Module: Altcha::V2

Defined in:
lib/altcha/v2.rb

Overview

V2 proof-of-work: find a counter C such that KDF(nonce+C) starts with keyPrefix. Supports SHA-, PBKDF2/SHA-, and SCRYPT algorithms via OpenSSL::KDF.

Defined Under Namespace

Classes: Challenge, ChallengeParameters, CreateChallengeOptions, Payload, ServerSignaturePayload, Solution, VerifyServerSignatureResult, VerifySolutionResult

Constant Summary collapse

DEFAULT_KEY_LENGTH = 
32
DEFAULT_KEY_PREFIX = 
'00'

Class Method Summary collapse

Class Method Details

.canonical_json(obj) ⇒ Object

Produces a canonical (sorted-key, compact) JSON string.



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# File 'lib/altcha/v2.rb', line 254

def self.canonical_json(obj)
  case obj
  when Hash
    pairs = obj.sort_by { |k, _| k.to_s }
               .map { |k, v| "#{k.to_s.to_json}:#{canonical_json(v)}" }
    "{#{pairs.join(',')}}"
  when Array
    "[#{obj.map { |v| canonical_json(v) }.join(',')}]"
  else
    obj.to_json
  end
end

.constant_time_equal?(a, b) ⇒ Boolean

Constant-time string comparison.

Returns:

  • (Boolean) 


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# File 'lib/altcha/v2.rb', line 345

def self.constant_time_equal?(a, b)
  return false if a.bytesize != b.bytesize

  OpenSSL.fixed_length_secure_compare(a, b)
rescue ArgumentError
  false
end

.create_challenge(options) ⇒ Challenge

Creates a v2 proof-of-work challenge.

Parameters:

Returns:



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# File 'lib/altcha/v2.rb', line 356

def self.create_challenge(options)
  key_length        = options.key_length        || DEFAULT_KEY_LENGTH
  key_prefix        = options.key_prefix        || DEFAULT_KEY_PREFIX
  key_prefix_length = options.key_prefix_length || (key_length / 2)
  expires_at        = options.expires_at.is_a?(Time) ? options.expires_at.to_i : options.expires_at

  parameters = ChallengeParameters.new(
    algorithm:   options.algorithm,
    nonce:       OpenSSL::Random.random_bytes(16).unpack1('H*'),
    salt:        OpenSSL::Random.random_bytes(16).unpack1('H*'),
    cost:        options.cost,
    key_length:  key_length,
    key_prefix:  key_prefix,
    memory_cost: options.memory_cost,
    parallelism: options.parallelism,
    expires_at:  expires_at,
    data:        options.data
  )

  derived_key_bytes = nil

  if options.counter
    nonce_bytes       = [parameters.nonce].pack('H*')
    salt_bytes        = [parameters.salt].pack('H*')
    password_bytes    = make_password(nonce_bytes, options.counter)
    derived_key_bytes = derive_key(parameters, salt_bytes, password_bytes)
    parameters.key_prefix = derived_key_bytes[0, key_prefix_length].unpack1('H*')
  end

  if options.hmac_signature_secret
    if derived_key_bytes && options.hmac_key_signature_secret
      parameters.key_signature = hmac_hex(
        derived_key_bytes,
        options.hmac_key_signature_secret
      )
    end
    signature = hmac_hex(
      canonical_json(parameters.to_h),
      options.hmac_signature_secret
    )
    Challenge.new(parameters: parameters, signature: signature)
  else
    Challenge.new(parameters: parameters)
  end
end

.derive_key(parameters, salt_bytes, password_bytes) ⇒ Object

Derives a key from the given parameters, salt, and password bytes.



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# File 'lib/altcha/v2.rb', line 274

def self.derive_key(parameters, salt_bytes, password_bytes)
  alg     = parameters.algorithm
  key_len = parameters.key_length || DEFAULT_KEY_LENGTH

