1

Tolga Acar24 Feb. 2011

Distributed Key ManagementandCryptographic Agility

2

Overview

• Distributed Key Lifecycle– Problem statement and status quo– Distributed Key Manager– Typical application scenario and architecture

• Hardware Rooted Key Management– How to use TPMs for key management– TPM Key hierarchy

• Diving into Cryptographic Theory– Security Definitions– Cryptographic Agility

3

Distributed Key Management

Node 1 Node 2 Node N

Storage(Replica)

C1=E(K,M1) C2=E(K’,M2) M1=D(K,C1)

Storage(Replica)

Save ciphertext C1

Read ciphertext

Encrypt M1 Decrypt C1

Replication Protocol

Where is the correct key?How is it protected?

4

Key Lifecycle Model• Creation. A key object is created on at least one replica, but its

attributes (e.g., key value) are not set. • Initialization. The key object has all its core key attributes set

on at least one replica. • Full Distribution. An initialized key is available on all replicas.• Active. An initialized key is available for cryptographic

operations on at least one replica. • Inactive. An initialized key is available for some cryptographic

operations on all replicas (e.g., decrypt, only). • Termination. An initialized key is permanently deleted from all

replicas.

5

Key State Transitions

Initialization

FullDistribution

Active Inactive TerminationCreation

Cryptographic operationsCreate and

initialize a key

6

DKM Problem Statement• No cross-user and cross-machine data protection

– Windows Data Protection API (DPAPI) is single-user, single-machine.– KeyCzar and PKCS#11 uses local keys; no distribution mechanism.

• Engineering problem– Ad-hoc key management groups (protection siloes)– Scalability & Availability (10Ks of machines)– Geo-redundancy (multiple data centers)– Key lifecycle management (automation)

• Cryptography problem– Protect arbitrary data (broad applicability)– Use existing algorithms (e.g. AES, HMAC-SHA2)– Automatically update group keys (key rollover)– Crypto agile (algorithm and key length changes)

7

DKM Architecture

Untrusted Storage(DKM-protected data)

DKM Repository ACL

DKM

Group A

ProtectUnprotect

API

Secure connection

DKM Container

Group B ACL

Client 1

Client 2

Client 3

ProtectUnprotect

API

ProtectUnprotect

APIDKM Group Container

KA

KB

KB

KA

KA

8

DKM Approach• Active Directory Approach

– Key storage is straightforward• Store group keys in AD objects• Protect keys with AD object ACLs• AD security groups correspond to principals / groups

– Rely on Active Directory replication for high availability– Network transport is secure (LDAP with Kerberos)

• DKM provides– Auto key update mechanism– Multiple groups and multiple keys per group– Cryptographic policy per domain and per group– Crypto agility

9

Walkthrough: DKM in Hosted E-Mail

• Scenario:– Hosting mail for multiple tenants in a datacenter– Product supports message aggregation from other

providers for users with multiple email accounts• User signs in once• E-Mail Server fetches and aggregates mail

– Tenant Admins must be able to perform Administrative tasks• But should NOT be able to read user credentials

10

Walkthrough: DKM in Hosted E-MailHosted E-Mail

Tenant 2Tenant 1

Mailbox Stores

User Settings

User’s Mailbox

User’s DKM encrypted ISP

password

DKM Keys

User’s Mailbox

User’s DKM encrypted ISP

password

User’s ISP

Internet

E-Mail Servers

DKM

ISP Mail Server

(Hotmail, Yahoo, Gmail, etc)

Tenant “2”

Admin 2

User 2

Tenant “1”

User 1

Admin 1 User Settings

Active Directory

Tenant Admin can administer Exchange

Tenant Admin can NOT access DKM keys

11

DKM in Hosted E-MailHosted E-Mail

Tenant 2Tenant 1

Mailbox Stores

User Settings

User’s Mailbox

User’s DKM encrypted ISP

password

DKM Keys

User’s Mailbox

User’s DKM encrypted ISP

password

User’s ISP

Internet

E-Mail Servers

DKM

ISP Mail Server

(Hotmail, Yahoo, Gmail, etc)

Tenant “2”

Admin 2

User 2

Tenant “1”

User 1

Admin 1 User Settings

Active Directory

E-Mail Servers can retrieve mail from the ISP on behalf

of the user

12

3

DKM-TPM MotivationSecret Protection Technology:

• Approach sits between a pure HSM solution and a full software solution.

