COEN 350

IPSec, SSL, SSH,

IPSec

RFC 1636 identified key areas where the internet needs to be made more secure. Spoofing: Creating packets with false

addresses. Eavesdropping / packet sniffing. Works for both IPv4 and IPv6.

IPSec

Implemented below the transport layer.

No application needs to be rewritten.

Is part of the OS.Applications

TCP

IPsec

IP

lower layers

IPSec An IPSec packet in tunnel mode

completely encapsulates the payload. IP Header is either an

Authentication Header ESP Encapsulating Security Payload that tells

the user which Security Association to use.

IP Header IPSec header Secure IP Payload

IPSec

Developed by the Internet Engineering Task Force IETF

Architecture ESP (Encapsulating Security Payload) AH (Authentication Header) Encryption Algorithm Authentication Algorithm Key Management DOI (Domain of Interpretation) (How to fit the

work together.)

IPSec Security Association

Cryptographically protected connection. Paradigm to manage authentication and

confidentiality between sender and receiver. Unidirectional. IPSec header contains SPI (Security

Parameter Index) that identifies the security association.

Allows partner to look up the necessary data such as the key in SA database.

IPSec Security Association Database

When X transmits to Y in IPSec, X looks up Y in the SA database.

Provides key Provides SPI Provides algorithms to be used Provides sequence number

When Y receives a transmission, Y uses the SPI and the destination address to find the SA.

IPSec

Security Policy Database Specifies what to do with packets:

Dropping Forwarded and accepted without IPSec

protection Forwarded and protected by IPSec

Decision based on fields in the IPsec packet.

IPSec Two types of IPsec headers. AH

Authentication header. Provides integrity protection only. Allows firewalls to peek at TCP ports.

ESP Encapsulating Security Payload

Optional integrity protection Optional encryption

IPSec

Two modes Transport mode

Adding IPsec information between IP header and remainder of package.

Tunnel mode Keeps the original IP packet intact,

but put it into a new packet with new IP header and IPsec data.

IPSec Transport mode

versus Tunnel mode

Original Packet IPsec Package in Transport Mode

IPSec Package in Tunnel Mode

IP header | rest IP header | IPsec header | rest new IP hdr | IPSec | IP header | rest

IPSec

IPsec in tunnel mode for a VPN:

IP: src=R1, dst=R2 | ESP | IP: src=A, dst=B | packet

IPSec NAT

Network address translation NAT boxes takes IP traffic from the

outside. Based on port number, repackages

packet to be send to an internal address and vice versa.

Allows organization to make to do with few IP addresses.

IPSec AH Header

Next header: protocol of encapsulated package Payload length: Size of AH header in words. SPI Sequence number: Used by AH to recognize replayed

packages Authentication data: Cryptographic integrity check on

the payload data.

1 1 2 4 4

Next header

Payload length

Unused SPI Sequence Number

Authentication data

IPSec AH

Some IP header fields get reset by NATs and routers.

Mutable fields can be changed: Type of service Flags Fragment offset Time to live Header checksum

Immutable fields cannot be changed: Payload length

Needed to reassemble fragmented AH packets.

IPSec ESP

SPI Sequence Number (same as for AH) IV Initialization Vector (used by some cryptographic algorithms Data: protected data, possibly encrypted Padding: needed to make data multiple of block size. Padding length Next header: Protocol field in IPv4 or next header in IPv6 Authentication data: Cryptographic integrity check.

4 4 var. var. var. 1 1 var.

SPI Sequence

number

IV data padding padding length

Next header / protocol

type

Authentication data

IPSec: IKE

Internet Key Exchange Needed for

mutual authentication to set up an SA …

Compromise based on Photuris and Skip

Photuris Uses Cookies

Different from web browser cookies. When Alice connects to Bob, Bob chooses a

cookie and sends it to Alice. Bob only honors further requests from Alice

with the cookie. Foils very simple DoS attacks. To keep cookie stateless, the cookie is a

function of Alice’s address and a secret known by Bob only.

PhoturisA

lice

Bob

CA

CA, CB, crypto

CA, CB, gb mod p, crypto selected

CA, CB, gb mod p

CA, CB, {Alice, sig of prev. message} gab mod p

CA, CB, {Bob, sig of prev. message} gab mod p

Photuris Alice chooses cookie CA in order to

keep different login attempts separated.

Bob uses a stateless cookie CB in order to keep DoD attacks at bay.

Messages 3 and 4 consists of a Diffie-Hellman encryption.

Messages 5 and 6 serve for authentication.

PhoturisA

lice

Bob

CA

CA, CB, crypto

CA, CB, gb mod p, crypto selected

CA, CB, gb mod p

CA, CB, {Alice, sig of prev. message} gab mod p

CA, CB, {Bob, sig of prev. message} gab mod p

SKIP Simple Key Management for Internet

Protocols Principals have

Certified Diffie-Hellman public keys ga mod p Private key a. Alice wants to talk to Bob:

Alice takes Bob’s public key gb and raises it to the ath power.

Bob takes Alice’s public key ga and raises it to the bth power.

Both share the secret gab mod p.

SKIP

SKIP derives a key KAlice,Bob from the mutually shared secret between Alice and Bob. Such as the lower bits of gab mod p.

Each packet is encrypted / authenticated with a randomly generated key Kpacket.

The key Kpacket is encrypted with KAlice, Bob and added to the packet.

The header of the packet is in clear text.

SKIP SKIP packet

Clear IP Header KAlice,Bob{Kpacket} Kpacket{payload}

SKIP

Changing a principal’s key is a difficult, but needed operation. Minimizes exposure of the key and

makes crypt-analysis more difficult. Updating the master key prevents

reusing compromised traffic keys. Each new key needs to be certified.

