Digital Signature Preliminary RSA signature scheme ElGamal signature scheme DSA signature scheme

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Digital Signature Preliminary RSA signature
scheme ElGamal signature scheme DSA
signature scheme
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Preliminary

• A signature is a commitment (or binding) of a
  person to a document

Dear John This is a notice to you about
. Sincerely,
Wen-Guey Tzeng Wen-Guey Tzeng

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Digital signature

• Goal Use the digital technique to emulate the
  hand-written signature
• Security requirements
• Unforgeability one cannot create a signature
  that is claimed to be anothers
• Undeniability the signer cannot later deny the
  validity of his signature

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Requirements

• The signature depends on the signer and the
  document to be signed.
• Easy to compute it is easy for a signer to sign
  a document
• Universal verifiability every one can verify
  validity of a signature (with respect to the
  signer and the document)
• Easy to store the signature should be short
  enough

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RSA signature scheme

• Key generation algorithm
• Input k (security parameter)
• Randomly generate two primes p and q of length
  k/2 bits
• Compute npq (n is k-bit long)
• Randomly select e, 2?e??(n)-1, withgcd(e,
  ?(n))1 (Note ?(n)(p-1)(q-1))
• Compute de-1 mod ?(n)
• The verification (public) key KU(e, n)
• The signing (private) key KR(d, n)

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RSA signature scheme (cont.)

• Let H be a hashing algorithm, publicly known
• Signing algorithm Sig
• Input ((d,n), M) (0?M?n-1)
• Compute sH(M)e mod n
• Output(s).
• Verification algorithm Ver
• Input ((e,n), M, s) (0?s?n-1)
• Compute hse mod n
• Output yes if and only if hH(M).

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RSA example

• Randomly select two primes p7, q17
• Calculate npq717119
• Calculate ?(n)(p-1)(q-1)96
• Randomly select e5, since gcd(e,?(n))1
• Calculate de-1 mod ?(n)77
• Public key KU(5, 119)
• Private key KR(77, 119)

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ElGamal signature scheme

• Key generation algorithm
• Input k (security parameter)
• Randomly generate a prime 2q1, where q is also
  prime
• Randomly select h?Zp and compute gh2 mod p?1.
  (Note gq1 (mod p))
• Randomly select a number x, 1?x?q-1
• Compute ygx mod p
• The verification (public) key KU(g, p, y)
• The signing (private) key KR(g, p, x)

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ElGamal signature scheme (cont.)

• Let H be a hashing algorithm, publicly known
• Signing algorithm Sig
• Input ((g,p,x), M) (0?M?p-1)
• Randomly select k, 1?k?q-1 and compute rgk mod p
• Compute sk-1 (H(m)-rx) mod q
• Output((r,s)).
• Note there are many signatures for a message
• Verification algorithm Ver
• Input ((g,p,y), M, (r,s))
• Compute hH(M)
• Output yes if and only if ghyrrs mod p.

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ElGamal signature scheme (cont.)

• Why ghyrrs (mod p) ?
• yrrs mod p
• (gx)r (gk)s mod p
• gxrks mod p
• gh(M) mod p

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Digital signature standard (DSS)

• Key generation algorithm
• Input L (security parameter)
• Randomly generate an L-bit prime kq1, where q
  is also prime, 2159ltqlt2160
• Randomly select h?Zp and compute gh(p-1)/q mod
  p?1. (Note gq1 (mod p))
• Randomly select a number x, 1?x?q-1
• Compute ygx mod p
• The verification (public) key KU(g, p, q, y)
• The signing (private) key KR(g, p, q, x)

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DSS signature scheme (cont.)

• Let H be a hashing algorithm, publicly known
• Signing algorithm Sig
• Input ((g,p,q,x), M) (0?M?p-1)
• Randomly select k, 1?k?q-1 andcompute r(gk mod
  p) mod q
• Compute sk-1(H(m)rx) mod q
• Output (r,s).
• Note there are many signatures for a message

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DSS signature scheme (cont.)

• Verification algorithm Ver
• Input ((g,p,g,y), M, (r,s))
• Compute w s-1 mod q
• Compute u1 H(M)w mod q
• Compute u2 rw mod q
• Compute v (gu1yu2 mod p) mod q
• Output yes if and only if vr

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DSS signature scheme (cont.)

• Why it works?
• gu1yu2 mod p mod q
• gH(M)w gxrw mod p mod q
• gw(H(M)xr) mod p mod q
• gk mod q mod p mod q
• gk mod p mod q
• r
• Note ordp(g)q.

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DSS signature scheme example

• Key generation
• p676?111, q11
• g2(p-1)/11 mod p36 mod 6759
• x5, ygx mod p62
• KU(59, 67, 11, 62)
• KR(59, 67, 11, 5)
• Signing
• Let H(M)4, k3
• rgk mod p mod q593 mod 67 mod 112
• sk-1(H(M)rx) mod q3-1(42?5) mod 111
• (r,s)(2,1)

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DSS signature scheme example

• Verification (r, s)(2, 1)
• ws-1 mod q 1-1 mod 11 1
• u1 H(M)?w mod q 4?1 mod 11 4
• u2 r?w mod q 2?1 mod 11 2
• vgu1 ? yu2 mod p mod q 594?622 mod 67 mod
  11 2
• Since vr, (2,1) is a signature to H(M)

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DSS signature scheme security

• Based on computing discrete logarithm over a
  subgroup of size q logg y mod p.Note ordp(g)q
• The per-message secret k cannot be used twice.
  Otherwise, given two signatures(r1,s1) for M1
  and (r2,s2) for M2, we have
• s1k-1(H(M1)r1x) mod q
• s2k-1(H(M2))r2x) mod q
• Solve x(s2H(M1)-s1H(M2))/(r2s1-r1s2) mod q

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Digital signature usage

• Off-line Signing a document for emulating the
  hand-written signature
• On-line identity authentication (session key
  distribution)

challenge c
Bob
Alice
rSig(KRBob, c)
If Ver(KUBob, r, c)true then accept that Bob is
talking to me