Differential Protocol Analysis & API Level Attacks

1. Differential Protocol Analysis& API Level Attacks Mike Bond Computer Security Group Security and Protection of Information 30th Apr ‘03

2. Summary • Security APIs • Hardware Security Modules • Introduction to Banking Security • Conventional Protocol Attacks • API-Level Attacks • Differential Protocol Analysis • Solution: Formal Methods & Evaluation ? • Conclusions

3. What is a Security API ? • A command set that uses cryptography to control processing of and access to sensitive data, according to a certain policy VDU Host PC or Mainframe Security Processor PCI Card or Separate Module I/O Devs Network Security API

4. Example Security API Commands U->C : { A }KM , { B }KM C->U : { A+B }KM U->C : GUESS , { ANS }KM C->U : YES (if GUESS=ANS else NO) U->C : { X }K1 , { K1 }KM , { K2 }KM C->U : { X }K2

5. Research into API Attacks • Some work in early 90’s using prolog style search to find attacks, but few documented atttacks • Work started in 2000 at University of Cambridge with analysis of hardware security modules used in banks to protect PINs for ATMs • New work found many more attacks, and produced first significant catalogue of API failures • Scope has been broadened to include security modules used by certification authorities and also general purpose crypto libraries (eg MSCAPI, PKCS#11) • Latest work revisiting financial APIs examining PIN generation and verification procedures…

6. Hardware Security Modules • An instantiation of a security API • Often physically tamper-resistant(epoxy potting, temperature & xray sensors) • May have hardware crypto acceleration(not so important with speed of modern PC) • May have special ‘trusted’ peripherals(key switches, smartcard readers, key pads) (referred to as HSMs subsequently)

7. Hardware Security Modules

8. Who Needs Security Modules ? • Those who need to enforce access policies to sensitive information Examples: Granting signing permission at a CertificationAuthority Enforcing split control policies on nuclear weapons & arming codes • Those who need to protect mission critical sensitive data Example: Protecting PIN generation keys at banks • Those who need to protect data in hostile environments Examples: Protecting Token Vending Machines (Electricity, Lottery etc…) Protecting communications keys in battlefield radios • Those with high crypto throughput requirements Example: SSL acceleration for webservers

9. Financial HSMs & API Attacks • Attacks discovered at Cambridge • VSM Type System attack • XOR to Null key attack • Meet in the Middle attack • Newly discovered attacks: • Decimalisation table attack • PAN modification attack • Prerequisites: • Financial security 101 • Conventional Protocol Attacks review

10. Why Financial Security? • Concrete and simple security policy for APIs “Only the customer should know her PIN.” “Keys protecting PINs may only be manipulated when authorised by two different employees.” • API manuals are often publicly available • IBM put 4758 CCA manual on its website • Diversity: many manufacturers have APIs performing same broad functionality – good for comparison • ATM security was the “killer-app” that brought cryptography into the commercial mainstream – so long history of financial API development

11. Introduction to ATM Security • The crucial secret is the customer PIN. The customer should be the only person that knows the value of this PIN • PINs need to be protected from malicious insiders and outsiders • PINs must be protected when generated, in storage, when issued to customers, when travelling via the international ATM network, and when being verified • To this end, banks use Hardware Security Modules (HSMs) to perform cryptography and implement a policy which prevents both insiders and outsiders from gaining unauthorised access to PINS.

12. ATM Network Security Modules in Banks Acquiring Bank Issuing Bank HSM HSM HSM HSM HSM HSM with keypad HSM Issuing Bank Regional HQ ATM HSM HSM

13. How are PINs Generated ? Start with your bank account number (PAN) 5641 8203 3428 2218 Encrypt with PIN Derivation Key (aka PMK – Pin Master Key) 22BD 4677 F1FF 34AC Chop off the (B->1) End 2213 (D->3) decimalise

14. What’s a Decimalisation Table ? • Remember encrypted result was in hexadecimal? • Encryption produces output that looks uniformly distributed, so 0-F are all equally likely • Decimalisation Table used to map 0-F back to 0-9 digit in 0123456789ABCDEF digit out 0123456789012345 e.g. 22BD -> 2213 • Because some numbers have several hexadecimal digits mapped to them, they are more likely to occur in issued PINs than others

15. Example Distribution : HSBC (Sample size: 45 people)

16. Conventional Protocol Attacks • The starting point for the study of API attacks • The protocol world has attacks similar to each of those found on APIs • Each example demonstrates a different aspect of protocol attack: • Needham Schroder Public Key • attacker knows how to encrypt/decrypt • TMN Protocol • attacker knows algebra of RSA • Bleichenbacher PKCS#1 Attack • attacker knows protocol data structures & understands RSA crypto

17. Protocol Attacks Needham Schroeder Public Key TMN Attack Manipulation Attacks Mathematical Attacks Bleichenbacher Attack

