •Requirements on R values: odd in one direction, even in the other, concatenate with senders’ name

Password/Key Guessing

• Also note, Trudy can get Bob to encrypt a value (or a several of values) and then try an offline attack to guess K • Have Bob return R1 value for Alice to encrypt

Alice

I’m Alice R1

Bob

R2, f(K,R1) f(K,R2) Now Bob would have to reuse R1 in order for Trudy, who eavesdrops, to be able to use f(K,R1)

Timestamps

Alice

I’m Alice, f(K,timestamp) f(K,timestamp+1)

Bob

• Same issues as before plus clock skew • Any modification to timestamp will work

Mutual Authentication with Public Keys

Alice

I’m Alice, [R2] Bpub [R1] Apub , R2 R1

Bob

• how to obtains/store/validate Bob’s public key

Session Key

• Once authentication occurs, want to encrypt data exchanged • Session key • If key to one session obtained, can’t decrypt all sessions • Don’t want known/easy to derive relationship among session keys

Agenda

• Authentication • Security Handshakes – One-way – Two-way •

Mediated Authentication

• Kerberos

Mediated Authentication

• Needham-Schroeder • Otway-Rees

Needham-Schroeder

Bob Alice

N1, Alice wants to talk to Bob

KDC

E Ka (N1, "Bob", Kab, ticket) N1 authenticates KDC to Alice ticket = E Kb (Kab, "Alice") E Kab (N2), ticket E Kab (N2 - 1, N3) E Kab (N3 - 1) Bob decrypts ticket to get Kab Nonces used to verify each end has Kab • N1, N2, N3 are nonces ("number used once")

Expanded Needham-Schroeder

• Issue - ticket doesn’t expire • If Trudy obtains Alice’s key and ticket, Trudy can connect to Bob even if Alice changes key.

Alice Bob

I want to talk to you E kb (Nb) N1, Alice wants to talk to Bob, E kb (Nb) E Ka (N1, "Bob", Kab , ticket)

KDC

ticket = E Kb (Kab, "Alice", E kb (Nb)) Nb is unique per session so can’t reuse ticket E Kab (N2), ticket E Kab (N2 - 1, N3) E Kab (N3 - 1)

Needham-Schroeder

• Reflection attack in last steps if ECB mode (and nonce size = block size) Trudy->Bob: E Bob->Trudy: E Trudy->Bob: E Kab Kab Kab (N2), ticket (N2 - 1, N4) (N4), ticket Bob->Trudy: E Kab (N4 - 1, N5) Extract E Kab (N4 - 1) and use in response of first run Trudy doesn’t have Kab to obtain N4, needs N4-1 in next step, so get Bob to encrypt N4-1 Extract 1 st block of ciphertext • CBC solves this

Otway-Rees

Alice

Nc, "Alice", "Bob", E Ka (Na, Nc, "Alice", "Bob")

Bob

Suspicious party should generate challenge E Ka (Na, Nc, "Alice", "Bob"), E Kb (Nb, Nc, "Alice", "Bob")

KDC

Nc, E Ka (Na, Kab), E Kb (Nb, Kab) E Ka (Na, Kab) E Kab (something recognizable) •Bob can’t decipher most of first message – forwards it to KDC •KDC verifies Nc matches in messages from Alice and Bob •KDC gives Bob message to forward to Alice •Alice trusts KDC and Bob are real - KDC would not have continued process if Bob did not have Kb to encrypt Nc and KDC was able to encrypt Na in message containing Kab •Bob trusts KDC – was able to encrypt Nb in message containing Kab •Last message proves to Bob that Alice knows Kab

Encrypted Key Exchange (EKE)

Shared weak secret W = hash(Alice’s password) Bob “Alice”, E W (g a mod p) Alice E W (g b mod p, C1) Stores Alice, W Both compute K = g ab mod p E K (C1,C2) Both compute K = g ab mod p E K (C2) Diffie-Hellman key exchange with exchange encrypted – removes man in middle Mutual authentication If try offline password attack, can’t determine correct plaintext – what is valid plaintext of g a mod p, g b mod p ?

Kerberos

• Based on Needham-Schroeder • Uses time and includes ticket expiration • Later in lecture

Nonces

• Random number – 128-bit value negligible chance of being repeated • Timestamp – Synchronization – Predictable • Sequence number – Maintain state – Predictable?

