Digital signature in automatic analyses for confidentiality against

active adversariesIlja Tšahhirov,

Peeter Laud

Goal of the analysis

• Problem statement– Given the protocol (set of programs making

calculations and exchanging messages)– It works with some secret data– No active adversary should be able to learn

anything about the secret data

• Automatically determine whether the protocol is secure or not.

Original technique

• Published in: Peeter Laud. Symmetric encryption in automatic analyses for confidentiality against active adversaries. 2004 IEEE Symposium on Security and Privacy, pages 71-85, May 2004.– Automatic analyzer present

– Programming language

– Single cryptographic primitive – symmetric encryption

– Definition of the adversary

– Definition of the security

– Protocol transformations

Programming language

• Instruction setP :: = k:=gen_key | y:=(x1,…,xm) | x:= πi

m(y)

| x:=encrk(y) | y:=decrk(x) | x:=random

| send(x) | x:=receivel | check(x=y)

| x:=constant(b) | x:=y

| kp:=gen_key_pair | pk:=public_key(kp)

| sm:=signkp(m) | testpk(sm)

| m:=get_signed_message(sm)

• The only cryptographic primitive in original analysis – symmetric encryption

• Our contribution is adding the digital signature primitive support (commands in bold) to the language.

Adversary

• Adversary is active - it schedules the participants and relays messages between them

• Can modify, create new, or not deliver sent messages

Security definition

The protocol is considered secure if the secret message is computationally independent from the adversary’s view.

Security against chosen-ciphertextattacks

• No PPT adversary should be able to distinguish second black box from the first

Without querying the second algorithm with the outputs from the first

Protocol transformations - encryption

• During the analysis protocols are transformed

• Protocols working with the first black box can be replaced to use the second (under certain conditions)

Information flow analysis

• If some participant of the protocol contains a statement of the form x:=E(x1,…,xn) there is an information flow from the variable xi to the variable x.

• The protocol is deemed secure if M * y holds for no y affecting the adversary’s view.

• The protocol transformation described above breaks some of those links.

Unforgeability under adaptive chosen message attack

• The property we require signature scheme to satisfy

• Adversary making queries to the signature oracle should not be able to create a valid signature for the message that has not previously been signed by it

S T

KP

Query

Answer

Query

Answer

Protocol transformations – digital signature

• Signature operations are replaced with checking whether the signed message being tested belongs to the set of the actually signed messages.

Running example

• Transmit the public key and signature from A to B

A generates KPA

A : public_key(KPA)

AB : enc(KAB: public_key(KPA))

AB : enc(KAB:sign(KPA:M)) B verifies the signature

B : OK

• KAB is a long-term key shared between A and B.

Data dependencies

B

A KAB

R2 R3

S1 S2 S3

enc

KPA

public_key

sign M

enc

dec

test

dec

S4

Control dependencies

KAB

R2 R3

S1 S2 S3

enc

KPA

public_key

sign M

enc

dec

test

dec

S4

B

A

Criterion for security

No path from M to any Si

The system is secure

Security does not follow

KAB

R2 R3

S1 S2 S3

enc

KPA

public_key

sign M

enc

dec

test

dec

S4

Encryptions replaced

KAB

R2 R3

S1 S2 S3

enc

KPA

public_key

sign

Menc

case

test

case

0

0

S4

Security still does not follow

KAB

R2 R3

S1 S2 S3

enc

KPA

public_key

sign

Menc

case

test

case

0

0

S4

Case handling – Case 1

KAB

R2 R3

S1 S2 S3

enc

KPA

public_key

sign

Menc

=?

test

=?

0

0

S4

Case 1 - Replacing the signature test

KAB

R2 R3

S1 S2 S3

enc

KPA

public_key

sign

Menc

=?

get_signed_message

=?

0

0

in

S4

Case 1 – in statement handling.

KAB

R2 R3

S1 S2 S3

enc

KPA

public_key

sign

Menc

=?

get_signed_message

=?

0

0

=?

S4

Case 1 – check statement handling

KAB

R2 R3

S1 S2 S3

enc

KPA

public_key

sign

Menc

=?

get_signed_message

=?

0

0

=?

S4

Sub-protocol is secure (result of check can be statically determined)

Case 2

KAB

R2 R3

S1 S2 S3

enc

KPA

public_key

sign

Menc

=?

test

=?

0

0

S4

2x

Sub-protocol is secure (test statement always fails)

Conclusions and future work

• Conclusions– The presented technique can be used in automated

analysis of the cryptographic protocols– Technique is published in Nordsec 2005

proceedings, p 29-41.

• Future work– Implementation of the automated analyser– Introducing the support for other cryptographic

primitives