coen 350 ipsec, ssl, ssh,. communication security decision: what layer? implemented at application...
TRANSCRIPT
COEN 350
IPSec, SSL, SSH,
Communication Security
Decision: What Layer? Implemented at application level
Application change OS does not change
Implemented at TCP/IP level OS changes Applications do not change
Communication Security Session Key Establishment
Threat: Session Hijacking Counter-measure: Encryption
Use Session key for each session Session key needs to be unpredictable
Implementation of SSL used time, process id, parent process id to concoct session key
Attacker could narrow search space to about 30b of key.
Both partners should contribute to session key Threat: Packet replay
Counter-measure: Sequence number
Communication Security Perfect Forward Security
Threat: Eavesdropper captures traffic. Eavesdropper later acquires master key for both
communicants. PFS: Eavesdropper can still not encrypt data.
Diffie Hellman key exchange provides PFS Counter-example:
Encrypting all messages with a public key of partner
Kerberos Session key is inside ticket, encrypted with
long-term secret key Sending session key encrypted with public key
Communication Security
Escrow-foilage Alice and Bob have to give their
private keys to an escrow agency. Passive listener with those keys can
still not decrypt traffic between Alice and Bob
Communication Security Denial of Service
Protections Cookies:
Server can generate random looking cookies.
Server can quickly verify that something is a cookie.
Server hands out cookies to requestors.
Requestors need to pass cookie along with all traffic.
Communication Security
Denial of Service Attack Protection Puzzles
Server creates puzzles Client needs to solve puzzle in order to
get work done. Client does more work than server DOS
attack is harder
Communication Security Replay prevention
Use session keys Session Resumption
Goal is avoiding costly initial encryption exchange
Lotus Notes: Server has secret that changes once a month. Server sends hash(client-name, server-secret)
to client after authentication. Session key is calculated from this hash plus
nonces.
Communication Security
Negotiation of crypto-parameters Systems evolve:
Crypto-systems become breakable Newer crypto-systems demand larger
resources. Potential Security Flaw
Negotiating in bad faith, insisting on breakable crypto-suites.
Communication Security
Endpoint identifier hiding Establish secure tunnel (via Diffie
Hellman) first. Then authenticate. Man-in-the-middle gets caught in the
second step. Can only find out one identity.
IPSec
RFC 1636 identified key areas where the internet needs to be made more secure. Spoofing: Creating packets with false
addresses. Eavesdropping / packet sniffing. True 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
Provides confidentiality for IP connections
Allows implementation of access policies
Authenticates source IP addresses But not users.
IPSec Transport Mode
Adds IPSec information between IP Header and remainder of packet.
Tunnel Mode Encapsulates the original IP header and packet. Adds new IP header and IPSec header
IP Header IPSec Header IP Header Secure IP Payload
IP Header IPSec Header IP-payload: Old rest of packet
IPSec An IPSec packet in tunnel mode
completely encapsulates the payload. IP Header is either an
AH: 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
Security Parameter Index 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
NAT Have difficulties with incoming calls to
dynamic hosts. Need to maintain routing table
dynamically. Usually, need to be application-aware.
Function as a limited, package-based firewall.
IPSec NAT
Have difficulties with programs like FTP. FTP uses normally two channels: command
channel and data channel. Client opens command channel.
Packet to port 21, informs server of port on which it is listening.
Server responds by opening a data channel from port 20 to the client’s listening port.
PASV mode: Client sends PASV command to server. Server starts to listen on random port, gives port to
client in respond to PASV. Client opens data channel to the new port.
IPSec AH Header
Next header: position of protocol field of encapsulated package
Payload length: Size of AH header in words. SPI (Security Parameter Index) Sequence number: Used by AH to recognize replayed
packages. Not identical with TCP package number. Authentication data: Cryptographic integrity check on
the payload data.
1B 1B 2B 4B 4B variable
Next header
Payload length
Unused SPI Sequence Number
Authentication data
IPSec AH
Some IP header fields get reset by NATs and routers.
Mutable fields are not covered by the integrity check and can be changed by routers:
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 AH
Immutable fields Destination address is protected by AH. NAT will change the destination address.
Hence, IPSec /AH and NAT do not work well together.
There is no way to predict the change at the source.
In source routing, routers change the destination address to the next field specified by source routing.
AH can predict the destination address. An example of a mutable, but predictable field.
