Transport Level
Security
Raj Jain
Washington University in Saint Louis
Saint Louis, MO 63130
Jain@cse.wustl.edu
Audio/Video recordings of this lecture are available at:
http://www.cse.wustl.edu/~jain/cse571-14/
Washington University in St. Louis CSE571S ©2014 Raj Jain
17-1
Overview
1. Secure Sockets Layer (SSL)
2. Transport Layer Security (TLS)
3. HTTPS
4. Secure Shell (SSH)
These slides are based partly on Lawrie Brown’s slides supplied with William Stallings’s
book “Cryptography and Network Security: Principles and Practice,” 6th Ed, 2013.
Washington University in St. Louis CSE571S ©2014 Raj Jain
17-2
Web Traffic Security Approaches
SSL/TLS provides the following services over TCP layer :
1. Crypto negotiation: Negotiate encryption and hash
methods
2. Key Exchange: Secret key exchange using public key
certificates
3. Privacy: Encryption using secret key
4. Integrity: Message authentication using a keyed hash
Washington University in St. Louis CSE571S ©2014 Raj Jain
17-3
History
SSL was developed by Netscape. V1 was never deployed. V2
had major issues.
SSL v3 is most commonly deployed protocol
IETF standardized SSL V3 with some upgrades as Transport
Layer Security (TLS) V1 in RFC 2246 1999
TLS is encoded as SSL V3.1
The differences are small but the protocols do not interoperate.
TLS v1.1 (SSL V3.2) added protection against CBC attacks
[RFC 4346 2006]
Ref: http://en.wikipedia.org/wiki/Transport_Layer_Security
Washington University in St. Louis CSE571S ©2014 Raj Jain
17-4
History (Cont)
TLS v1.2 (SSL V3.3) in RFC 5246 August 2008 added:
MD5-SHA-1 pseudorandom function (PRF) replaced with
SHA-256
MD5-SHA-1 Finished message hash replaced with SHA-
256
MD5-SHA-1 in digitally-signed element replaced with a
single hash negotiated during handshake, default=SHA-1.
Enhanced Client's and server's specification for hash and
signature algorithms
Expansion of support for authenticated encryption ciphers
TLS Extensions definition and Advanced Encryption
Standard Cipher Suites
RFC 6176 updated TLS v1.2 by requiring that SSL V2 is never
accepted.
Ref: http://en.wikipedia.org/wiki/Transport_Layer_Security#TLS_1.3_.28draft.29
Washington University in St. Louis CSE571S
Must Read ©2014 Raj Jain
17-5
SSL Architecture
SSL has 4 components in two layers
1. Handshake protocol: Negotiates crypto parameters for a
“SSL session” that can be used for many “SSL/TCP
connections”
2. Record Protocol: Provides encryption and MAC
3. Alert protocol: To convey problems
4. Change Cipher Spec Protocol: Implement negotiated crypto
parameters
Washington University in St. Louis CSE571S ©2014 Raj Jain
17-6
SSL Handshake Protocol
Allows server and client to:
Authenticate each other
To negotiate encryption & MAC algorithms
To negotiate cryptographic keys to be used
Comprises a series of messages in phases
1. Establish Security Capabilities
2. Server Authentication and Key Exchange
3. Client Authentication and Key Exchange
4. Finish
Washington University in St. Louis CSE571S ©2014 Raj Jain
17-7
SSL Handshake Protocol Actions
Client Client Hello: Crypto Choices (Protocol Version, Cipher Suite, Compression, RClient Server
Server Hello: Crypto Selected, RServer
Certificate: Server Certificate (Optional)
Server Key Exchange (Optional)
Certificate Request (Optional)
Server Hello Done
Generate Certificate: Client Certificate
random
PMS S Client Key Exchange: E(Kserver Public Key, PreMasterSecret)
Compute Compute
MS K
Certificate Verify MS K
Change Cipher Spec
Handshake Finished: Hash and MAC of Previous messages
Change Cipher Spec
Handshake Finished
Washington University in St. Louis CSE571S ©2014 Raj Jain
17-8
Handshake Messages
All messages are Type-Length-Value (TLV) encoded.
Types
1 = Client Hello: Highest Version Supported, RClient, Session ID, Cipher Suites,
Compressions
2 = Server Hello: Version Accepted, RServer, Session ID, Chosen Cipher,
Chosen Compression
14 = Server Hello Done
16 = Client Key Exchange: Encrypted pre-master key
12 = Server Key Exchange: Modulus p, Exponent g, Signature (export only)
13 = Certificate Request: CA Names (requested by server)
11 = Certificate: sent by server
15 = Certificate Verify: Signature of Hash of messages
20 = Handshake Finished: MD5 and SHA Digest of message halves
Washington University in St. Louis CSE571S ©2014 Raj Jain
17-9
Security Capability Negotiation
Key-Exchange Methods:
RSA
Fixed D-H: Shared secret generated using fixed public keys
Ephemeral D-H: Ephemeral = Temporary, one-time secret key is
generated after certificate exchange and authentication
Anonymous D-H: No authentication. Only public key exchange.
