Secure Protocols and VPNs

Secure Protocols and VPNs

Stefek ZabaHewlett-Packard Labs, Bristol

[email protected]

What we’ll cover

• This lecture:– network layering revision– “secure channel” concept– substantial example: IPSec

• Next lecture:– substantial example 1: TLS/SSL– substantial example 2: SSH– summaries and random rants

Network layers

• You heard this in Lecture 1!• Simplified Internet model:

applicationtransportnetworklink

Network layersapplication datale

nTCP headerport, seqnum,

SYN, ACK, FIN, …

application datalenTCP header

port, seqnum,SYN, ACK, FIN, …

lenIP header

srcIP, destIP,TTL, pktID, …



lenIP header


Ethernet headerether-addr, checksum, … le

n



lenIP header’

srcIP, destIP,TTL-n, pktID, …

X.25 headerVirtual-circuit-ID, … le

n

Application layer: lots of protocols, e.g. HTTP carrying Web traffic, SMTP carrying email, NNTP carrying News, …

Transport layer: TCP - reliable connections (sequence nums, retransmission), carries HTTP, FTP, Telnet, …; UDP – unreliable datagrams, e.g. streaming audio/video

Network (IP) layer: “is” Internet. Carries: TCP, UDP, ICMP (“ping”, router control), …

Link layer: carries IP (and others, e.g. Appletalk, NetBUI, IPX, ARP, ...): hop-by-hop LAN.Examples: IEEE802.3, PPP, RF LAN

Where shall we “put security”?

• Link level:covers all traffic on that link, e.g. RF only one hop

• Network (IP) level:covers “all” traffic, end-to-endtransparent to applications little application control “unnatural”, since IP is stateless packets

but channel is stateful

Where shall we “put security”?

• Transport (TCP) level:end-to-endapps can control when it’s used apps must be modified (unless proxied)

• Application level:can be tuned to payload requirements no “leverage” - must rework for every

app

What “security” are we providing?

• A “secure channel”, typically offering– Origin authentication (but of what: OS? App?

User?)

– Integrity– Confidentiality– …

• Not:– Non-repudiation– Any services once data received

Crypto primitives used

• Symmetric ciphers• Asymmetric ciphers & signatures• (Keyed) hash functions• (Keyed) pseudo-random functions• Key agreement protocols: mainly

DH

Typical goals

• At least one party authenticated• Shared secret established• MAC & bulk cipher keys derived

from shared secret• Further traffic “protected”• Optional: session re-use, rekeying,

…

IPSec: overview

• Network-level: all IP datagrams covered• Mandatory for next-generation IP (v6),

optional for current-generation (v4)• Authentication-only or confidentiality too• Two “modes”

– “transport” mode: for IPSec-aware hosts as endpoints

– “tunnel” mode: for IPSec-unaware hosts, established by intermediate gateways or host OS

References: RFC2401..2412; FreeSWAN

Authentication: AH

• Authenticates whole payload and most of header– vitally, covers source IP address– omits only fields modified in transit

• e.g. TTL/hop-limit, fragmentation fields, some options

Encryption + auth: ESP

• Encrypts and optionally authenticates payload, but not IP header

• Combine with AH for “full” conf+auth• Use alone for payload conf+auth• There are both engineering and

political reasons for their separate existence!

AH & ESP, Tunnel & Transport



lenIP header


application datalenTCP headerlenIP header


AHSPI, seqnum,

MAC, …

MAC scope



ESPheader:

SPI,seqnum

MAC scope

ESPtrailer: pad,padlen, …

encryption scope



ESPheader:

SPI, seqnum

MAC scope

ESPauth: MAC

application datalenTCP headerlenInner IP header

host-srcIP, host-destIP,TTL, pktID, …

AHSPI, seqnum,

MAC. …

MAC scope

Outer IP headergw-srcIP, gw-destIP,

TTL, pktID, …

len



AHSPI,seqnum,

MAC, …

Inner AH MAC scope


TTL, pktID, …

len ESP

trailer: pad,padlen…

Outer ESP MAC scope

ESPheader:

SPI,seqnum

encryption scope

Original unprotected datagram

AH in transport mode

ESP (conf and auth)in transport mode

ESP auth-onlyin transport mode

AH in tunnel mode

ESP I(conf and auth) in tunnel mode

carrying AH in transport mode

ESPauth: MAC

ESPauth: MAC

Why so many combinations!?

