Password Enabled Public-Key Infrastructure (PKI): Virtual Smartcards vs. Virtual Soft Tokens

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Ravi SandhuChief Scientist

SingleSignOn.Net&

Professor, George Mason University

Mihir BellareChief Cryptographer

SingleSignOn.Net&

Professor, Univ. of California--San Diego

Ravi GanesanChief Executive

OfficerSingleSignOn.Net

11417 Sunset Hills Rd., Reston, VA 20190

Password Enabled Public-Key Infrastructure (PKI):Virtual Smartcards vs. Virtual Soft Tokens

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Why Password-Enabled PKI

• Smartcards have not happened– It’s the smartcard readers stupid!– Roaming capability is critical– Even DoD is stretched in large-scale deployment

• Trends are not in favor of smartcards– Deployment scale of 10’s or even 100’s of millions of

users– Computing devices are proliferating– Large installed base of reader-less computers

Smartcards are likely to remain a high-assurance niche application

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Solve PKI Gap and Silo Problem

Weak Password Systems

Strong PKI Systems

PKI Hardened Passwords

PKI with Password

Convenience

Result

• Phased migration path

• No “quantum jump”

• PKI integral, not silo’d

Password Usability

PKI Capability

No change for users

No change for issuer

Eliminate weaknesses

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A Common Misperception

• Fact: Password based systems are often vulnerable to attacks

• Myth: Passwords are inherently insecure.• Fact: It is completely possible to design a

sufficiently secure password system.

Designing sufficiently secure password-based systems is non-trivial but it is possible.

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Another Common Misperception

• Fact: Users hate current password systems that require– too many passwords and – force too many changes

• Myth: Users inherently hate passwords. • Fact: It is completely possible to design a user

friendly password system with PKI-enabled Single Sign On.

Designing user-friendly and sufficiently secure password-enabled PKI systems is non-trivial but it is possible.

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Password Vulnerabilities and Counter-Measures

• Bad password selection– enforce complexity rules

• On-line guessing attack– throttling mechanism

• Off-line guessing (dictionary attacks)– don’t reveal required information (we know how to design such protocols)

• Undetected theft and sharing– online intrusion detection to discover– deter sharing, e.g., sharing reveals sensitive user information

• Use of same password at strong and weak servers– user awareness and education

• Password reuse– don’t force unnecessary password changes

• Server spoofing– use secure protocols to prove knowledge of password w/o sending it– limit password exposure to trusted servers

• Server compromise– use hardened servers or multiple servers

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Password Benefits

• Instant roaming capability• Proven user acceptance

– 100’s of millions of passwords usages per day in cyberspace

• Cheap• Self-maintained

– Password resets– Password change

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Traditional Public-Key Infrastructure (PKI)

• How to distribute public-keys– Digital Certificates– Certificate Revocation Lists

• How to distribute private-keys (long-term)– Smartcards

• The private key never leaves the smartcard• Often called a hard token

• How to distribute private-keys (short-term)– Password protected on the hard disk

• Not very mobile– Password protected on a floppy disk

• Often called a soft token

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“Modern” Public-Key Infrastructure (PKI)

• How to distribute public-keys– Digital Certificates– Certificate Revocation Lists– On-line servers for certificate validation

• How to distribute private-keys (long-term)– Smartcards

• The private key never leaves the smartcard• Often called a hard token

• How to distribute private-keys (short-term)– Password protected on the hard disk

• Not very mobile– Password protected on a floppy disk

• Often called a soft token– On-line servers for password-enabled mobility

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Approaches

How to marry PKI and Passwords?

• Approach 1: Virtual Soft TokenUse password to encrypt private key and store it on remote server(s).

Need password to RETREIVE private key. • Approach 2: Virtual Smartcard

The password is part of the composite private key.

Need password to USE private key.

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Trivial Insecure Virtual Soft Token

• Private key encrypted with user’s password is stored on an on-line serverEpwd(private-key)

• Anyone is allowed to retrieve the encrypted private key

• Only the user can decrypt it using the password

• Unacceptable risk due to dictionary attack

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Cryptographic Camouflage, Hoover and Kausik

• Epwd(private-key)

• Dictionary attack– Knowledge of public key allows attacker

to obtain known plaintext– So prohibit knowledge of public key

resulting in closed public-key system

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EKE Roaming, Bellovin-Merritt et al

• Store Epwd(private-key) on server

• Transmit EK(Epwd(private-key)) where K is a strong symmetric key

• K is established using password-based authenticated key exchange protocol (such as EKE or SPEKE)– Immune to off-line dictionary attack

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Hardened Password Roaming, Kaliski-Ford

• User’s “hardened password” is retrieved at any computer from two on-line servers– Compromise of both servers is required to

compromise “hardened password”– Successful retrieval of “hardened password”

requires knowledge of user’s password

• User’s private key is retrieved by means of “hardened password”

• Once retrieved the user’s private key can be freely used on this computer

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Step 5: Ask for C

redentials

Long term private key is lockedwith ‘hardened password’ H.