Random Numbers

• Be careful with seed • Size • Easily known value: timestamp • Divulging seed – don’t use some value included unencrypted in message

Performance

• Number of messages exchanged • Number of operations – using secret key algorithm – using public key algorithm – using hash • And number of bytes involved

Checklist

• Eavesdropping • Initiation of conversation/partial conversations • Impersonation by accepting a connection • Access to disk/database at either end • Man-in-middle

Agenda

• Authentication • Security Handshakes – One-way – Two-way • • Mediated Authentication

Kerberos

Needham-Schroeder

Bob Alice

N1, Alice wants to talk to Bob

KDC

E Ka (N1, "Bob", Kab, ticket) N1 authenticates KDC to Alice ticket = E Kb (Kab, "Alice") E Kab (N2), ticket E Kab (N2 - 1, N3) E Kab (N3 - 1) Bob decrypts ticket to get Kab Nonces used to verify each end has Kab • N1, N2, N3 are nonces ("number used once")

Overview

• Originally developed at MIT • An essential part of Windows servers • Authentication infrastructure – Designed to authenticate users to servers – Users must use their password as their initial key and must not be forced to retype it constantly • Based on Needham-Schroeder, with timestamps to limit key lifetime

Versions

• MIT support: version 4 end-of-life in 2006 – DES – Protocol flaw • Original Needham-Schroeder protocol implicitly requires nonmalleable encryption: prevent an attacker, given a ciphertext, from producing a different ciphertext whose plaintext is meaningfully related to the plaintext of the original ciphertext. • Kerberos version 4 fails to provide by inadequately authenticating its messages. Nonmalleability concept was not well-developed at the time.

– nonstandard checking.

• Version 5 overhead

propagating cipher block chaining

– Implementation flaws – Encoding changes – Optional, variable-length fields, types (PCBC) mode. ciphertext block swaps being undetectable without additional integrity – Fixes/improvements (delegation, ticket lifetime, key versions …) • Adds flexibility, but increases number of bytes per message and processing

Design Goals

• Users only have passwords to authenticate themselves • The network is completely insecure • It’s possible to protect the Kerberos server • The workstations have not been tampered with (not a safe assumption)

Resources Protected

• Network access to home directory • Printer • IM system • Remote login • Anything else that requires authentication

Principals

• A Kerberos entity is known as a

principal

• Could be a user or a system service • Principal names are tuples: V4: <

primary name

,

instance

,

realm

> V5: • The

realm

identifies the Kerberos server

How Kerberos Works

• Users present

tickets

— cryptographically sealed messages with session keys and identities — to obtain a service.

• Use Needham-Schroeder (with password as Alice’s key) to get a

Ticket-Granting Ticket

(TGT); this ticket (and the associated key) are retained for future use during its lifetime.

• Use the TGT (and TGT’s key) in a Needham Schroeder dialog to obtain keys for each actual service

Shared Secret

• Each principal (user, device) shares a secret (master key) with the Kerberos KDC • For users, this is their password (actually, a key derived from the password) • The KDC is assumed to be secure and trustworthy; anything it says can be believed

Kerberos Data Flow

TGT is ticket granting ticket

Obtaining TGT

Alice Alice, password Workstation AS_REQ Alice needs a TGT KDC AS_REP E KA (SA,TGT) Creates session key SA Looks up Alice’s master key KA Create TGT E K_KDC (Alice,SA,expire time) AS_REQ: authentication server request AS_REP: authentication server reply TGT: ticket granting ticket K_KDC is KDC’s master key KDC stateless – sends ticket, does not need to save a copy Workstation sends TGT when Alice needs to access remote resource

Ticket Request

Alice Login to Bob Workstation TGS_REQ Alice wants to talk to Bob TGT Authenticator = E SA (timestamp) TGS_REP E SA (Bob, KAB, ticket to Bob) KDC Creates key KAB Decrypts TGT to get SA Decrypts authenticator to verify timestamp Looks up Bob’s key KB Creates ticket E KB (Alice, KAB)

Access Remote Resource

Workstation (Alice) AP_REQ Ticket to Bob = E KB (Alice, KAB) Authenticator = E KAB (timestamp) AP_REP E KAB (timestamp+1) Bob

Messages

• Message fields listed in text • Part of assigned readings • Know what they mean/are for • Don’t need to memorize list of fields for midterm

Service Tickets

• Service tickets are almost identical to ticket-granting tickets • The differences is that they have the name of a different service — say, “email” — rather than the ticket-granting service • They’re encrypted in a key shared by the KDC and the service

Using Service Tickets

• The client sends the service ticket and an authenticator to the service • The service decrypts the ticket, using its own key • The service knows it’s genuine, because only the KDC knows the key used to produce it • The service verifies that the ticket is for it and not some other service • It uses the enclosed key to decrypt and verify the authenticator • The net result is that the service knows the client’s principal name, extracted from the ticket

Authentication, Not Authorization

• Kerberos is an

authentication

service • It does not (usually) provide authorization • The services know a genuine name for the client, vouched for by the KDC • They then make their own authorization decision based on this name