• To support different VPN arrangements, to meet different security and deployment-practicality requirements

Simple transport-mode usage:

host-to-host(no singing, please)

app datalenTCPlenIP

app datalenTCPlenIP

app datalenTCPlenIP lenAH lenESP ESP

app datalenTCPlenIP lenAH lenESP ESP

Without transport-mode IPSec

With transport-mode IPSec

Simple tunnel-mode usage:

gateway-to-gateway

app datalenTCPlenIP

host-to-host app datale

nTCPlenIP

host-to-host

app datalenTCPlenIP

gw-to-gw

lenESP lenIP

host-to-host

ESP app datalenTCPlenIP

gw-to-gw

lenESP lenIP

host-to-host

ESP

Other combinations for other requirements



AHSPI, seqnum,

MAC, …

MAC scope



ESPheader:

SPI,seqnum

MAC scope

ESPtrailer: pad,padlen, …

encryption scope



ESPheader:

SPI, seqnum

MAC scope

ESPauth: MAC



AHSPI, seqnum,

MAC. …

MAC scope


TTL, pktID, …

len



AHSPI,seqnum,

MAC, …

Inner AH MAC scope


TTL, pktID, …

len ESP

trailer: pad,padlen…

Outer ESP MAC scope

ESPheader:

SPI,seqnum

encryption scope

AH in transport modeHost-to-host auth-only,

e.g. network management?

ESP (conf and auth) in transport mode

Host-to-host secure channel,

e.g. encapsulated legacy apps?

ESP auth-only in transport mode

As for AH-transport; probably worse in all

cases…AH in tunnel modeBetween-gateway VPN

with no overall confidentiality provision

(may be good performance choice)ESP in tunnel modecarrying AH in transport

modeCommon “road warrior”

VPN: secure channel across public network, and strong, continuing authentication to end

system(s)

ESPauth: MAC

ESPauth: MAC

But where did the keys come from?

• “SPI” (security parameters index) refers to an “SA” (security association)

• SAs set up manually or by IKE – IPSec Key Exchange

• Policy “databases” define how fine- or coarse-grained SAs are– anything from “all traffic shall use this key” to

individual combinations of source and destination addresses and ports

– even “user-based” keying supported… but binding a user to an IP address is (very) problematic…

Outbound processing

• Lookup policy for this datagram– drop, pass through, or process

• Create a new SA if none exists• Apply keys from SA for MAC and enciphering• Add explicit IV for each datagram

– because they can be lost and arrive out-of-order

• Pass assembled datagram down to link layer– or to next instance of IPSec processing!– Let’s ignore fragmentation, PMTU discovery, …

Inbound processing

• Lookup policy for this datagram– drop, pass through, or process

• SA should already exist (we’re the responder)

• Apply keys from SA for MAC-check and deciphering (using datagram’s IV too)

• Raise security error if needed; otherwise,• Pass assembled datagram up to rest of

normal IP processing– or to next instance of IPSec processing!

What can be MAC’ed?• Immutable or predictable fields and options in IPv4 and IPv6• AH-controlled all immutable, so MACed:

– payload, payload length, next-header, SPI, sequence number, reserved fields

• IP header immutables and predictables v6:– version, payload length, next-header, source and destination IP

addrs,– BUT NOT: class, flow label, or hop limit (= v4 TTL)– all v6 extension headers self-describing as to mutability

• IP header immutables and predictables v4:– version, header and payload lengths, packetID, protocol type,

source and destination IP addrs– BUT NOT: ToS, flags, fragment offset, TTL, header checksum

• All unpredictable fields zero-filled for MAC calculation– so overall length and alignment still protected

IPsec Key Exchange – IKEor, time for a deep breath!

