Hash Days 11 Slides

8/3/2019 Hash Days 11 Slides

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Cryptanalysis vs. Reality

Jean-Philippe Aumasson

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Cryptanalysis is the study of methods forobtaining the meaning of encrypted informationwithout access to the secret information that is normallyrequired to do so. Wikipedia


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Cryptanalysis is the study of methods forobtaining the meaning of encrypted informationwithout access to the secret information that is normallyrequired to do so. Wikipedia


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The fundamental goal of a cryptanalyst is toviolate one or several security notionsfor algorithms that claim, implicitly or explicitly,to satisfy these security notions.

Antoine Joux, Algorithmic Cryptanalysis


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Reality noun (pl. realities)1. the state of things as they actuallyexist, as opposed to an idealistic or

notional idea of them.2. a thing that is actually experiencedor seen.3. the quality of being lifelike.4. the state or quality of having exis-tence or substance.Compact Oxford English Dictionary


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Cryptanalysis relies on an ATTACKER MODEL= assumptions on what the attacker can and cannot do

All models are in simulacra , that is, simplied reections

of reality, but, despite their inherent falsity, they arenevertheless extremely useful G. Box, N. Draper, Empirical Model-Building and Response Surfaces

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Cryptanalysis usually excludes methods of attack that donot primarily target weaknesses in the actualcryptography, such as bribery, physical coercion,

burglary, keystroke logging, and socialengineering , although these types of attack are animportant concern and are often more effectiveWikipedia

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Cryptanalysis used to be tightly connected to reality


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But times have changed

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Broken in a model does not

imply broken in reality!


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Models language overlaps with real-world language:attacks, broken have multiple meanings

Has cryptanalysis lost connection with reality?


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Cryptography is usually bypassed. I am notaware of any major world-class security systememploying cryptography in which the hackers penetratedthe system by actually going through the cryptanalysis.(. . . ) Usually there are much simpler ways of penetratingthe security system.Adi Shamir, Turing Award lecture, 2002

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Is cryptanalysis relevant at all??


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Remainder of this talk

PART 1: PHYSICAL ATTACKSBypass and misuseSide channels

PART 2: ALGORITHMIC ATTACKSState-of-the-ciphersWhy attacks arent attacksCognitive biases

What about AES?

CONCLUSIONS + REFERENCES


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PART 1: PHYSICAL ATTACKSBypass and misuse

Side channels


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HTTPS protection uses (say) 2048-bit RSA toauthenticate servers, and to avoid MitM attacks

100-bit security (see http://www.keylength.com/ ) 2100 ops to break RSA by factoring the modulus

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http://www.keylength.com/http://www.keylength.com/http://www.keylength.com/


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Or 233 using a quantum computerimplementing Shors algorithm

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Or 233 using a quantum computerimplementing Shors algorithm

Or 20 by compromising a CA. . .

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AES-256 provides 256-bit security (does it really?)

FIPS 140-2 is supposed to inspire condence. . .

Yet secure USB drives by Kingston, SanDisk, Verbatimwere easily broken

The aw: password validation on host PC+ static unlock code

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How NOT to use decent cryptographic primitives:

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ECDSA signing with a constantinstead of a random numberto nd SONY PS3s private key

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RC4 stream cipher with part of the key public andpredictable (as found in the WEP WiFi protection)

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RC4 stream cipher with part of the key public andpredictable (as found in the WEP WiFi protection)

TEA block cipher in hashing mode

to perform boot code authenticationEquivalent keys lead to collisions

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S f id h l k


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Software side-channel attacksPractical attacks exploiting non-constant-time AES implementations

Breaking the secure AES of OpenSSL 0.9.8n :

Breaking AES on ARM9 :

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H d id h l k


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Hardware side-channel attacksPower analysis (SPA/DPA)Electromagnetic analysisGlitches (clock, power supply, data corruption)MicroprobingLaser cutting and fault injection

Focused ion beam surgery, etc.

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PART 2: ALGORITHMIC ATTACKSState-of-the-ciphersWhy attacks arent attacks

What about AES?Cognitive biases


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ALGORITHMIC ATTACKS = attacks targetting acryptographic function seen as an algorithm anddescribed as algorithms rather than physicalprocedures

ALGORITHMIC ATTACKS are thus independent of theimplementation of the function attacked

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Well focus on symmetric cryptographic primitives:Block ciphersStream ciphersHash functions

PRNGsMACs

Though thered be a lot to say about public-key encryption/signatures,authentication protocols, etc.

