distributed key management cryptographic agility · untrusted storage (dkm-protected data) dkm...
TRANSCRIPT
Tolga Acar 24 Feb. 2011
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Distributed Key Management and Cryptographic Agility
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
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Distributed Key Management
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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?
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.
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Key State Transitions
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Initialization
Full
Distribution
Active Inactive TerminationCreation
Cryptographic operations Create and
initialize a key
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)
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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
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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
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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
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Walkthrough: DKM in Hosted E-Mail Hosted 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
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DKM in Hosted E-Mail Hosted 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
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DKM-TPM Motivation
Secret Protection Technology:
• Approach sits between a pure HSM solution and a full software solution.
Expensive Moderate Inexpensive Cost:
Very Secure More Secure Moderate (OS-Dependent) Security:
Hard Easier Easy Deployment:
HSM Hardware Security Module
TPM-based
Crypto
Software 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 List Store PK List Node PK List Configuration
Store
DKM Keys Policies
Master PK List Store PK List Node PK List Configuration
Node
Master PK List Store PK List Node PK List Configuration
CommServer CommClient CommClient CommClient
TPM
KM & Crypto Repository
TPM
KM & Crypto Repository
TPM
KM & Crypto Repository
Node Logic & API Store Logic & API Master Logic & API
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
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y {0, 1}* If (x in A) y Fetch(A,x) Else Store(x,y) in A Return y
Random Oracle
x
y
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
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b {0, 1} C = Enc(K, mb) Return C
Left-Right Oracle m0, m1
C
Guess b?
IND-CPA Game
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
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IND-CCA/CCA2 Game
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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
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.
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F1 F2
K
Pseudo Random Function Agility
Facts
• 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.
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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?
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>
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⊥ (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)