Subject to MITM attack
Fortezza: Using PC-Cards (http://en.wikipedia.org/wiki/Fortezza )
CipherSpec:
Cipher Algorithm: RC4, RC2, DES, 3DES, DES40, IDEA, or Fortezza
MAC Algorithm: MD5 or SHA-1
CipherType: Stream or Block
IsExportable: True or False
HashSize: 0, 16 (for MD5), or 20 (for SHA-1) bytes
Key Material: info used to generate keys
IV Size: Size of IV for CBC
Ref: http://en.wikipedia.org/wiki/Cipher_suite
Washington University in St. Louis CSE571S ©2014 Raj Jain
17-10
Cryptographic Computations
Master secret creation
A one-time 48-byte value based on nonces
A 48-byte pre-master secret is exchanged/generated using
secure key exchange (RSA / Diffie-Hellman) and then
hashing:
Master_Secret = MD5(Pre_master_Secret || SHA(‘A’) ||
pre_master_secret || clientHello.random ||
ServerHello.random)) ||….
Generation of cryptographic parameters
Client write MAC secret, a server write MAC secret, a
client write key, a server write key, a client write IV, and a
server write IV
Generated by hashing master secret
Washington University in St. Louis CSE571S ©2014 Raj Jain
17-11
SSL Change Cipher Spec Protocol
A single 1-byte message
Causes negotiated parameters to become current
Hence updating the cipher suite in use
Washington University in St. Louis CSE571S ©2014 Raj Jain
17-12
SSL Alert Protocol
Conveys SSL-related alerts to peer entity
Two byte message: Level-Alert, level = warning or fatal,
fatal Immediate termination
0 Close notify (warning or fatal)
10 Unexpected message (fatal)
20 Bad record MAC (fatal)
21 Decryption failed (fatal, TLS only)
22 Record overflow (fatal, TLS only)
41 No certificate (SSL v3 only) (warning or fatal)
42 Bad certificate (warning or fatal)
43 Unsupported certificate (warning or fatal)
44 Certificate revoked (warning or fatal)
45 Certificate expired (warning or fatal)
….
Washington University in St. Louis CSE571S ©2014 Raj Jain
17-13
SSL Record Protocol Services
Confidentiality
Using symmetric encryption with a shared secret key
defined by Handshake Protocol
AES, 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
Washington University in St. Louis CSE571S ©2014 Raj Jain
17-14
SSL Record Protocol Operation
Washington University in St. Louis CSE571S ©2014 Raj Jain
17-15
Encoding
All exchanges are in records up to 214B or 216-1B.
Standard allows multiple messages in one record or multiple
records.
Most implementations use one message per record.
Four Record Types:
20 = Change Cipher Spec
21 = Alerts (1 = Warning, 2 = Fatal)
22 = Handshake
23 = Application Data
Record header: Record Type Version # Rec Length
1B 2B 2B
Each message starts with a 1B message-type and 3B message
length. Msg Type Msg Len Msg
Washington University in St. Louis CSE571S ©2014 Raj Jain
17-16
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
Based on HMAC using SHA-1 or MD5
Has additional alert codes
Some changes in supported ciphers
Changes in certificate types & negotiations
Changes in crypto computations & padding
Washington University in St. Louis CSE571S ©2014 Raj Jain
17-17
HTTPS
HTTPS (HTTP over SSL)
Combination of HTTP & SSL/TLS to secure
communications between browser & server
Documented in RFC2818
No fundamental change using either SSL or TLS
Use https:// URL rather than http://
And port 443 rather than 80
Encrypts URL, document contents, form data, cookies, HTTP
headers
Ref: http://en.wikipedia.org/wiki/HTTP_Secure
Washington University in St. Louis CSE571S ©2014 Raj Jain
17-18
HTTPS Use
Connection initiation
TLS handshake then HTTP request(s)
Connection closure
Have “Connection: close” in HTTP record
TLS level exchange close_notify alerts
Can then close TCP connection
Must handle abnormal TCP close before alert exchange sent
or completed
Washington University in St. Louis CSE571S ©2014 Raj Jain
17-19
Secure Shell (SSH)
Secure remote login
SSH1 provided secure remote logon facility
Replace TELNET & other insecure schemes
Also has more general client/server capability
SSH2 fixes a number of security flaws
Documented in RFCs 4250 through 4254
SSH clients & servers are widely available
Ref: http://en.wikipedia.org/wiki/Secure_Shell
Washington University in St. Louis CSE571S ©2014 Raj Jain
17-20
SSH Protocol Layers
IP: Routes messages to destination
TCP: end-to-end reliable delivery
SSH Transport Layer Protocol:
Server authentication, confidentiality, integrity
May optionally provide compression
SSH User Authentication Protocol: Authenticates client
SSH Connection Protocol: Provided multiple logical channels
2. SSH User Authentication Protocol 3. SSH Connection Protocol
1. SSH Transport Layer Protocol
TCP
IP
Washington University in St. Louis CSE571S ©2014 Raj Jain