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Null- to low-impact attacks (examples)


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Null- to low-impact attacks (examples)

Block ciphers:AESGOST (Russian standard, 1970s!)

KASUMI(3GPP)

Triple DESHash functions:

SHA-1

Whirlpool (ISO)

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Medium- to high-impact attacks (examples)


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Medium to high impact attacks (examples)

Block cipher:DES (56-bit key): practical break by. . . bruteforce

Stream cipher:A5/1 (GSM): attacks on GSM facilitated

Hash function:MD5 : famous rogue certicate attack PoC

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Unattacked primitives (examples)


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Unattacked primitives (examples)Block ciphers

CAST5 (default cipher in OpenPGP)IDEA (1991!)IDEA-NXT (aka FOX)Serpent (AES nalist)

Twosh (AES nalist)Stream ciphers:

Grain128a (for hardware)Salsa20 (for software)

Hash functions:SHA-2 (SHA-256, . . . , SHA-512)RIPEMD-160 (ISO)

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Despite the large amount of research andnew techniques, breaks almost never happen:Why?

High-complexity attacks


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High complexity attacks

Example: preimage attack on MD5 with time complexity

2123 . 4

against 2 128 ideally

High-complexity attacks do not matter as long asthe effort is obviously unfeasible, or

overwhelms the cost of other attacksYet MD5 can no longer be sold as 128-bit security hash

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Back-to-reality interlude

2 GHz CPU1 sec = 2 10 9 233 clocks

1 year 2 58 clocks1000 years 2 68 clockssince the Big-Bang 2 116 clocks

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The encryption doesnt even have to be very strong to beuseful, it just must be stronger than the otherweak links in the system. Using any standardcommercial risk management model, cryptosystem failureis orders of magnitude below any other risk.Ian Griff, Peter Gutmann, IEEE Security & Privacy 9(3), 2011

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Attacks on building blocks


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g

Example: 296 collision attack on the compressionfunction of the SHA-3 candidate LANE

Did not lead to an attack on the hashInvalidates the security reduction compression hashDisqualied LANE from the SHA-3 competition!

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Attacks on building blocks


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g

Example: 296 collision attack on the compressionfunction of the SHA-3 candidate LANE

Did not lead to an attack on the hashInvalidates the security reduction compression hashDisqualied LANE from the SHA-3 competition!

How to interprete those attacks?1. We attacked something

crypto must be weak!2. We failed to attack the full function

crypto must be strong!

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Strong models: ex of related-key attacks


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g y

Attackers learn encryptions with a derived keyK = f (K )

One of the rst attacks: when Enigma operators set rotorsincorrectly, they sent again with the correct key. . .

Modern version introduced by Knudsen/Biham in 1992

Practical on weak key-exchange protocols (EMV, 3GPP?),but unrealistic in most decent protocols

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Related-key attacks example


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y

Key-recovery on AES-256 with time complexity

2119

against 2256

ideally!

Needs 4 related keys . . . actually, related sub keys !attacks are still mainly of theoretical interest and do notpresent a threat to practical applications using AESthe authors (Khovratovich / Biryukov)

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Model from reality: pay-TV encryption


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MPEG stream encrypted with CSACommon Scrambling Algorithm , 48b or 64b key

Useful break of CSA needs

Unknown- xed-key attacksCiphertext-only , partially-known plaintext (no TMTO)Key recovery in < 10 seconds (cryptoperiod)

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Theres not only time!


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Back to our previous examples:

MD5 : time 2123.4 and 2 50 B memory (1024 TiB)

LANE : time 296 and 2 93 B memory (2 53 TiB)AES-256 : time 2119 and 2 77 B memory (2 37 TiB)

Memory is not free! ($$$, infrastructure, latency)

Practical cost of access to memory neglected

New attacks should be compared to genericattacks with a same budget

See cracking machines in Understanding bruteforce http://cr.yp.to/papers.html#bruteforce

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http://cr.yp.to/papers.html#bruteforcehttp://cr.yp.to/papers.html#bruteforce


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Distinguishing attacks


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aka distinguishers

Used to be statistical biases

Now distinguishers areKnown- or chosen-key attacks

Sets of input/outputs satisfying some relationExample: differential q -multicollision distinguisher on AES

E K 1 (P 1) E K 1 (P 1 ) = E K 2 (P 2) E K 2 (P 2 )

= E K 3 (P 3) E K 3 (P 3 ) = . . .