• Documentation hard to follow– IKE is specific adaptation of more general

protocols (“Oakley” and “ISAKMP”)

• Two levels of SA negotiated– an initial context (bidirectional, with heavy-

duty authentication and negotiation)– then several client SAs, negotiated quickly

using initial SA as secure channel; one for each direction and each AH and ESP

– initial SA also used for error traffic and similar management traffic

IKE security goals

• Authentication of parties (by digital signature, proof of knowledge of private key, or shared key)

• Establishment of a fresh shared secret• Shared secret used to derive keys for channel

confidentiality and authentication• “Perfect Forward Secrecy”, at cost of using up shared

material• (partial) anti-clogging, against denial-of-service

attacks• Secure negotiation of algorithms: asymmetric (e.g.

RSA, elliptic curve), symmetric (e.g. 3DES, Blowfish, AES), and hash (e.g. MD5, SHA-1)

IKE details

• Phase 1 is the heavyweight exchange to establish a secure key management channel– “Main mode” variant: slower, more cautious,

hides details of credentials used and allows forward secrecy (independence of short-term keys)

– “Aggressive mode”: less negotiation, fewer round trips, more information disclosed

• Phase 2 (“quick mode”) established SAs for IPSec itself, using the Phase 1 channel

Main Mode IKE Phase 1using digital signatures

(see RFC2409, p.11)

Initiator Responder

HDR, SA_i -->

<-- HDR, SA_rHeaders (HDR) include “cookies” CKY-I and CKY-R respectively. Initiator’s SA has one or more “proposals”, in preference order, for algorithms to be used for ISAKMP, the key management channel we’re building. The responder chooses exactly one of these proposals. These SAs may refer to one of a few standard Diffie-Hellman groups (both integer and ECC), or may define new DH groups.

HDR, KE, Ni -->

<-- HDR, KE, Nr

Ni, Nr are initiator’s and responder’s nonces, respectively; KE are Diffie-Hellman g^x and g^y valuesHDR*{IDii, [CERT,] SIG_I} -->

<-- HDR*{IDir, [CERT,] SIG_R}

HDR* denotes remaining ISAKMP traffic is encrypted. IDii, IDir are IDentifiers, typically IP addresses.SIG_I is over HASH_I = prf( SKEYID, g^x | g^y | CKY-I | CKY-R | SA_i | IDii )SIG_R is over HASH_R = prf(SKEYID, g^y | g^x | CKY-R | CKY-I | SA_i | IDir )where SKEYID = prf( Ni_b | Nr_b, g^xy ), and prf is the negotiated keyed pseudo-random function

Note 3 round-trips, 4 DH modular exponentiations, 2 signature-generations and 2 signature-verifications

Aggressive Mode IKE Phase 1using public-key proof

(see RFC2409, p.14)

HDR, SA_i, <Ni>Pubkey_r, <KE>Ke_i, <IDii>Ke_i -->

HDR includes CKY-I. SA_i has exactly one “take-it-or-leave-it” proposal. The nonce Ni is encrypted in the responder’s public key; KE (that’s g^x) and IDII are encrypted under Ke_i = prf( Ni, CKY-I ). So, the responder can decrypt Ni and so derive Ke_i only if it has the private complement to Pubkey_r.

<-- HDR, SA_r, <Nr>Pubkey_i, <KE>Ke_r,<IDir>Ke_r, HASH_R

HDR includes CKY-R. SA_r must equal SA_i. Similarly to the initiator’s message, the nonce Nr is encrypted in the initiator’s public key, while KE (that’s g^y) and IDir are encrypted under KE_r = prf( Nr, CKY-R ), requiring the initiator to have the private complement of Pubkey_i. HASH_R is as on the previous page.

HDR, HASH_I -->

The hashes sent in each direction aren’t signed; but the ability to generate them proves receipt and successful decryption of the nonce received from the other party.

Note 1.5 round-trips only, still 4 DH modular exponentiations, 2 public-key encrypts and 2 public-key decrypts. Careful analysis shows “plausible deniability”: the absence of digital signatures allows either party to disown the exchange.

