![Secure Authen`cated Key Exchange
• Desired proper`es:
– Authen7ca7on of communica`on partners
– Good cryptographic keys
• Several security models formalizing this no`on
– We use enhanced version of Bellare-Rogaway [BR’93]
• Adopted to the public-key sewng
• Cf. Blake-Wilson et al. [BJM’99]
• Model described by two components:
– Execu7on model
– Security defini7on
13](https://image.slidesharecdn.com/2016bar-ilantls-dhe-160502175231/85/On-the-Security-of-TLS-DHE-in-the-Standard-Model-13-320.jpg)
![Secure Authen`cated Key Exchange
• Desired proper`es:
– Authen7ca7on of communica`on partners
– Good cryptographic keys
• Several security models formalizing this no`on
– We use enhanced version of Bellare-Rogaway [BR’93]
• Adopted to the public-key sewng
• Cf. Blake-Wilson et al. [BJM’99]
• Model described by two components:
– Execu7on model
– Security defini7on
14](https://image.slidesharecdn.com/2016bar-ilantls-dhe-160502175231/85/On-the-Security-of-TLS-DHE-in-the-Standard-Model-14-320.jpg)
![Secure Authen`cated Key Exchange
• Desired proper`es:
– Authen7ca7on of communica`on partners
– Good cryptographic keys
• Several security models formalizing this no`on
– We use enhanced version of Bellare-Rogaway [BR’93]
• Adopted to the public-key sewng
• Cf. Blake-Wilson et al. [BJM’99]
• Model described by two components:
– Execu7on model
– Security defini7on
15](https://image.slidesharecdn.com/2016bar-ilantls-dhe-160502175231/85/On-the-Security-of-TLS-DHE-in-the-Standard-Model-15-320.jpg)
![Unsa`sfying Situa`on
• TLS is the most important security protocol
in prac`ce
• TLS Handshake is insecure in any AKE security
model based on key-indis`nguishability
• Two approaches to resolve this issue:
1. Consider “truncated” TLS Handshake [MSW’10],
without encryp7on of FINISHED messages
2. Develop a new security model
25](https://image.slidesharecdn.com/2016bar-ilantls-dhe-160502175231/85/On-the-Security-of-TLS-DHE-in-the-Standard-Model-25-320.jpg)
![Unsa`sfying Situa`on
• TLS is the most important security protocol
in prac`ce
• TLS Handshake is insecure in any AKE security
model based on key-indis`nguishability
• Two approaches to resolve this issue:
1. Consider “truncated” TLS Handshake [MSW’10],
without encryp7on of FINISHED messages
2. Develop a new security model
26](https://image.slidesharecdn.com/2016bar-ilantls-dhe-160502175231/85/On-the-Security-of-TLS-DHE-in-the-Standard-Model-26-320.jpg)
![TLS-DHE is a Secure ACCE Protocol
35
Theorem:
TLS-DHE is a secure ACCE protocol, if
• the PRF is a secure pseudo-random func7on,
• the digital signature scheme is EUF-CMA secure,
• the DDH assump7on holds in the Diffie-Hellman group,
• the PRF-ODH assump7on holds, and
• the Record Layer cipher is secure (sLHAE)
Stateful Length-Hiding Authen7cated Encryp7on [PRS’11]:
• Security no`on for symmetric ciphers
• Captures exactly what is expected from TLS Record Layer
• Achieved by CBC-based ciphersuites in TLS 1.1 and 1.2](https://image.slidesharecdn.com/2016bar-ilantls-dhe-160502175231/85/On-the-Security-of-TLS-DHE-in-the-Standard-Model-35-320.jpg)
![The PRF-ODH Assump`on
• PRF-ODH assump7on: no efficient aaacker can
dis`nguish PRF(guv,m) from random
– Variant of Oracle Diffie-Hellman assump`on [ABR’01] 41
Adversary A Challenger C
m∈M
U,V∈G
PRF(guv,m) or rand∈R
“real” or “random”
W∈G,m’∈M
PRF(Wu,m’)
• Let G = <g> be a group with order p,
let PRF : G x M à R be a func`on
U := gu, V := gv
where u,v ß Zp](https://image.slidesharecdn.com/2016bar-ilantls-dhe-160502175231/85/On-the-Security-of-TLS-DHE-in-the-Standard-Model-41-320.jpg)
![The PRF-ODH Assump`on
• PRF-ODH assump7on: no efficient aaacker can
dis`nguish PRF(guv,m) from random
– Variant of Oracle Diffie-Hellman assump`on [ABR’01] 42
Adversary A Challenger C
m∈M
U,V∈G
PRF(guv,m) or rand∈R
“real” or “random”
W∈G,m’∈M
PRF(Wu,m’)
• Let G = <g> be a group with order p,
let PRF : G x M à R be a func`on
U := gu, V := gv
where u,v ß Zp](https://image.slidesharecdn.com/2016bar-ilantls-dhe-160502175231/85/On-the-Security-of-TLS-DHE-in-the-Standard-Model-42-320.jpg)
![Is PRF-ODH really necessary?