IPSec ESP (Encapsulating Security Payload)
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 AH protects the IP header itself. ESP protects everything beyond the
ESP header. Hence: AH provides additional (but
useless?) protection. AH is less likely to fall under export
restrictions.
IPSec TF-ESP (Transport-friendly ESP)
Proposal to copy fields of interest of the original header in clear.
Firewalls and routers can look at these information.
Potential for information leak. Firewalls should not look at any data above
layer 3. But of course, they now do.
IPSec protection is end-to-end, and intermediate routers / firewalls cannot trust the cleartext copies of these fields.
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. Encrypted with Diffie-Hellman key.
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 Long-time use
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:
AliceBob
ga, Alice, crypto-proposal
gb, crypto-choice, Proof that I’m Bob.
Proof that I’m Alice
IPSec: IKE Phase 1 Main Mode:
Alice
Bob
crypto-suites I support
Crypto suites I choose.
ga
gb
gab{Alice, Proof that I’m Alice}
gab{Bob, Proof that 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
IP SEC Resumen
Transport Mode IPSec protects the message passed
down from the message layer by adding AH or ESP header.
Tunnel Mode IPSec completely encapsulates IP
datagram.
IP SEC Resumen
Security Policy Rule programmed into IPSec
Implementation. Tells implementation how to process
particular packets.
Security Policy Database: Stores security policies.
IP SEC Resumen Security Association
Set of security information regarding a particular connection between two devices.
Security Association Database Database containing security associations of
a specific device. Selectors
Set of rules for choosing datagrams that a SA applies to.
IP SEC Resumen
Security Association Triples Define an SA:
Security Parameter Index (SPI) 32 b number placed in AH or ESP
IP destination address Security Protocol Identifier
Specifies whether this association is AH or ESP
IP SEC Resumen
IPv4 AH Datagram Format
IPSec Transport Mode
IP SEC Resumen
IPv4 AH datagram IPSec Tunnel Mode
IP SEC Resumen
IKE overview: Blend of Internet Security and Key
Management Protocol (ISAKMP) Oakley key exchange SKEME key exchange
IP SEC Resumen
IKE phases Phase 1: Mutual Authentication and
establishment of session keys. AKA: ISAKMP SA, IKE SA
Phase 2: Multiple SAs based on phase 1
IP SEC Resumen IKE Phase 1
Aggressive mode Diffie-Hellman exchange plus mutual
authentication 3 messages
Main mode crypto suite choosing Diffie Hellman Authentication protected by Diffie
Hellman
IP SEC Resumen
IKE Phase 2 (Quick Mode) Assumes a phase 1 SA
IP SEC Resumen
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. Chosen by Alice.
K is the master secret, calculated from RAlice, RBob, S
Bob has authenticated himself to Alice, but not vice versa!
Alice specifies lists of ciphers and a random
number
Bob gives certificate of his public key. Bob
picks cipher.
Alice calculates the pre-master secret and sends it to Bob, protected by Bob’s public key. She also creates a (complicated) hash of the hand-shake message. She also calculates a session key K
Bob responds with his version of the hash of the session key.
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:Session Resumption
Session resumption is not stateless.
Server Bob needs to maintain a database entry of session id and master secret.
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
Provides Authentication Encryption Integrity
Secure Shell: SSH SSH provides
scp: secure file transfer Secure remote command execution Automatic authentication:
Place public key files on remote computers Enable SSH clients (scp, ssh) to access remote
accounts Invoke ssh-agent program Choose keys needed for remote logins. Load private keys with ssh-add (invoking passphrase) ssh-agent keeps private keys in memory.
Delegate limited authentication. Secretary can only read the email.
Port forwarding
Secure Shell: SSH1 Client contacts server.
By going to port 22 by convention. 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: SSH1 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
At this point, a secure communication channel has been established.
Client is sure of the authenticity of server.
Server now authenticates the client.
Secure Shell: SSH SSH2 changes:
SSH2 consists of modules: SSH Transport layer protocol SSH Authentication protocol SSH Connection protocol
SSH2 allows for additional parameter negotiation
More general session key exchange possibilities
Secure Shell: SSH SSH-2 changes:
SSH-2 uses better integrity checking for messages.
Supports password changes. User-authentication methods more
restricted: Public key (DSA, RSA, OpenPGP) Hostbased password