The Relationship between Decisional Second-Preimage Resistance and Preimage Resistance in Cryptographic Hash Functions

This work delves into the subtle question of when Decisional Second-Preimage Resistance (SPR) implies Preimage Resistance (PRE) in hash functions. It presents a tool for enabling tight security proofs for hash-based signatures by exploring the success probability of adversaries against collision resistance, second-preimage resistance, and one-wayness. The study also discusses the implications of stronger assumptions such as Collision Resistance in relation to easier attack scenarios compared to Second-Preimage Resistance. Additionally, it addresses reductions and challenges in proving that Collision Resistance implies Second-Preimage Resistance and Second-Preimage Resistance implies Preimage Resistance.

• Cryptographic Hash Functions
• Security Properties
• SPR
• PRE
• Hash-Based Signatures

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1. Decisional second- preimage resistance When does SPR imply PRE? Daniel J. Bernstein, Andreas H lsing

2. Motivation This work answers a long standing subtle question about the relation of hash function properties provides a tool that enables tight security proofs for hash-based signatures 11/29/2019 https://sphincs.org 2

3. Cryptographic hash functions Efficient function h: 0,1? 0,1?(?) 0,1? We write h k,x = ?(?) Key ? in this case is public information. Think of function description. 3

4. Collision resistance Success probability of an adversary ? against collision resistance (CR) of h is defined as ??(?) = Pr[? ? 0,1?, ?1,?2 ? ? : Succ ??1 = ??2 ?1 ?2] 4

5. Second-preimage resistance (SPR) Success probability of an adversary ? against second-preimage resistance (SPR) of h is defined as ???? = Pr[? ? 0,1?,? ? 0,1? ?, Succ ? ? ?,? : ?? = ?? ? ? ] 5

6. Security properties: Preimage resistance / One-wayness Success probability of an adversary ? against preimage resistance (PRE) of h is defined as ???? = Pr[? ? 0,1?,? ? 0,1? ?, Succ ? ?? ,? ? ?,? : ?? = ?] 6

7. Relations Stronger assumption / easier to break Collision-Resistance Assumption / 2nd-Preimage- Resistance Attacks weaker assumption/ harder to break One-way 11/29/2019 https://sphincs.org 7

8. CR implies SPR? ????(?): Reduction ??? 1. ? ? 0,1? ? 2. ? ????(?,?) 3. Return (?,? ) ????= ????? ??????? Succ Succ 8

9. SPR implies PRE? ????(?,?): Reduction ???? 1. ? = ?(?) 2. ? ????(?,?) 3. Return ? ???? ? ??????? ??????? Succ 0.5 Succ Where is the problem? 9

10. Positive result Rogaway-Shrimpton show that for ?(?) much bigger then ? we are fine 11/29/2019 https://sphincs.org 10

11. Negative result The identity function demonstrates that SPR cannot generally imply PRE. 11/29/2019 https://sphincs.org 11

12. The gap Functions with ?(?) ? (especially length preserving) Exactly the ones we use in hash-based OTS Are we doomed? 11/29/2019 https://sphincs.org 12

13. The general case SHA-X identity function SHA-X random function 11/29/2019 https://sphincs.org 13

14. Fooling the reduction Reductions have to work for all ?! ?(?,?): 1. Compute ? = ?? 2. If ? > 1, abort 3. Else ? = {?}, return ? 1(?) For ??: 0,1? 0,1? 0,1?random, ???? =1 Succ ? ????= 0 ??????? Succ 11/29/2019 https://sphincs.org 14

15. Decisional second-preimage resistance to the rescue! 1(??(?)) = 1 1, otherwise ??? = 0, if ?? ????if Can salvage reduction ???? 1. SPprob(fk) = Pr ??? = 1 ? ? 0,1? is non- negligible , and 2. it is hard to reliably determine ??? We show that SPprob fk 1 1 majority of all functions. E.g., for a random 256bit hash fk ?for the overwhelming 11/29/2019 https://sphincs.org 15

16. DSPR = It is hard to reliably determine ??? ????? = max{0, Adv? Pr[? ? 0,1?,? ? 0,1?,? ? ?,? :??? = ?] SPprob(fk) } 11/29/2019 https://sphincs.org 16

17. Some intuition about DSPR ????? 0 SPprob fk 1 Adv? If fkis strongly compressing, it is information-theoretically DSPR. ????? = max{0, Adv? Pr[? ? 0,1?,? ? 0,1?,? ? ?,? :??? = ?] SPprob(fk) } 11/29/2019 https://sphincs.org 17

18. Some intuition about DSPR Given SPR adversary ???? define: ????? 1. If ????(?,?) succeeds, return 1 2. Return 0. Requires Succ? non-zero advantage! ?????,? : ??????? > 2 SPprob fk 1 for ????? = max{0, Adv? Pr[? ? 0,1?,? ? 0,1?,? ? ?,? :??? = ?] SPprob(fk) } 11/29/2019 https://sphincs.org 18

19. DSPR at work ????(?,?): Reduction ???? 1. ? = ?(?) 2. ? ????(?,?) 3. Return ? We show ??????? ????????? ??????? Succ? Adv? 3 Succ? ????+ ???? ????(?,?): Reduction ????? 1. ? = ?(?) 2. ? ????(?,?) 3. Return 1 if ? ? 4. Return 0 11/29/2019 https://sphincs.org 19

20. Application to hash-based signatures Interactive Game T-OpenPRE: 1. Generate T pairs ??,?? = ??,???? ?? ? 0,1? 2. Give pairs to ? and allow ? to ask for up to ? 1 of the ?? 3. Output 1 if (?,?) ?( ) is a preimage (???? = ??)for unopened image ?? , Variants of this naturally arise in security proof of WOTS, and L-OTS 11/29/2019 https://sphincs.org 20

21. Pre T-OpenPRE is non-tight! Given ?? ??????? build ? ?,? : 1. Play T-OpenPRE game but replace random pair (??,??) by challenge ?,? 2. If ? asks to open position ?, abort 3. If ? returns (?,?), output ? Reduction loss of 1/T! 11/29/2019 https://sphincs.org 21

22. Multi-target DSPR 11/29/2019 https://sphincs.org 22

23. T-DSPR + T-SPR T-OpenPRE, tightly! ????, and ????? ???? Use T-target versions of ???? Can replace all pairs by challenges We do know a preimage If ?? ??????? always returns known image, ?? ???? DSPR ?? ??????? succeeds with high probability ?? ??????? will have advantage in breaking T- Else, ?? ??? 11/29/2019 https://sphincs.org 23

24. More in paper DSPR is quantum-hard for random functions Detailed analysis of SPprob Full proofs See The SPHINCS+ Signature Framework (CCS 19) for application to SPHINCS+ and other hash-based signatures. 11/29/2019 https://sphincs.org 24

25. Questions? Paper(s) available at https://sphincs.org/resources.html 11/29/2019 https://sphincs.org 25