Distributed Key Management and Cryptographic Agility. Tolga Acar 24 Feb. 2011

Transcription

1 Distributed Key Management and Cryptographic Agility Tolga Acar 24 Feb

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 2

3 Distributed Key Management Encrypt M 1 Decrypt C 1 C 1 =E(K,M 1 ) C 2 =E(K,M 2 ) M 1 =D(K,C 1 ) Read ciphertext Node 1 Node 2 Node N Save ciphertext C 1 Replication Protocol Storage (Replica) Storage (Replica) Where is the correct key? How is it protected? 3

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. 4

5 Key State Transitions Initialization Creation Active Inactive Termination Full Distribution Create and initialize a key Cryptographic operations 5

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) 6

7 DKM Architecture API Client 1 Protect Unprotect K A Secure connection DKM Container API DKM Group Container Client 2 Protect Unprotect K B DKM Repository DKM Group A K A ACL API Client 3 Protect Unprotect K A Group B K B ACL Untrusted Storage (DKM-protected data) 7

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 8

9 Walkthrough: DKM in Hosted Scenario: Hosting mail for multiple tenants in a datacenter Product supports message aggregation from other providers for users with multiple accounts User signs in once Server fetches and aggregates mail Tenant Admins must be able to perform Administrative tasks But should NOT be able to read user credentials 9

10 Walkthrough: DKM in Hosted Tenant 1 Tenant Admin can administer Exchange Hosted Tenant 1 Tenant 2 User s ISP Active Directory Admin 1 User 1 Tenant Admin can NOT access DKM keys DKM Keys Mailbox Stores User Settings User s Mailbox User Settings User s Mailbox Tenant 2 Internet User s DKM encrypted ISP password User s DKM encrypted ISP password Admin 2 Servers ISP Mail Server User 2 DKM (Hotmail, Yahoo, Gmail, etc) 10

11 DKM in Hosted Tenant 1 Hosted Tenant 1 Tenant 2 User s ISP Active Directory Admin 1 DKM Keys User Settings User Settings User 1 Mailbox Stores User s Mailbox User s Mailbox Tenant 2 Internet User s DKM encrypted ISP password User s DKM encrypted ISP password Admin Servers ISP Mail Server User 2 Servers can retrieve mail from the ISP on behalf of the user DKM 3 (Hotmail, Yahoo, Gmail, etc) 11

12 Secret Protection Technology: DKM-TPM Motivation HSM Hardware Security Module TPM-based Crypto Software Crypto No Hardware Cost: Security: Deployment: Expensive Moderate Inexpensive Very Secure More Secure Moderate (OS-Dependent) Hard Easier Easy Approach sits between a pure HSM solution and a full software solution.

13 DKM-TPM Key Hierarchy Keys Storage: TPM Processing: TPM Protection: TPM EK (Endorsement Key) SRK (Storage Root Key) Keys Storage: External Processing: TPM Protection: TPM TLSK (TLS Key) AIK (Attestation Identity Key) SK (Signing Key) WK (Wrapping Key) Keys Storage: External Processing: Memory Protection: TPM DKMK (DKM Key)* Seal * There are one or more DKM Keys.

14 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

15 DKM-TPM Roles Node Store Master Node Logic & API Store Logic & API Master Logic & API CommClient CommClient CommServer CommClient Repository KM & Crypto Repository KM & Crypto Repository KM & Crypto TPM TPM TPM Master PK List Store PK List Node PK List Configuration DKM Keys Policies Master PK List Store PK List Node PK List Configuration Master PK List Store PK List Node PK List Configuration

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 x y Random Oracle y {0, 1}* If (x in A) y Fetch(A,x) Else Store(x,y) in A Return y 16

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 m 0, m 1 C Left-Right Oracle b {0, 1} C = Enc(K, m b ) Return C Guess b? IND-CPA Game 17

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 18

19 IND-CCA/CCA2 Game Free Oracle Access Queries {m,c} Responses {C,m} Encrypt C = Enc(K, m) Decrypt m = Dec(K, C) Challenge m 0, m 1 C Left-Right Oracle b {0, 1} C = Enc(K, m b ) Guess b? Adaptive (CCA2) Adversary C m Decrypt m = Dec(K, C) 19

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 F 1 and F 2 have the same key space and length Informal Definition: A primitive Π is agile if any F 1, F 2 Π can securely use the same key. K F 1 F 2 20

21 Pseudo Random Function Agility Facts PRF: F is a PRF if no efficient adversary can distinguish F(K,.) from a random function. F 1 (K 1,x) and F 2 (K 2,x) are not distinguishable from a pair of random functions. K Definition: A set {F 1,F 2 } is agile if F 1 (K,x) and F 2 (K,x) are not distinguishable from a pair of random functions. x F 1 F 2 F 1 (K,x) x F 2 (K,x) Question: Are PRFs agile? Yes, if every {F 1,F 2 } is agile. Answer: No. Example: F 2 (K,x) = NOT (F 1 (K,x)) Now, what? 21

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,f 1 (K,x)) K F 1 F 2 (x,f 1 (K,x)) PRF-based security for Authenticated Encryption: CCM, GCM, etc. Pick a PRF from a small agile set Encryption of M with K, with PRF K ae = PRF(K,<ae>) C = E(K ae, M) Decryption K ae = PRF(K,<ae>) M = D(K ae, C) 22