18. Needham-Schroeder Public Key Protocol A -> B : { NA , A}KB B -> A : { NA , NB }KA A -> B : { NB }KB A -> B : { NA , A}KB B -> D : { NA , A}KD D -> B : { NA , ND }KA B -> A : { NA , ND }KA A -> B : { ND }KB B -> D : { ND }KD attackers abilities: Construct messages, Encrypt & decrypt

19. TMN Protocol Explanation: Cubing modulo N represents RSA encryption under public key of server S A->S: RA3 mod N B->S: RB3 mod N S->A: RA xor RB C->S: RA3 * X3 mod N D->S: RD3 mod N S->A: RA*X xor RD Explanation: Server will not accept same random number twice, so C cannot simply replay RA3

20. Bleichenbacher PKCS#1 Attack Explanation: Alice sends key K encrypted using RSA with PKCS#1 v1.5 padding A->S: Ke mod N S->A: {data}K C->S: Ke * Xe mod N S->A: {data}K*X or S->A: decryption error S->A: format error Explanation: Server tries to decrypt message from Charlie, and leaks information as to whether the decrypted message satisfied PKCS#1 format requirement

21. API Attacks & Protocol Attacks • API Attacks already found in each category • Manipulation (VSM Type System) • Mathematical (XOR to null key) • Cryptographic (MIM attack)(both manipulative and mathematical) • Differences between API and Protocol Attacks: • API is a ‘dumb adversary’ • APIs are more complex – contain many protocols • APIs (usually) have only two principals, HSM and User

22. XOR to Null Key Attack • Top-level crypto keys exchanged between banks in several parts carried by separate couriers, which are recombined using the exclusive-OR function • A single operator could feed in the same part twice, which cancels out to produce an ‘all zeroes’ test key. PINs could be extracted in the clear using this key U->C : {KP1}KM , {KP2}KM C->U : {KP1 xor KP2}KM U->C : {KP1}KM , {KP1}KM C->U : {KP1 xor KP1}KM ( = {0}KM ) (Anderson 2000)

23. VSM Type System Attack • Encrypting communication keys for transfer to an ATMs used exactly the same process as calculating a customer PIN • Customer PINs could be generated by re-labelling an account number as a communications key, and using the same encryption process (Bond 2000)

24. The Visa Security Module

25. VSM Type Diagram

26. VSM Type System Attack

27. Type System Attack (Protocol Notation) U->C : 5641 8203 3428 2218 C->U : {5641 8203 3428 2218}TC U->C : {5641 8203 3428 2218}TC , { PMK }TMK C->U : {5641 8203 3428 2218}PMK {5641 8203 3428 2218}PMK = 22BD 4677 F1FF 34AC So customer PIN is 22BD i.e. 2213

28. Car Park Analogy • A thief walks into a car park and tries to steal a car... • How many keys must he try?

29. Car Park Analogy 1900

30. Car Park Analogy 2000

31. The Meet in the Middle Attack • Common sense statistics • Attack multiple keys in parallel • Need the same plaintext under each key • Encrypt this plaintext to get a ‘test vector’ • Typical case: A 256 search for one key becomes a 240 search for 216 keys • Poor implementations of 3DES key storage allow 3DES key halves to be attacked individually

32. MIM Attack on DES Security Modules • Generate 216 keys • Encrypt test vectors • U->C : { KEY1 }KM • C->U : { 0000000000000000 }KEY1 • Do 240 search Cryptoprocessor’s Effort Search Machine’s Effort 16 bits 40 bits 56 bit key space

34. Protocol Attacks Manipulation Attacks Needham Schroeder Public Key Decimalisation Table Attack TMN Attack PAN Modification Attack Mathematical Attacks Differential Attacks Bleichenbacher Attack

35. Decimalisation Table Attack • Remember PINs derived from account numbers • Hexadecimal raw PIN is converted to decimal using decimalisation table • Most APIs allow the decimalisation table to be specified with each PIN verification command • A normal verification command eliminates one of 10,000 combinations of PIN for the attacker. • If the table is altered, whether or not the alteration affects correct verification leaks much more information about the PIN examples… (Bond/Clulow 2002)

36. Decimalisation Table Attack (1) Encrypted PMK 48CCA975F4B2C8A5 PAN 5641820334282218 Trial PIN 0000 0123456789ABCDEF 0123456789012345 PIN_Verify 1. Encrypt PAN Raw PIN = 22BD 2. Decimalise Natural PIN = 2213 3. Verify 0000 != 2213 Yes/No (eliminates 1 combination)

37. Decimalisation Table Attack (2) Encrypted PMK 48CCA975F4B2C8A5 PAN 5641820334282218 Trial PIN 0000 0123456789ABCDEF 0000000100000000 PIN_Verify 1. Encrypt PAN Raw PIN = 22BD 2. Decimalise Natural PIN = 0000 3. Verify 0000 = 0000 Yes/No (eliminates all PINs containing digit 7)