17-21
SSH Transport Layer
Server Authentication,
Privacy and Integrity
Client must know the
servers public key in
advance
Padding Length
Packet Length
Ref: RFC 4253
Washington University in St. Louis CSE571S ©2014 Raj Jain
17-22
SSH User Authentication Layer
Authenticates client to server
Three message types:
SSH_MSG_USERAUTH_REQUEST
SSH_MSG_USERAUTH_FAILURE
SSH_MSG_USERAUTH_SUCCESS
Authentication methods used: Public-key, password, host-based
Client Server
SSH_MSG_USERAUTH_REQUEST
Method=None
SSH_MSG_USERAUTH_FAILURE
Accept public_key, password
SSH_MSG_USERAUTH_REQUEST
Method=Password, my password
SSH_MSG_USERAUTH_SUCCESS
Ref: RFC 4252
Washington University in St. Louis CSE571S ©2014 Raj Jain
17-23
SSH Connection Layer
Runs on SSH Transport Layer Protocol
Assumes secure authentication
connection
Used for multiple logical channels
SSH communications use separate
channels
Either side can open with unique id
number
Flow controlled
Have three stages:
Opening a channel, data
transfer, closing a channel
Four types:
Session, x11, forwarded-tcpip
(remote port forwarding),
direct-tcpip (local port
forwarding).
Ref: RFC 4254
Washington University in St. Louis CSE571S ©2014 Raj Jain
17-24
Port Forwarding
Application Application Application Application
Firewall Firewall
Host H Host W Host S Host H Host W Host S
SSH SSH SSH SSH
x a b c d y x a b c d y
TCP TCP TCP TCP TCP TCP
(a) Local Forwarding (a) Remote Forwarding
ssh –La:S:y W ssh –Ra:S:y H
Port forwarding or tunneling allows insecure applications to run over secure SSH.
SSH tells location application to connect to H:a rather than S:y. SSH listens to H:a,
encrypts the traffic and sends to other side where SSH sends to S:y.
Note: All TCP connections are bidirectional. Arrows show the TCP connect
message direction. If application server is on W, “localhost” is used in place of S.
Local forwarding: Client SSH (Host H) starts the tunnel, informs the server SSH
(Host W): “Please forward the traffic on this channel to S:y”
Remote Forwarding: Client SSH (Host W) starts the tunnel, informs the server SSH
(Host H): “I will forward the traffic on this channel to S:y”
Ref: http://docstore.mik.ua/orelly/networking_2ndEd/ssh/ch09_02.htm
Washington University in St. Louis CSE571S ©2014 Raj Jain
17-25
Summary
1. SSL provides security at transport layer. TLS is a
standardization of SSL V3.
2. SSL consists of 4 protocols: Handshake (Crypto Negotiation),
Change Cipher, Alert, and Record (Encryption and MAC)
3. HTTPS is simply http over SSL.
4. SSH provides secure remote login and consists of 3 protocols:
User authentication, Connection (Channels), Transport layer
(Encryption, MAC, Server authentication)
5. SSH port forwarding (tunneling) allows insecure applications
to run in a secure mode.
Washington University in St. Louis CSE571S ©2014 Raj Jain
17-26
Homework 17
Consider the following threats to Web security and describe how each is encountered by a
particular feature of SSL.
A. Brute-Force Cryptanalytic Attack: An exhaustive search of the key space for a conventional
encryption algorithm
B. Know Plaintext Dictionary Attack: Many messages will contain predictable plain text, such as
the HTTP GET command. An attacker constructs a dictionary containing every possible
encryption of the known-plaintext message. When an encrypted message is intercepted, the
attacker takes the portion containing the encrypted known plaintext and looks up the ciphertext
in the dictionary. The ciphertext should match against an entry that was encrypted wit the same
secret key. If there are several matches, each of these can be tried against the full ciphertext to
determine the right one. This attack is especially effective against small key sizes (e.g., 40-bite
keys).
C. Replay Attack: Earlier SSL handshake messages are replayed.
D. Man in the middle Attack: AN attacker interposes during key exchange, active as the client to
the server and as the server to the client.
E. Password Sniffing: Passwords in HTT or other application traffic are eaves dropped.
F. IP Spoofing: Uses forced IP addresses to fool a host into accepting bogus data.
G. IP Hijacking: An active, authenticated connection between two hosts is disrupted and the
attacker takes the place of one of the hosts.
H. SYN Flooding: An attacker sends TCP SYN messages to request a connection but does not
respond to the final message to establish the connection fully. The attacked TCP module
typically leaves the “half-open connection” around for a few minutes. Repeated SYN messages
can clog the TCP module.
Washington University in St. Louis CSE571S ©2014 Raj Jain
17-27