Bidirectional Authentication

• Sometimes, the client wants to be sure of the server’s identity • It asks the server to prove that it, too, knows the session key • The server replies with E KAB (

timestamp

+1) using the same timestamp as was in the authenticator

Ticket Lifetime

• TGTs typically last about 8–12 hours — the length of a login session • Service tickets can be long- or short-lived, but don’t outlive the TGT • Live tickets are cached by the client • When service tickets expire, they’re automatically and transparently renewed

Inter-Realm Tickets

• A ticket from one realm can’t be used in another, since a KDC in one realm doesn’t share secrets with services in another realm • Realms can issue tickets to each other • A client can ask its KDC for a TGT to another realm’s KDC • The remote realm trusts the user’s KDC to vouch for the user’s identity • It then issues service tickets

with the original realm’s name

for the principal, not its own realm name

Putting Authorization in Tickets

• Under certain circumstances, tickets can contain authorization information known or supplied to the KDC • Windows KDCs use this, to centralize authorization data • As a result, Windows and open source Kerberos KDCs don’t interoperate well. • Users can supply some authorization data, too, to restrict what other services do with

proxy tickets

Proxy Tickets

• Suppose a client wants to print a file • The print spooler doesn’t want to copy the user’s file; that’s expensive • The user obtains a

proxy ticket

granting the print spooler access to its files • The print spooler uses that ticket to read the user’s file

Restricting the Print Spooler

• The client doesn’t want the spooler to have access to all of its files • It lists the appropriate file names in the proxy ticket request; the KDC puts that list of names into the proxy ticket • When the print spooler presents the proxy ticket to a file server, it will only be given those files • Note: the file server must verify that the client has access to those files.

Limitations of Kerberos

• Ticket cache security • Password-guessing • Subverted login command

Ticket Cache Security

• Where are cached tickets stored?

• Often in /tmp — is the OS protection good enough?

• Less of an issue on single-user workstations; often a threat on multi-user machines • Note: /tmp needs to be a local disk, and not something mounted via NFS. . .

Subverting Login

• No great solutions.

• Keystroke loggers are a real threat today • Some theoretical work on secure network booting

Version 5 Changes

Delegation

• Delegation of rights – Alice wants Bob to access resource X on behalf of Alice for time t.

• Example: Alice logs into host Bob then wants to log into host X from Bob – Alice can request ticket with Bob’s address or a list of addresses – Ticket can include application specific data – not used by Kerberos but used by host • Can set to not allow delegation

Ticket Lifetime

• V4: 21 hours max • V5: up to Dec. 31, 9999 • Lifetime in seconds • Not revocable – be careful • Time ticket granted, start time and stop time • Renew until – instead of long lifetime, give option to keep renewing – If stop using/needing ticket, won’t remain open • Postdating • Grant ticket to run some process in future – Batch job at end of week but requested ticket at beginning of week

Key Version

• Suppose Alice has ticket to Bob • Bob changes his key with KDC • KDC keeps versions both versions of Bob’s key (key, version) • Alice’s ticket keeps working until it expires • Any other renewable or post-dated ticket will work with old key

Master Keys and Realms

• Master keys – key between entity (such as Alice) and KDC • Alice registered to realms R1, R2 – Uses same password • Hash password with realm name to get master key • If attacker gets Alice’s password, still can compute both master keys • But R1 and R2 don’t have the other’s master key for Alice. If attacker breaks into one, won’t get both keys.

Repairs

• Insert new cipher (AES) to replace DES • Checksum fix for integrity – replaced by choice of algorithm • PCBC – replaced by choice (AES-CBC common)

Hierarchy of Realms

A B C D E F G H I J H to D, go through E then B List transited realms in ticket, don’t give transited realms power to impersonate others If don’t trust one realm, all realms visited after that one are suspect

Hierarchy of Realms

A May have short cut links F talks directly to B instead of C then A then B B C D G E H I F J

Password Guessing

• V4: anyone can send ticket request to KDC Alice wants to talk to Bob Trudy KDC • V5: include encrypted timestamp Alice Alice wants to talk to Bob, E Ka (timestamp) KDC

Password-Guessing

• Kerberos tickets have verifiable plaintext • An attacker can run password-guessing programs on intercepted ticket-granting tickets (Merritt and Bellovin invented EKE while studying this problem with Kerberos.) • Kerberos uses

passphrases

instead of

passwords

• Does this make guessing harder? Not sure

Password Guessing

• On many Kerberos systems, anyone can ask the KDC for a TGT • There’s no need to eavesdrop to get them — you can get all the TGTs you want over the Internet.

• Solution:

preauthentication

• The initial request includes a timestamp encrypted with Kc • It’s still verifiable plaintext, but collecting TGTs becomes harder again

Multiple Sessions – Added Security

• Alice opens two sessions to Bob • Don’t want Trudy swapping messages between sessions • Alice specifies different session key to use for each

Other Protocols in Practice

• SSH • TLS • IPsec • Application aware – transport layer or above • Application not aware - IP layer