Tamper-Evident Digital Signatures: Protecting Certification Authorities Against Malware

1
Tamper-Evident Digital SignaturesProtecting
Certification Authorities Against Malware

• Jong Youl Choi
• Computer Science Dept.
• Indiana University at Bloomington

Philippe Golle Palo Alto Research Center CA, USA
Markus Jakobsson School of Informatics Indiana
University at Bloomington
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Threats to Certificate Authorities

• Certificate repudiation
• A user chooses weak private key
• Intentionally let his private key be leaking
  discretely for forgery
• Certificate private key leaking
• Malicious attack such as Trojan horse
• Leaking CAs private via covert-channel

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What is a covert channel?

• Hidden communication channel
• Steganography Information hiding

Original Image
Extracted Image
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Prisoners' problem Simmons,93

• Two prisoners want to exchange messages, but must
  do so through the warden
• Subliminal channel in DSA

What Plan?
Plan A
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Leaking attack on RSA-PSS

• Random salt is usedfor padding string in
  encryption
• In verification process, salt is extracted from
  EM
• Hidden informationcan be embedded insalt value

RSA-PSS PKCS 1 V2.1
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Approaches

• Detect leaking
• A warden observes outputs from CA

Something hidden?

• Malicious attack
• Replacement of function

Pseudo Random Number Generator
Certificate Authority
mk
Sigk
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Approaches (Contd)

• Observing is not so easy because random number
  ...
• looks innocuous
• Or, doesnt reveal any state
• A warden (observer) can be attacked

Something hidden?
Pseudo Random Number Generator
Certificate Authority
mk
Sigk
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Undercover observer

• Signer outputs non-interactive proof as well as
  signature
• Ambushes until verification is invalid

Pseudo Random Number Generator
mk
Sigk
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Tamper-evident Chain

• Predefined set of random values in lieu of
  random number on the fly
• Hash chain verification

Hash()
Hash()
Hash()
Hash()
Hash()
x3
.
xn
x1
x2
Xn1
x3
Sig1
Sig2
.
Sign
Sig3
? X1Hash(X2)
? Xn-1Hash(Xn)
? X2Hash(X3)
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DSA Signature Scheme

• Gen x ? y gx mod p
• Sign m ? (s, r) where r (gk mod p) mod q
  and s k-1(h(m) x r) for random
  value k
• Verify For given signature (s, r), u1 h(m)
  s-1 u2 r s-1 and check rgu1 yu2 mod p mod q

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Hash chain construction
Hash()
Hash()
Hash()
Hash()
Hash()
k1
k2
k3
.
kn
kn1
k3
.
rgk1
rgk2
rgkn
rgk3
rgk3
P1
P2
Pn
P3
Pn1
.
Sign
.
Sig2
Sig3
Sig1
? X1Hash(X2)
? Xn-1Hash(Xn)
? X2Hash(X3)
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Conclusion

• Any leakage from CAs is dangerous
• CAs are not strong enough from malicious attacks
• We need observers which are under-cover
• A small additional cost for proofs

Or, Send me email jychoi_at_cs.indiana.edu