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A1256656640MfGZV

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The client resumes the previously negotiated TLS session by sending the session ID. ... This mechanism is used for resuming the sessions. ... – PowerPoint PPT presentation

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Title: A1256656640MfGZV

1
Transport Layer Security (TLS)
Team Techno Gautami Parulkar Hariharan
Venkataraman Jigar Patel karthik
gottiparthy Neeraj Sharma
2
Presentation Agenda

Introduction
TLS Protocol Stack
TLS Handshake
Improvement
Demo

3
TLS

Cryptographic protocol to provide secure
communications on the internet
Similar to SSL with slight differences
Preventions of eavesdropping, tampering, and
message forgery
Current approved version of TLS is 1.1

4
TLS Protocol Stack
1
2
3
5
TLS Record Protocol
Data
Encrypted Data MAC
Record header
6
Full TLS Handshake
7
Limitations of TLS

Time consuming Handshake messages during
connection establishment.
Limitation of the session caching period at the
servers cache.
Access security threat (web jacking).
Time consuming and expensive RSA private key
decryption.
Denial of Service Attacks (DoS).

8
Limitations condt

Denial of Service Attacks (DoS).

9
Counter Measures

Resume Handshake Mechanism
The client resumes the previously negotiated TLS
session by sending the session ID.
The server stores the session for certain period
of time before evicting the session (aborting the
session) causing the client to perform the full
Handshake.
Amazons servers evict the session every 2mins
.As a result the clients had to perform full
handshake even tough the client wanted to resume
the session

Eric Rescorla,Dan Boneh,Hovav Shacham
proposed two mechanism.
1. Fast Track Handshake Message.
2. Client side session caching.
Fast Track Handshake Message
The ordinary TLS handshake mechanism follows
1) negotiating the session parameters
2) authentication of the server and optionally
the client.
3) Establishing the shared cryptographic secret

11
Ordinary TLS Handshake

The ordinary TLS handshake follows
negotiating the session parameters
authentication of the server and optionally the
client.
Establishing the shared cryptographic secret

12
Fast Track Handshake

Specifically used for short lived TLS session ,no
resumption done.
The peers have not communicated at all.
The client and the server well established
session got expired in the middle.
Eg. Amazon clients placed multiple requests in
the 2min window of the servers cache.

13
Fast Track HS condt.

The Fast track clients store the long-lived
server information such as the server certificate
and the preferred cipher suite.
The HS parameters are called the determining
Parameters.
The long-lived parameters do not change unless
the configuration of either the client or the
server changes.

14
Handshake Parameters

1st Category
The servers certificate chain
The servers Diffie-Hellman group, if any and
Whether client authentication is required if
so,
Acceptable client certificate types and
Acceptable certificate authorities.

2nd Category
The preferred clientserver cipher suite and
The preferred clientserver compression method

15
Fast Handshake Diagram
16
Description

The clientHelloFT message must include the
fast-track hash function with the Hash of
determining parameters.
Once the client validates the server certificate
,it is not required to again revalidate it.
Verification of the SHA-1 hash of the determining
parameters is required.

The server uses the information of clientHello
msg and its own configuration to form a version
of determining parameters.
These determining parameters are exchanged using
the cryptographic hash and for more security
handshake MAC is included in the finished
message.
The 3 msg Server Certificate,Certificate Request,
Server Hello done are not included in the Fast
Track Handshake.

18
Performance Table
19
Details

The first cipher suite, called TLS RSA WITH 3DES
EDE CBC SHA in RFC 2246 (and called
DES-CBC3-SHA in OpenSSL), uses RSA for key
exchange. It does not require a ServerKeyExchange
message to be sent.
The second cipher suite, TLS DHE RSAWITH 3DES
EDE CBC SHA (EDH-RSA-DES-CBC3-SHA in OpenSSL),
employs EDH for key exchange, with RSA
authentication .

20
Advantages Disadvantages

Advantages
It implements 3 flows instead of 4 flows.
It saves bandwidth and time , improves network
traffic.
Disadvantages
Does not support RSA key exchange.
Though the 3 msg are not included in the HS ,they
do not involve any significant computational
intensive operations.

21
Client Side Caching

To reduce the overload of session caching on the
server, the client is forced to store the session
information in its cache.
This mechanism is used for resuming the sessions.
The cached data sent to client is encrypted by
symmetric cipher key called the enc-key.

22
Client side caching condt.

The integrity is still not guaranteed ,server
uses MAC with the fixed server key called
mac-key.
A token is formed using enc-key and mac-key.
Token enc-key cache data mac
Mac mac-key cache data

The Authentication Token retains all the
information about the session .Only that the
master key has to be shared.
This token is included in the session ID .
Problem
The session Id is 32 bytes but the master secret
in the token is 48 bytes, cannot fit in the ID.
The server provides the session ID in the server
Hello message, which is transmitted before it
knows the master secret.

To overcome this problem the normal zero length
session ID is sent in the server hello message.
Then the extended session ID message is sent
containing the authentication token is sent
immediately before the change cipher spec
message.

25
Connection Establishment

During the connection establishment the client
signals the server about its capability of CSSC
using the client side cache capable extension in
the client Hello.
If the server wants to request it sends a client
side cache capable request extension.
For the resumption of the session the client send
the authentication token, if it is less than 256
bytes, it is placed in the session ID.

27
Disadvantage

The session can become invalid due to expiration,
improper closer and error. Authentication tokens
are self-authenticating and hence it is difficult
to invalidate the session easily, if an error is
found .
Solution To invalidate the session is to have a
blacklist consisting of sessions that have been
invalidated but may not have been expired.

28
Performance Improvement by Re-balancing

Clients send simultaneous requests flooding the
server causing it to crash or result in Denial of
Service.
DoS attacks cause the server to deny service to
legitimate clients.
The server consumes a lot of CPU processing time
to perform the costly computation like decrypting
key.

Solution to this problem is Load Distribution
where the client performs more work and the
server performs commensurately less work.
A mechanism called the Client Aided RSA speeds up
the processing by the factor of 11 to 19.

The server can encrypt the session key thus
shifting the decryption burden to the client. But
for this the server has to authenticate the
client and if the server cannot verify it will
lead to DOS attacks.
The server needs to verify all the certificate
chain. Therefore, the Client Aided RSA is used.

31
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32
CA-RSA

33
Attacks on TLS

Denial of Service is an incident in which a user
or organization is deprived of the services of a
resource they would normally expect to have.
Man in the middle (MITM) is an attack in which an
attacker is able to read, insert and modify at
will, messages between the two communicating
peers.

34
Conclusion