SKIP

Make the master key KAlice,Bob dependent on a version that automatically updates:KAlice,Bob = hash(gab,counter-value) Allows still principals to get a brand-

new certified key. Prevents some replay attacks.

IPSec: IKE Phases

Phase 1: Does mutual authentication and establishes session

keys. Known as KSAKMP SA / IKE SA

Phase 2: Establishes an ESP or AH SA

Phase 1 is necessarily expensive. The two phases try to have phase 2 profit

from a phase 1 interchange used for another protocol, connection, …

IPSec: IKE

Phase 1 IKE: Aggressive mode

Use a single crypto-proposal Main mode

Negotiate the strongest crypto-proposal that both parties can agree to.

IPSec: IKE

Phase 1 Aggressive Mode:

Alice Bob

ga, Alice, crypto-proposal

gb, crypto-choice, proof I’m Bob.

Proof I’m Alice

IPSec: IKE Phase 1 Main Mode:

Alice

Bob

crypto-suites I support

Crypto suites I choose.

ga

gb

gab{Alice, proof I’m Alice}

gab{Bob, proof I’m Bob}

IPSec: IKE

Key Types Pre-shared secret Public key for encryption / decryption Public key for signing

8 variants of Phase 1!!!

IPSec: IKE

Phase 1 establishes two session keys: Integrity key Encryption key for the last exchange

in phase 1 and all exchanges in phase 2.

Establishes a pair of cookies to keep different sessions different.

IPSec: IKE

Phase 1 protocols Read them!

IPSec: IKE Phase 2: A.k.a. quick mode.

Uses a pair X of cookies generated in phase 1.

Session nonce for phase 2 session. All messages are encrypted with Phase 1

encryption key SKEYID_e All messages are integrity protected with

Phase 1 intergrity key SKEYID_a. Can be initiated by either participant of

Phase 1.

IPSec: IKE

Alice BobX,Y, Crypto-protocol, SPIA, nonceA,

SPI: Security Parameter Index

X,Y, Crypto-protocol accepted, SPIB, nonceB

X, Y Ack

Secure Socket Layer 1995: deployed in Netscape Navigator

as SSLv2. 1995: Microsoft fixes SSLv2 and

introduces a similar protocol Private Communication Technology (PCT)

1996: Netscape introduces SSLv3 1999: IETF introduces Transport Layer

Security.

SSLv3 remains the most implemented protocol.

Secure Socket Layer SSL is built on top of TCP.

TCP provides reliable packet delivery. Rogue packet problem:

Maliciously introduced TCP packet. Easy to do, since it only needs to satisfy the

non-cryptographic TCP checksum. SSL disregards the package. TCP however will not accept the true

packet, because it looks like a double to it. SSL will have to start over.

Secure Socket Layer

Various keys are formed from various random numbers exchanged during the protocol.

Negotiate crypto-protocols.

Secure Socket Layer

SSL sessions are long-lived. Many SSL connections can be

derived from an SSL session.

Secure Socket Layer:Session Connection

Alice BobHello. Ciphers I support. RAlice

Certificate. Ciphers I choose. RBob

{S}Public Key of Bob. {Keyed Hash of Messages}

{Keyed Hash of Messages}

S is a random number, the pre-master secret.

K is the master secret, calculated from RAlice, RBob, S

Secure Socket Layer:Session Resumption If Bob wants to have multiple connections

per session, he sends in Message 2 a session id.

If Alice presents in Message 1 a session id, they skip the handshake.

Alice can still negotiate ciphers with Bob who might have changed policies.

Alice BobSession ID. Ciphers I support. RAliceSession ID. Certificate. Ciphers I choose.

RBob

{Keyed Hash of Messages}

Secure Socket Layer

SSL comes deployed with public keys of various trusted organizations.

User can modify this list. User verifies public keys by

sending certificate requests to the organizations in the list.

Secure Socket Layer

SSLv3 upgrades: Protects against the “downgrade attack”

Active attacker replaces the initial messages with ones containing weak crypto.

Protects against the “truncation attack” Active attacker sends a TCP close (FIN)

message. TCP is not protected, so the connection is abnormally

terminated without SSL being aware of it.

Secure Shell: SSH

SSH client and server are applications (running on top of OS).

SSH consists of a bunch of applications.

But SSH is not a UNIX shell.

Secure Shell: SSH Client contacts server. Client and server disclose the SSH

versions they support. Client and server switch to a packet

based protocol. Packet consists of

4B length, 1-8B of random padding, one-byte packet type code, packet payload data, four-byte integrity check field.

Secure Shell: SSH Server identifies itself by sending

Host key Server key 8 random bytes (use as cookie) List of encryption, compression,

authentication methods. Both sides compute a 128b session

identifier.

Secure Shell: SSH When the client receives the host key, the

client looks into the known host database.

If the host key matches the one in the database then the client proceeds.

If the host is in the database but with a different key, then the client queries the user.

Otherwise, the client warns the user and proposes to add host and key to the known host database.

Secure Shell: SSH Client randomly generates a session key.

Clients sends the session key encrypted with the server key and then with the host’s public key.

Together with the choice of crypto-suites. Both sides now use the session key for

encryption. Server sends confirmation message encrypted

with the session key. This proves the server’s authenticity to the

client.

Secure Shell: SSH

Authentication phase starts: SSH1 tries out

Kerberos Rhosts RhostsRSA Public key TIS Password

Secure Shell: SSH

Secure Shell: SSH