NO IMPACT ON SECURITY in a large majority of cases

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Attacks (high-complexity, strong model, high-memory,distinguishers, etc.) vs. Reality

2 general interpretations:1. This little thing is a sign of bigger things!2. This little thing is a sign of no big things!

Why are we biased? (towards 1. or 2.)


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Cryptographic Num3rol0gy

The basic concept is that as long as your encryption keysare at least this big, youre ne, even if none of thesurrounding infrastructure benets from that size or evenworks at allIan Griff, Peter Gutmann, IEEE Security & Privacy 9(3), 2011


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Cryptographic Num3rol0gy

The basic concept is that as long as your encryption keysare at least this big, youre ne, even if none of thesurrounding infrastructure benets from that size or evenworks at allIan Griff, Peter Gutmann, IEEE Security & Privacy 9(3), 2011

Choosing a key size if fantastically easy, whereas making

the crypto work effectively is really hardIbid


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Zero-risk bias


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= Preference for reducing a small risk to zeroover a greater reduction in a larger riskExample: reduce risk from 1% to 0% whereas anotherrisk could be reduced from 50% to 30% at the same cost

Cryptographic numerology (examples)1% = scary-new attack threatMove from 1024- to 2048-bit (or 4096-bit!) RSACascade-encryption with AES + Serpent + Twosh

+ Unintended consequences :Crypto is slower less deployed less security

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Survivorship bias


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We only remember/see the unbroken , deployedand/or standardized, algorithms

Not the numerous experimental designs broken

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Survivorship bias


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We only remember/see the unbroken , deployedand/or standardized, algorithms

Not the numerous experimental designs broken

Example: of the 56 SHA-3 submissions published14 implemented attacks (e.g. example of collision)3 close-to-practical attacks ( 260 )14 high-complexity attacks

Practical attacks kill ciphers before they areused and known to the public

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What about AES?


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What about AES?


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Groundbreaking attack bogeyman!

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What about AES?


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The facts :

AES-128 : 2126 complexity, 2 88 plaintext/ciphertextagainst 2128 and 20 for bruteforce AES-256 : 2254 complexity, 2 40 plaintext/ciphertextagainst 2256 and 21 for bruteforce

See Bogdanov, Khovratovich, Rechberger:http://research.microsoft.com/en-us/projects/cryptanalysis/aesbc.pdf

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http://research.microsoft.com/en-us/projects/cryptanalysis/aesbc.pdfhttp://research.microsoft.com/en-us/projects/cryptanalysis/aesbc.pdfhttp://research.microsoft.com/en-us/projects/cryptanalysis/aesbc.pdfhttp://research.microsoft.com/en-us/projects/cryptanalysis/aesbc.pdf


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CONCLUSIONS + REFERENCES

Conclusions


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Algorithmic attacks on deployed schemes are (almost)never a threat to security , due toHigh complexities, unrealistic models, etc.Weak ciphers are broken earlier and forgotten

We dont break ciphers, we evaluate their securityOrr Dunkelman

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Conclusions


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Beware cryptographic numerology!

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Conclusions


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Beware cryptographic numerology!

AES is ne, weak implementations are the biggest threat

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Related works


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Leakage-resilience vs. RealityLeakage Resilient Cryptography in Practice Standaert et al. http://eprint.iacr.org/2009/341

Bruteforce vs. RealityUsing the Cloud to Determine Key Strengths Kleinjung et al. http://eprint.iacr.org/2011/254

Crypto libs vs. RealityOpen-Source Cryptographic Libraries and Embedded Platforms Junod http://crypto.junod.info/hashdays10_talk.pdf

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Thank you for your attention
http://eprint.iacr.org/2009/341http://eprint.iacr.org/2011/254http://crypto.junod.info/hashdays10_talk.pdfhttp://crypto.junod.info/hashdays10_talk.pdfhttp://eprint.iacr.org/2011/254http://eprint.iacr.org/2009/341


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