Use of Phase 1 agreed material

Key material for the underlying ISAKMP key-management SA we’re building first is derived from the shared-secret quantity g^xy and the nonces securely exchanged during Phase 1 as follows:

SKEYID = prf( Ni | Nr, g^xy )

SKEYID_d = prf( SKEYID, g^xy | CKY-I | CKY-R | “0” )

SKEYID_a = prf( SKEYID, SKEYID_d | g^xy | CKY-I | CKY-R | “1” )

SKEYID_e = prf( SKEYID, SKEYID_d | g^xy | CKY-I | CKY-R | “2” )

where _a refers to Authenticator (MAC) material for the ISAKMP channel, and _e is for Encrypting material for the ISAKMP channel. _d is dual-purpose; firstly, it’s used as input for the _a and _e pseudo-random streams; secondly, it’s the main source of key material for the Phase 2 SAs which are the ones used by IPSec itself. SKEYID is used directly as the prf key for HASH_I and HASH_R, used to authenticate the parties.

Particular “transforms” (symmetric encryption algorithms, MACs, and so on) specify exactly how SKEYID_a, SKEYID_e, and SKEYID_d is to be used. For example, the specification for single-key DES uses at minimum the first 8 bytes of the PRF, forcing the parity bits to appropriate values, throwing away any bytes which would give rise to the known weak or semi-weak keys. (There are only 16 out of 2^56 such keys, so this isn’t likely to occur in practice!) The Triple-DES definition uses at least 24 bytes of the prf output, and the prf definition “stretches” its initial result by repeated application to produce as many bytes as are needed.

Phase 2: “Quick Mode”Now that we have an ISAKMP SA to define a secure key-management channel, doing algorithm and key agreement for client SAs such as AH and ESPis cheap and easy (relatively speaking). As example, this is how you get 4 SAs (one for each direction of an AH + ESP pair) - see RFC2409 p.19:

HDR*{HASH(1), SA0, SA1, Ni} -->

As before, HDR*{} means that all further material is encrypted (under SKEYID_e, remember?). SA0, SA1, etc. are “proposals” for client SAs for the AH and ESP transforms – each one is a preference-ordered list of possible algorithm combinations. Ni is a new initiator nonce. HASH(1) = prf( SKEYID_a, M-ID | SA0 | SA1 | Ni ); see how SKEYID_a is the MAC key. M-ID is the unique message-ID from HDR.

<-- HDR*{HASH(2), SA0, SA1, Nr}

Back come single algorithm choices for each SA, and a new responder nonce Nr. HASH(2) is similar to HASH(1): HASH(2) = prf( SKEYID_a, M-ID | Ni | SA0 | SA1 | Nr ); it has Ni added as a liveness proof.

HDR*{HASH(3} -->

This is a simple acknowledgement that the responder’s message has been received;HASH(3) = prf(SKEYID_a, “0” | M-ID | Ni | Nr )

Now key material for each IPSec SA is defined as follows:

KEYMAT = prf( SKEYID_d, protocol | SPI | Ni | Nr )

Since protocol and SPI are unique to AH/ESP and direction respectively, this gives 4 separate chunks of KEYMAT. If necessary, they are “stretched” as before by applying prf iteratively. Note the single Phase 1 DH exchange and public-key operations have been used to derive key material for all four IPSec SAs, spreading the cost of those expensive operations. There’s an option to include a fresh DH exchange in each Quick Mode if you prefer Forward Secrecy to computational efficiency...

Final notes on IPSec

• IKE is carried over UDP; hence unreliable (may need to be restarted) and blocked by some firewalls

• Managing IPSec policy and deployments isn’t easy, and getting it wrong can be embarassing in losing connectivity, e.g. by making exchanges of routing updates unreadable

• After trying to roll-its-own with PPTP, MS has put IPSec into WinXP

• See FreeS/WAN for implementation (and contribute too, unless you’re a US citizen):http://www.xs4all.nl/~freeswan/

Documents

Secure Protocols and VPNs