Expensive Moderate InexpensiveCost:

Very Secure More Secure Moderate (OS-Dependent)Security:

Hard Easier EasyDeployment:

HSMHardware Security Module

TPM-based

CryptoSoftware Crypto

No Hardware

DKM-TPM Key Hierarchy

KeysStorage: ExternalProcessing: MemoryProtection: TPM

KeysStorage: ExternalProcessing: TPMProtection: TPM

KeysStorage: TPMProcessing: TPMProtection: TPM

EK(Endorsement Key)

SRK(Storage Root Key)

AIK(Attestation Identity Key)

WK(Wrapping Key)

TLSK(TLS Key)

SK(Signing Key)

DKMK(DKM Key)*

* There are one or more DKM Keys.

Seal

DKM-TPM Roles

1. Master (Root of Trust)• Root of Trust for TPM public keys• Role assignment to TPM public keys• Push to Stores

2. Store (Repositories)• DKM repository (keys, policies, and metadata)• DKM Responder• Responds to requests from Masters, Stores, and Nodes

3. Node (Application servers)• Cryptographic operations with DKM keys• Client API• Sends requests to Stores

DKM-TPM Roles

Master

Master PK ListStore PK ListNode PK ListConfiguration

Store

DKM KeysPolicies

Master PK ListStore PK ListNode PK ListConfiguration

Node

Master PK ListStore PK ListNode PK ListConfiguration

CommServerCommClient CommClientCommClient

TPM

KM & CryptoRepository

TPM

KM & CryptoRepository

TPM

KM & CryptoRepository

Node Logic & API Store Logic & API Master Logic & API

16

Cryptosystem Security Definitions• Probabilistic Polynomial-Time (PPT) adversaries– Probabilistic randomized algorithm that gives the

correct answer with > ½ probability.• Random Oracle Model (RO or ROM)– Black box with a stateful uniform random response

y {0, 1}*If (x in A) y Fetch(A,x)Else Store(x,y) in A

Return y

Random Oraclex

y

17

Attack Game• Encryption scheme security definitions

– IND-R: Indistinguishability from Random– IND-CPA: Indistinguishability under Chosen Plaintext Attack (a.k.a. semantic

security)– IND-CCA: Indistinguishability under Chosen Ciphertext Attack

• IND-CPA IND-CCA⊂

b {0, 1}C = Enc(K, mb)Return C

Left-Right Oraclem0, m1

C

Guess b?

IND-CPA Game

18

Ciphertext Attacks

• IND-CCA2: Indistinguishability under adaptive chosen ciphertext attack– Decryption Oracle access (non-trivial)

• Non-adaptive– Query the decryption oracle till the challenge

ciphertext is received• Adaptive– Continuous queries to the oracle (max q queries)

• IND-CPA IND-CCA IND-CCA2⊂ ⊂

19

IND-CCA/CCA2 Game

m = Dec(K, C)

Decrypt

m0, m1

C

Queries {m,C}

Responses {C,m}

C = Enc(K, m)

Encrypt

b {0, 1}C = Enc(K, mb)

Left-Right Oracle

C

m

m = Dec(K, C)

Decrypt

Guess b?

Challenge

Adaptive (CCA2) Adversary

Free Oracle Access

20

Cryptographic Agility• Cryptographic primitives as sets:

– PRF = {F : F is a secure pseudorandom function}– AE = {F : F is a secure authenticated encryption scheme}

• Assume F1 and F2 have the same key space and length• Informal Definition: A primitive Π is agile if any F1, F2 Π ∈

can securely use the same key.

F1 F2

K

21

Pseudo Random Function AgilityFacts• PRF: F is a PRF if no efficient adversary can distinguish F(K,.) from a random

function.• F1(K1,x) and F2(K2,x) are not distinguishable from a pair of random functions.

x F1(K,x)

F1 F2

x F2(K,x)

K • Definition: A set {F1,F2} is agile if F1(K,x) and F2(K,x) are not distinguishable from a pair of random functions.

• Question: Are PRFs agile?– Yes, if every {F1,F2} is agile.

• Answer: No.– Example: F2(K,x) = NOT (F1(K,x))

• Now, what?

22

Agility in Practice• Certain primitives are agile: collision-resistant hash functions• Strong agility is achievable in practice: Authenticated Encryption

– Don’t use the key directly in the encryption algorithm <ae>– Use a derived subkey in <ae>

⊥ (x,F1(K,x))

F1 F2

⊥ (x,F1(K,x))

K• PRF-based security for Authenticated

Encryption: CCM, GCM, etc.– Pick a PRF from a small agile set

• Encryption of M with K, with PRF– Kae = PRF(K,<ae>)– C = E(Kae, M)

• Decryption– Kae = PRF(K,<ae>)– M = D(Kae, C)