Lecture 6: Web security: SSL

Anish Arora CSE 5473

Introduction to Network Security

Web security needs

• Web now widely used by business, government, individuals, but Internet & Web are vulnerable

• Threats: Revealing private information on server Interception of client information (client card fraud and abuse) Content hijacking (mimics of well known sites) and defacement Hostile content (recursive window pop ups) Cookies reveal where users have been, how many times ads have been

displayed (user profiling), allow impersonation Web server attacks:

Buffer overflow (longer than expected parameters) Dot dot URL traversal allows file system access Allowing attacker to execute programs (editors, xterm), root access

Denial of service

Web security needs

• CGI scripts clients can invoke services via HTML macros run on server side scripts allow encoding of commands server runs at root (port 80)

• Requirements include: integrity confidentiality dealing with denial of service authentication

Web security

Need added security mechanisms authentication can be based on message digest cookies can help access control via network addresses multi-layered: SHTTP (just for http) + SSL (generic for TCP)

+ IPSec (host to host)

SSL (Secure Socket Layer)

• transport layer security service, yields secure channel secure byte stream optional public-key server authentication optional client authentication

• originally developed by Netscape hash: combined MD5 & SHA encryption: Diffie Helman, RSA & DES, RC4

• version 3 designed with public input; subsequently became Internet standard TLS (Transport Layer Security)

• uses TCP to provide a reliable end-to-end service useful for HTTP, NNTP, telnet, telephony signaling

• SSL has two layers of protocols

SSL architecture

SSL architecture

• SSL session an association between client & server created by the Handshake Protocol define a set of cryptographic parameters may be shared by multiple SSL connections

• SSL connection a transient, peer-to-peer, communications link associated with 1 SSL session

SSL record protocol

• confidentiality using symmetric encryption with a shared secret key defined by

Handshake Protocol stateful protocol

IDEA, RC2-40, DES-40, DES, 3DES, Fortezza, RC4-40, RC4-128 message is compressed before encryption

• message integrity using a MAC with shared secret key similar to HMAC but with different padding

SSL record protocol operation

SSL record format

Crypto for Record Protocol

• Use IV (one of the parts of the keys exchanged) for seed in CBC encryption mode

• IV is used for first message• Last ciphertext of previous message is used as IV for new

message

• HMAC is appended to M; padding is used

SSL record protocol payload

SSL change cipher specification protocol

• one of 3 SSL specific protocols which use the SSL record protocol

• a single message• causes pending state to become current• hence updating the cipher suite in use

SSL alert protocol

• conveys SSL-related alerts to peer entity

• severity warning or fatal

• specific alert unexpected message, bad record mac, decompression failure,

handshake failure, illegal parameter close notify, no certificate, bad certificate, unsupported certificate,

certificate revoked, certificate expired, certificate unknown

• compressed & encrypted like all SSL data

SSL handshake protocol

• allows server & client to: authenticate each other to negotiate encryption & MAC algorithms to negotiate cryptographic keys to be used

• comprises a series of messages in phases establish Security Capabilities Server Authentication and Key Exchange Client Authentication and Key Exchange finish

What’s wrong with this handshake protocols?

Alice Bob

Can we talk?, cipher list cipher, certificate (Bob)

{K}Bob

Data protected with key K

Simplified SSL protocol

Alice Bob

Can we talk?, cipher list, NA

cipher, certificate (Bob), NB

{S}Bob, E(h(msgs,CLNT,K),K)

Data protected with key K

• S is pre-master secret, derived from random with PKCS padding and including client version number

• Key K = h(S,NA,NB)• K has three parts: (i) for encryption, (ii) for IV, (iii) for hash• msgs = all previous messages; CLNT and SRVR are constant

h(msgs,SRVR,K)

Certificate Verification

• Look at a browser, the certificate associated with a secure server, and its certificate chain

• Browser has embedded in it some certificate along this chain

• Basis for checking that certificate received in the second step is valid

SSL handshake protocol

• Colored messages are optional• Phase 1-3 messages are plaintext

SSL handshake protocol

Phase 1CS: Client hello (to establish security enhancement capabilities)

random challenge, which algorithms are supported server chooses encryption, compression algorithms

S C: Server hello: acknowledges algorithms established random connection id

Phase 2S C: Server certificate

server returns a X.509 certificate chain optionally asks for client certificate

SSL handshake protocol

Phase 3• Client key exchange

client verifies certificate chain against that in web browser if not in list of CAs, may trust the new certificate client generates 48 byte pre-secret

CS: session key exchange: pre-secret encrypted w/ server’s public key in certificate both sides can now generate master secret session key now generated from master secret client hello random provides “salt”

SSL handshake protocol

SC: optionally ask for request certificate (in phase 2) send challenge phrase, encrypted with server write key (in phase 3) client responds with encrypted

MD5(server challenge and certificate), client certificate server verifies certificate and hash certificate used to associate with username&pwd, for future logins

Phase 4:• Finished message exchange, encrypted with session key

verifies success of key exchange and authentication processes

Phase 5• Now encrypted application data exchanged between C, S

TLS (Transport Layer Security)

• IETF standard RFC 2246 similar to SSLv3

• with minor differences in record format version number uses HMAC for MAC a pseudo-random function expands secrets has additional alert codes some changes in supported ciphers changes in certificate negotiations changes in use of padding