• Not if
– no corrup7ons of long-term secrets are allowed, or
– small changes are made to TLS-DHE Handshake
• E.g. making it more similar to Σ0 [CK’02]
• Impossible to avoid, if
– security model with corrup7ons is considered, and
– reduc`on uses aFacker and PRF as black-box
43](https://image.slidesharecdn.com/2016bar-ilantls-dhe-160502175231/85/On-the-Security-of-TLS-DHE-in-the-Standard-Model-43-320.jpg)
![Is PRF-ODH really necessary?
• Not if
– no corrup7ons of long-term secrets are allowed, or
– small changes are made to TLS-DHE Handshake
• E.g. making it more similar to Σ0 [CK’02]
• Impossible to avoid, if
– security model with corrup7ons is considered, and
– reduc`on uses aFacker and PRF as black-box
44](https://image.slidesharecdn.com/2016bar-ilantls-dhe-160502175231/85/On-the-Security-of-TLS-DHE-in-the-Standard-Model-44-320.jpg)
On the Security of TLS-DHE in the Standard Model
- 2. TLS and SSL Versions 2 SSL 1.0 and 2.0 (Netscape) 1994 1995 SSL 3.0 (Netscape & MicrosoY PCT) 1999 TLS 1.0 (=SSL 3.1) (IETF standard) 2006 2008 TLS 1.2 TLS 1.1 • This talk: TLS ≈ TLS 1.0 ≈ TLS 1.1 ≈ TLS 1.2 • This talk is not about TLS 1.3! 2016? TLS 1.3
- 4. TLS Sessions: Handshake + Record Layer 4 1. Handshake Handshake: • Nego`a`on of cryptographic parameters (selec`on of Cipher Suite) • Authen7ca7on • Establishment of session key k Client Server
- 5. TLS Sessions: Handshake + Record Layer 5 1. Handshake 2. Record Layer Handshake: • Nego`a`on of cryptographic parameters (selec`on of Cipher Suite) • Authen7ca7on • Establishment of session key k Record Layer: • Data encryp7on and authen7ca7on using key k Client Server
- 13. Secure Authen`cated Key Exchange • Desired proper`es: – Authen7ca7on of communica`on partners – Good cryptographic keys • Several security models formalizing this no`on – We use enhanced version of Bellare-Rogaway [BR’93] • Adopted to the public-key sewng • Cf. Blake-Wilson et al. [BJM’99] • Model described by two components: – Execu7on model – Security defini7on 13
- 14. Secure Authen`cated Key Exchange • Desired proper`es: – Authen7ca7on of communica`on partners – Good cryptographic keys • Several security models formalizing this no`on – We use enhanced version of Bellare-Rogaway [BR’93] • Adopted to the public-key sewng • Cf. Blake-Wilson et al. [BJM’99] • Model described by two components: – Execu7on model – Security defini7on 14
- 15. Secure Authen`cated Key Exchange • Desired proper`es: – Authen7ca7on of communica`on partners – Good cryptographic keys • Several security models formalizing this no`on – We use enhanced version of Bellare-Rogaway [BR’93] • Adopted to the public-key sewng • Cf. Blake-Wilson et al. [BJM’99] • Model described by two components: – Execu7on model – Security defini7on 15
- 20. Security Defini`on 20 • Aaacker breaks the protocol if 1. C (or S) “accepts” with partner S (or C), but S and C do not have matching conversa,ons, or 2. it dis7nguishes “real” from “random” key Protocol Π C (pkC,skC) S (pkS,skS)
- 21. Security Defini`on 21 • Aaacker breaks the protocol if 1. C (or S) “accepts” with partner S (or C), but S and C do not have matching conversa,ons, or 2. it dis7nguishes “real” from “random” key Protocol Π C (pkC,skC) S (pkS,skS) “accept” with key k and partner S
- 22. Security Defini`on 22 • Aaacker breaks the protocol if 1. C (or S) “accepts” with partner S (or C), but S and C do not have matching conversa,ons, or 2. it dis7nguishes “real” from “random” key “Test” k / rnd “real” / “random” Protocol Π C (pkC,skC) S (pkS,skS) “accept” with key k and partner S