38. Decimalisation Table Attack (3) Encrypted PMK 48CCA975F4B2C8A5 PAN 5641820334282218 Trial PIN 0000 0123456789ABCDEF 0010000000000000 PIN_Verify 1. Encrypt PAN Raw PIN = 22BD 2. Decimalise Natural PIN = 1100 3. Verify 0000 != 1100 Yes/No (shows PIN contains digit 2)

39. Decimalisation Table Attack (4) Encrypted PMK 48CCA975F4B2C8A5 PAN 5641820334282218 Encrypted Trial PIN {2213}KM 0123456789ABCDEF 0123456789012345 PIN_Verify 1. Encrypt PAN Raw PIN = 22BD 2. Decimalise Natural PIN = 2213 3. Verify 2213 = 2213 Yes/No (no information)

40. Decimalisation Table Attack (5) Encrypted PMK 48CCA975F4B2C8A5 PAN 5641820334282218 Encrypted Trial PIN {2213}KM 0123456789ABCDEF 0123456089012345 PIN_Verify 1. Encrypt PAN Raw PIN = 22BD 2. Decimalise Natural PIN = 2213 3. Verify 2213 = 2213 Yes/No (eliminates PINs containing digit 7)

41. PAN Modification Attack (1) • Encrypted PINs transferred from ATM to issuing bank via ATM network using point to point encryption • At each node PIN block must be decrypted with incoming key, and re-encrypted with outgoing key • Common ISO standard “binds” PIN to particular customer by exclusive-ORing PAN with PIN before encryption • Attack: specifying incorrect PAN may make deduced PIN contain hexadecimal digit ‘A’-’F’, which causes formatting error. Conditions under which formatting error arises leaks information about PIN. (Clulow 2002)

42. PIN Block Formats IS0-0 padding Primary Account Number (PAN) PIN PIN length 5461 8203 6345 2239 Format ID 041234FFFFFFFFFF xor 0000820363452239 = 0412A6FC9CBADDC6 IS0-2 241234FFFFFFFFFF

43. PAN Modification Attack (2) Format Info {AWK}KM {IWK}KM {PIN Block}IWK PAN PIN_Translate {PIN Block}AWK (or FORMAT ERROR)

44. PAN Modification Attack (3) PIN 041234FFFFFFFFFF xor 0000820363452239 = 0412B6FC9CBADDC6 construction of PIN block 0412B6FC9CBADDC6 xor 0000820363452239 = 041234FFFFFFFFFF correct PAN removed PAN 0412B6FC9CBADDC6 xor 0000720363452239 = 0412C4FFFFFFFFFF modified PAN Removed – PIN contains ‘C’ – error

45. Differential Protocol Analysis • Phrase coined by Anderson & Bond in “Protocol Analysis, Composability and Computation” in Feb 2003 • Differential Protocol Analysis refers to attacks which use multiple runs of a protocol to gradually discover a secret key protected by the protocol • Input differentials (pairs of inputs) are carefully chosen such that a difference in output will be observed dependent upon some secret key material • Before this work, such attacks were rare – dumb adversaries were thought unlikely

46. Dectab Attack during PIN Generation Encrypted PMK 48CCA975F4B2C8A5 PAN 5641820334282218 0123456789ABCDEF 0123456089012345 PIN_Generate 1. Encrypt PAN Raw PIN = 22BD 2. Decimalise Natural PIN = 2213 3. Store as ISO PIN Block 042213FFFFFFFFFF FD29DA10029726DC

47. “DPA” - Decimalisation Table

48. “DPA” – PAN Modification

49. Protecting against API Attacks: Formal Methods and Evaluation? • New protocol attacks show formal methods now cover just one of many aspects of protocol attacks • Automated information-theoretic reasoning about protocols and APIs could be an interesting new direction for research and evaluation tools, but is years away • API attacks are already having an effect on the real world – especially in financial security, yet evaluation standards e.g. FIPS 140 series are not keeping up • API attacks have defeated many different modules certified FIPS 140-1 Level 4 – why? Because the evaluation does not cover the whole system, just a few components. • Be careful when buying entire systems to establish what has been evaluated, and what has not.

50. Conclusions • Protocol Analysis is far from dead • API security complements and extends the study of security protocols • “Differential Protocol Analysis” attacks were previously novelties, but may become important against ‘dumb adversaries’ as Security APIs become more common • API analysis used to only be relevant to a few specialist applications such as banking security and certification authorities; TCPA and Palladium APIs when deployed will bring targets into everybody’s homes • Formal methods tools have a long way to go to provide assurance for using systems • Independent evaluation can protect against these attacks, but it must cover the whole system.