Need duplicate credentials serverfor redundancy.

Credential Servers 1 & 2S

tep 6: C

heck if Cert

is revoked

OCSP server to check for revocation

Revocation Servers 1 & 2

Step 7: R

eturn C

ert an

d H (D

)

Step 8: Use H to decrypt

private key D

Step 2: Client Computer starts process

ClientComputer

Security server with partial knowledge of ‘H’ (H1). Need duplicate server

for redundancy.

Security Servers 1 & 2

Step 3: Get H

1

Security server with remaining knowledge of ‘H’ (H2). Need duplicate server

for redundancy.

Security Servers 3 & 4

Step

4: G

et H

2

Step 9: Finally get around to logonor sign operation!

Alice knows Password, Pa

Step 1: Alice sends Pa

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Approaches

How to marry PKI and Passwords?

• Approach 1: Virtual Soft TokenUse password to encrypt private key and store it on remote server(s).

Need password to RETREIVE private key. • Approach 2: Virtual Smartcard

The password is part of the composite private key.

Need password to USE private key.

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Trivial Insecure Virtual Smart Card

• Keep the private key on an on-line server

• Use the password as authentication to enable use of the private key on the server

• Lose non-repudiation

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We want:

2. Alice takes A

And creates A A

1. Appliance takes

And creates A

3. But (presto!)

A A A

ACC C

A

ID: Castle CorpFN: CastleLN: Corp..

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The Practical PKITM Approach

Password

Secure Identity

Appliance

A

A• A Secure Identity Appliance has key d2 for Alice which ONLY it knows.

CA

CC C

A

ID: Castle CorpFN: CastleLN: Corp..

• As before, Alice has public cert, with public key e, signed by a CA.

Process1. Alice authenticates to appliance, sets up

secure channel and sends M.2. Appliance performs partial signature on M

with its key for Alice d2.3. Alice completes signature with her key d1.

• Alice has password P which ONLY she knows. Password P expands to key d1 on computer.

A

A

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ComparisonTraditional PKI

Keys:a) Alice Public = eb) Alice Private = d

c) Alice Cert = C

Signing:a) S = Sign (M,d)

Send [S, C] to Bob

Bob:Gets e from CDoes Verify(S,e) = M?

Practical PKITM

Keys:a) Alice Public = eb) Alice PKCS5(password, salt,

iteration count) = d1c) Alice Cert = Cd) Alice appliance key = d2

Signing:a) Alice logs on to appliance using

d1 and creates secure channela) Spartial = Sign(M,d2)b) S = Sign(Spartial,d1)

Send [S, C] to Bob


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c) Alice Cert = C


Send [S, C] to Bob


Practical PKITM





Send [S, C] to Bob


Difference #1: Alice has short convenient password

Difference #2: Alice has to interact with appliance to

sign.

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c) Alice Cert = C


Send [S, C] to Bob


Practical PKITM





Send [S, C] to Bob


NOTHING ELSE CHANGES!!!!

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Strong Fraud Management

ID stolen

Theft detected

Theft reported

CA revokes ID

A

CA

ID: AliceFN: AliceLN: SmithEmail:[email protected] ..

CA

Recipient (we hope)stops accepting ID

Velocity Checking

Easy to report

ID CANNOT BE USED ANY FURTHER!INSTANT, COMPLETE, REVOCATION

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Strong Fraud Management

ID stolen

Theft detected

Theft reported

CA revokes ID

A

CA

ID: AliceFN: AliceLN: SmithEmail:[email protected] ..

CA

Recipient (we hope)stops accepting ID

Velocity Checking

Easy to report

ID CANNOT BE USED ANY FURTHER!INSTANT, COMPLETE, REVOCATION

Consumer or CSR can use password to revoke instantly!

Every signature requires appliance interaction. So appliance logs can be used for velocity

checking.

Every signature requires appliance interaction. Once

revoked key cannot be used further! Instant, complete revocation!

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SingleSignOn.Net

• Practical PKITM solution– Ease of use: password based– Quick to deploy– Simple to manage with least privilege– Velocity checking and instant revocation– Reusable for multiple applications

• Web, Wireless, VPN, email, etc.

– Use existing standards and widely deployed technologies

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Summary

• Password enabled solutions are poised to jump start the stalled PKI car.

• Major vendors jumping into password enabled solutions using on-line servers is a good sign.

• “Many servers” are not all good, and have quality/security downside.

• Making password a part of the composite private key (virtual smartcards) provides substantial advantages over using password to retrieve private key (virtual soft tokens).

Documents

Password Enabled Public-Key Infrastructure (PKI): Virtual Smartcards vs. Virtual Soft Tokens