- 24. The TLS Handshake is not a Provably Secure AKE Protocol • Enc(k;constS,finS) allows to dis7nguish real key k from random – Applies to TLS-DHE, TLS-DHS, and TLS-RSA – Theore7cal aFack in the security model 24 Enc(k;constS, finS) finS = PRF(ms; L3,prev. data) finC = PRF(ms; L4,prev. data) Enc(k;constC, finC) 2. Key exchange 1. Cipher suite agreement 3. FINISHED messages: “Accept” key k with partner S “Accept” key k with partner C
- 25. Unsa`sfying Situa`on • TLS is the most important security protocol in prac`ce • TLS Handshake is insecure in any AKE security model based on key-indis`nguishability • Two approaches to resolve this issue: 1. Consider “truncated” TLS Handshake [MSW’10], without encryp7on of FINISHED messages 2. Develop a new security model 25
- 26. Unsa`sfying Situa`on • TLS is the most important security protocol in prac`ce • TLS Handshake is insecure in any AKE security model based on key-indis`nguishability • Two approaches to resolve this issue: 1. Consider “truncated” TLS Handshake [MSW’10], without encryp7on of FINISHED messages 2. Develop a new security model 26
- 28. 1st Approach: “Truncated TLS” 28 finS finS = PRF(ms; L3,prev. data) finC = PRF(ms; L4,prev. data) finC 2. Key exchange 1. Ciphersuite agreement 3. FINISHED messages: Theorem: Truncated TLS-DHE Handshake is a secure AKE protocol, if • the PRF is a secure pseudo-random func7on, • the digital signature scheme is EUF-CMA secure, • the DDH assump7on holds, and • the PRF-ODH assump7on holds “Accept” key k with partner S “Accept” key k with partner C
- 29. Comparison to Previous Work Morrissey, Smart, Warinschi ‘10 Our work Bellare-Rogaway Model Bellare-Rogaway Model TLS_DHE, TLS_DH, TLS_RSA1 TLS_DHE Random Oracle Model Standard Model2 29 1 Assumes different RSA encryp`on scheme 2 Requires PRF-ODH assump`on Modular analysis Monolithic analysis Truncated TLS: Morissey, Smart, Warinschi ‘10
- 30. Comparison to Previous Work Morrissey, Smart, Warinschi ‘10 Our work Bellare-Rogaway Model Bellare-Rogaway Model TLS_DHE, TLS_DH, TLS_RSA1 TLS_DHE Random Oracle Model Standard Model2 30 1 Assumes different RSA encryp`on scheme 2 Requires PRF-ODH assump`on Modular analysis Monolithic analysis Truncated TLS: Morissey, Smart, Warinschi ‘10 Both results do not consider the real TLS Handshake…!
- 31. 2nd Approach: New Security Model • Secure AKE provides indis7nguishable keys – Key can be used in any further applica7on – Too strong for TLS Handshake – Stronger than necessary: TLS uses keys for Record Layer • Can we describe a new security model which is – strong enough to provide security, but – weak enough to be achievable by TLS? 31
- 32. 2nd Approach: New Security Model • Secure AKE provides indis7nguishable keys – Key can be used in any further applica7on – Too strong for TLS Handshake – Stronger than necessary: TLS uses keys for Record Layer • Can we describe a new security model which is – strong enough to provide security, but – weak enough to be achievable by TLS? 32 but
- 34. TLS-DHE is a Secure ACCE Protocol 34 Theorem: TLS-DHE is a secure ACCE protocol, if • the PRF is a secure pseudo-random func7on, • the digital signature scheme is EUF-CMA secure, • the DDH assump7on holds in the Diffie-Hellman group, • the PRF-ODH assump7on holds, and • the Record Layer cipher is secure (sLHAE)
- 35. TLS-DHE is a Secure ACCE Protocol 35 Theorem: TLS-DHE is a secure ACCE protocol, if • the PRF is a secure pseudo-random func7on, • the digital signature scheme is EUF-CMA secure, • the DDH assump7on holds in the Diffie-Hellman group, • the PRF-ODH assump7on holds, and • the Record Layer cipher is secure (sLHAE) Stateful Length-Hiding Authen7cated Encryp7on [PRS’11]: • Security no`on for symmetric ciphers • Captures exactly what is expected from TLS Record Layer • Achieved by CBC-based ciphersuites in TLS 1.1 and 1.2
