Class: SecretSharing::Shamir::Share

Inherits:
Object
  • Object
show all
Defined in:
lib/secretsharing/shamir.rb

Overview

A SecretSharing::Shamir::Share object represents a share in the Shamir secret sharing scheme. The share consists of a point (x,y) on a polynomial over Z/Zp, where p is a prime.

Constant Summary collapse

FORMAT_VERSION = 
'0'

Instance Attribute Summary collapse

Class Method Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(x, y, prime, prime_bitlength) ⇒ Share

Create a new share with the given point, prime and prime bitlength.



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# File 'lib/secretsharing/shamir.rb', line 240

def initialize(x, y, prime, prime_bitlength)
  @x = x
  @y = y
  @prime = prime
  @prime_bitlength = prime_bitlength
end

Instance Attribute Details

#prime ⇒ Object (readonly)

Returns the value of attribute prime.



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# File 'lib/secretsharing/shamir.rb', line 235

def prime
  @prime
end

#prime_bitlength ⇒ Object (readonly)

Returns the value of attribute prime_bitlength.



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# File 'lib/secretsharing/shamir.rb', line 235

def prime_bitlength
  @prime_bitlength
end

#x ⇒ Object (readonly)

Returns the value of attribute x.



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# File 'lib/secretsharing/shamir.rb', line 235

def x
  @x
end

#y ⇒ Object (readonly)

Returns the value of attribute y.



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# File 'lib/secretsharing/shamir.rb', line 235

def y
  @y
end

Class Method Details

.from_string(string) ⇒ Object

Create a new share from a string format representation. For a discussion of the format, see the to_s() method.



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# File 'lib/secretsharing/shamir.rb', line 249

def self.from_string(string)
  version = string[0,1]
  if version != '0' then
    raise "invalid share format version #{version}."
  end
  x = string[1,2].hex
  prime_bitlength = 4 * string[-2,2].hex + 1
  p_x_str = string[3, string.length - 9]
  checksum = string[-6, 4]
  computed_checksum = Digest::SHA1.hexdigest(p_x_str)[0,4].upcase
  if checksum != computed_checksum then
    raise "invalid checksum. expected #{checksum}, " + \
          "got #{computed_checksum}"
  end
  prime = SecretSharing::Shamir. \
          smallest_prime_of_bitlength(prime_bitlength)
  self.new(x, OpenSSL::BN.new(p_x_str, 16), prime, prime_bitlength)
end

Instance Method Details

#==(share) ⇒ Object

Shares are equal if their string representation is the same.



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# File 'lib/secretsharing/shamir.rb', line 294

def ==(share)
  share.to_s == self.to_s
end

#to_s ⇒ Object

A string representation of the share, that can for example be distributed in printed form. The string is an uppercase hexadecimal string of the following format: ABBC*DDDDEEEE, where

  • A (the first nibble) is the version number of the format, currently fixed to 0.

  • B (the next byte, two hex characters) is the x coordinate of the point on the polynomial.

  • C (the next variable length of bytes) is the y coordinate of the point on the polynomial.

  • D (the next two bytes, four hex characters) is the two highest bytes of the SHA1 hash on the string representing the y coordinate, it is used as a checksum to guard against typos

  • E (the next two bytes, four hex characters) is the bitlength of the prime number in nibbles.



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# File 'lib/secretsharing/shamir.rb', line 283

def to_s
  # bitlength in nibbles to save space
  prime_nibbles = (@prime_bitlength - 1) / 4 
  p_x = ("%x" % @y).upcase
  FORMAT_VERSION + ("%02x" % @x).upcase \
    + p_x \
    + Digest::SHA1.hexdigest(p_x)[0,4].upcase \
    + ("%02x" % prime_nibbles).upcase
end