Class: H2C::HashToPoint

Inherits:
Object
  • Object
show all
Defined in:
lib/h2c/hash_to_point.rb

Overview

Complete and secure function for hashing strings to points.

Instance Attribute Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(suite) ⇒ HashToPoint

Returns a new instance of HashToPoint.

Parameters:



9
10
11
 
# File 'lib/h2c/hash_to_point.rb', line 9

def initialize(suite)
  @suite = suite
end

Instance Attribute Details

#suite ⇒ Object (readonly)

Returns the value of attribute suite.



6
7
8
 
# File 'lib/h2c/hash_to_point.rb', line 6

def suite
  @suite
end

Instance Method Details

#digest(msg) ⇒ ECDSA::Point

Hash returns a point on an elliptic curve given a message.

Parameters:

  • msg (String) —

    Message with binary to be hashed.

Returns:

  • (ECDSA::Point) —

    point



16
17
18
19
20
21
22
23
24
25
26
27
28
 
# File 'lib/h2c/hash_to_point.rb', line 16

def digest(msg)
  p =
    if suite.ro
      u = hash_to_field(msg, 2)
      p0 = suite.map.map(u[0])
      p1 = suite.map.map(u[1])
      p0 + p1
    else
      u = hash_to_field(msg, 1)
      suite.map.map(u[0])
    end
  suite.curve.cofactor ? p.multiply_by_scalar(suite.curve.cofactor) : p
end

#hash_to_field(msg, count) ⇒ Array

Hashes a msg of any length into an element of a finite field. www.ietf.org/archive/id/draft-irtf-cfrg-hash-to-curve-16.html#name-hash_to_field-implementatio

Parameters:

  • msg (String) —

    A byte string containing the message to hash.

  • count (Integer) —

    The number of elements of Field to output.

Returns:

  • (Array) 


35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
 
# File 'lib/h2c/hash_to_point.rb', line 35

def hash_to_field(msg, count)
  field = suite.curve.field
  len = count * suite.m * suite.l
  pseudo = suite.exp.expand(msg, len)
  u = []
  (0...count).each do |i|
    v = []
    (0...suite.m).each do |j|
      offset = suite.l * (j + i * suite.m)
      t = pseudo[offset, (offset + suite.l)]
      vj = t.unpack1("H*").to_i(16)
      v[j] = field.mod(vj)
    end
    u[i] = v
  end
  u.flatten
end