- 36. The PRF-ODH Assump`on 36 • Let G = <g> be a group with order p, let PRF : G x M à R be a func`on
- 41. The PRF-ODH Assump`on • PRF-ODH assump7on: no efficient aaacker can dis`nguish PRF(guv,m) from random – Variant of Oracle Diffie-Hellman assump`on [ABR’01] 41 Adversary A Challenger C m∈M U,V∈G PRF(guv,m) or rand∈R “real” or “random” W∈G,m’∈M PRF(Wu,m’) • Let G = <g> be a group with order p, let PRF : G x M à R be a func`on U := gu, V := gv where u,v ß Zp
- 42. The PRF-ODH Assump`on • PRF-ODH assump7on: no efficient aaacker can dis`nguish PRF(guv,m) from random – Variant of Oracle Diffie-Hellman assump`on [ABR’01] 42 Adversary A Challenger C m∈M U,V∈G PRF(guv,m) or rand∈R “real” or “random” W∈G,m’∈M PRF(Wu,m’) • Let G = <g> be a group with order p, let PRF : G x M à R be a func`on U := gu, V := gv where u,v ß Zp
- 43. Is PRF-ODH really necessary? • Not if – no corrup7ons of long-term secrets are allowed, or – small changes are made to TLS-DHE Handshake • E.g. making it more similar to Σ0 [CK’02] • Impossible to avoid, if – security model with corrup7ons is considered, and – reduc`on uses aFacker and PRF as black-box 43
- 44. Is PRF-ODH really necessary? • Not if – no corrup7ons of long-term secrets are allowed, or – small changes are made to TLS-DHE Handshake • E.g. making it more similar to Σ0 [CK’02] • Impossible to avoid, if – security model with corrup7ons is considered, and – reduc`on uses aFacker and PRF as black-box 44
- 45. Subsequent Works using ACCE (This work: CRYPTO 2012) • Further TLS cipher suites: – Krawczyk et al., Crypto 2013 – Li et al., PKC 2014 • Secure re-nego`a`on of TLS session keys: – Giesen et al., CCS 2013 • Other prac`cal protocols: – EMV channel establishment (Brzuska et al., CCS 2013) – SSH (Bergsma et al., CCS 2014) – QUIC (Lychev et al., S&P 2015). 45
- 46. Subsequent Works using ACCE (This work: CRYPTO 2012) • Further TLS cipher suites: – Krawczyk et al., Crypto 2013 – Li et al., PKC 2014 • Secure re-nego`a`on of TLS session keys: – Giesen et al., CCS 2013 • Other prac`cal protocols: – EMV channel establishment (Brzuska et al., CCS 2013) – SSH (Bergsma et al., CCS 2014) – QUIC (Lychev et al., S&P 2015). 46 ACCE: Not beau`ful, but useful
- 48. Subsequent Works on the Provable Security of TLS <= 1.2 (This work: CRYPTO 2012) For example: • Krawczyk, Paterson, Wee (CRYPTO 13): ACCE-based analysis of TLS-RSA and TLS-sDH • Bhargavan e.a. (Oakland 13): Formally-verified implementa`on of TLS (miTLS) • Alterna`ves to ACCE: – Brzuska et al. (Informa`on Security 2013): Relaxed yet composable security no`ons for key exchange – Bharghavan e.a. (CRYPTO 14): Use real-or-random key for encryp`on of FIN-message 48
- 49. Subsequent Works on the Provable Security of TLS 1.3 For example: • Krawczyk, Wee (Euro-S&P 16): Theore`cally-sound founda`on of TLS 1.3 • Dowling, Fischlin, Günther, Stebila (ACM CCS 2015): Formal analysis of TLS 1.3 handshake candidates • Cremers, Horvat, Scoa, Van Der Merwe (Oakland 16): Automated Analysis of TLS 1.3 • … 49
- 50. Summary • New ACCE security model • First full security proof for TLS-DHE • Many follow-up works, but also many open problems: – TLS is much more complex – we (and subsequent “pencil-and-paper” proofs) considered only the cryptographic core – Other promising approaches: • Verified implementa`ons (miTLS) • Automated Analysis (see Thyla’s talk) 50
- 51. Summary • New ACCE security model • First full security proof for TLS-DHE • Many follow-up works, but also many open problems: – TLS is much more complex – we (and subsequent “pencil-and-paper” proofs) considered only the cryptographic core – Other promising approaches: • Verified implementa`ons (miTLS) • Automated Analysis (see Thyla’s talk) 51 Thank you!