  case alg
  when 'ARGON2ID'
    begin
      require 'argon2/kdf'
    rescue LoadError
      raise LoadError, "Add 'argon2-kdf' to your Gemfile to use the ARGON2ID algorithm"
    end
    # argon2-kdf's `m` is log2(memory_cost_in_KiB) — convert from KiB.
    m_kib = parameters.memory_cost || 65536
    Argon2::KDF.argon2id(
      password_bytes,
      salt:   salt_bytes,
      t:      parameters.cost,
      m:      Math.log2(m_kib).round,
      p:      parameters.parallelism || 1,
      length: key_len
    )
  when /\APBKDF2\//
    digest = case alg
             when 'PBKDF2/SHA-512' then 'SHA512'
             when 'PBKDF2/SHA-384' then 'SHA384'
             else 'SHA256'
             end
    OpenSSL::KDF.pbkdf2_hmac(
      password_bytes,
      salt:       salt_bytes,
      iterations: parameters.cost,
      length:     key_len,
      hash:       digest
    )
  when 'SCRYPT'
    OpenSSL::KDF.scrypt(
      password_bytes,
      salt:   salt_bytes,
      N:      parameters.cost,
      r:      parameters.memory_cost || 8,
      p:      parameters.parallelism || 1,
      length: key_len
    )
  else
    # SHA-256 / SHA-384 / SHA-512 (iterative)
    digest     = case alg
                 when 'SHA-512' then 'SHA512'
                 when 'SHA-384' then 'SHA384'
                 else 'SHA256'
                 end
    iterations = [parameters.cost, 1].max
    buf        = salt_bytes.b + password_bytes.b
    derived    = nil
    iterations.times do |i|
      derived = OpenSSL::Digest.digest(digest, i.zero? ? buf : derived)
    end
    derived[0, key_len]
  end
end

.hmac_hex(data, key, algorithm = 'SHA-256') ⇒ Object

Computes an HMAC hex digest using the specified algorithm ('SHA-256' etc.).



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# File 'lib/altcha/v2.rb', line 335

def self.hmac_hex(data, key, algorithm = 'SHA-256')
  digest = case algorithm
           when 'SHA-384' then 'SHA384'
           when 'SHA-512' then 'SHA512'
           else 'SHA256'
           end
  OpenSSL::HMAC.hexdigest(digest, key, data)
end

.make_password(nonce_bytes, counter) ⇒ Object

Builds the password buffer (nonce bytes + counter) used for key derivation. Counter is encoded as a 4-byte big-endian unsigned integer.



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# File 'lib/altcha/v2.rb', line 269

def self.make_password(nonce_bytes, counter)
  nonce_bytes + [counter].pack('N')
end

.parse_verification_data(data, array_fields: %w[fields reasons])) ⇒ Object

Parses a URL-encoded verification_data string into a typed Hash. Booleans, integers, and floats are auto-detected; comma-separated fields listed in array_fields are converted to arrays.



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# File 'lib/altcha/v2.rb', line 507

def self.parse_verification_data(data, array_fields: %w[fields reasons])
  result = {}
  URI.decode_www_form(data).each do |key, value|
    result[key] = if value == 'true'
                    true
                  elsif value == 'false'
                    false
                  elsif /\A\d+\z/.match?(value)
                    value.to_i
                  elsif /\A\d+\.\d+\z/.match?(value)
                    value.to_f
                  elsif array_fields.include?(key) && !value.empty?
                    value.strip.split(',')
                  else
                    value.strip
                  end
  end
  result
rescue StandardError
  nil
end

.solve_challenge(challenge, max_counter: nil, counter_start: 0, counter_step: 1) ⇒ Solution?

Solves a v2 challenge by brute-forcing counter values.

Parameters:

  • challenge (Challenge)
  • max_counter (Integer, nil) (defaults to: nil) —

    Safety cap; nil means no limit.

  • counter_start (Integer) (defaults to: 0)
  • counter_step (Integer) (defaults to: 1)

Returns:



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# File 'lib/altcha/v2.rb', line 408

def self.solve_challenge(challenge, max_counter: nil, counter_start: 0, counter_step: 1)
  parameters  = challenge.parameters
  nonce_bytes = [parameters.nonce].pack('H*')
  salt_bytes  = [parameters.salt].pack('H*')
  key_prefix  = parameters.key_prefix
  start_time  = Time.now
  counter     = counter_start

  loop do
    return nil if max_counter && counter > max_counter

    password_bytes    = make_password(nonce_bytes, counter)
    derived_key_bytes = derive_key(parameters, salt_bytes, password_bytes)
    derived_key_hex   = derived_key_bytes.unpack1('H*')

    if derived_key_hex.start_with?(key_prefix)
      return Solution.new(
        counter:     counter,
        derived_key: derived_key_hex,
        time:        ((Time.now - start_time) * 1000).round
      )
    end

    counter += counter_step
  end
end

.verify_fields_hash(form_data:, fields:, fields_hash:, algorithm: 'SHA-256') ⇒ Boolean

Verifies the SHA hash of selected form fields.

Parameters:

  • form_data (Hash)
  • fields (Array<String>)
  • fields_hash (String) —

    Expected hex digest.

  • algorithm (String) (defaults to: 'SHA-256') —

    Defaults to 'SHA-256'.

Returns:

  • (Boolean) 


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# File 'lib/altcha/v2.rb', line 535

def self.verify_fields_hash(form_data:, fields:, fields_hash:, algorithm: 'SHA-256')
  digest = case algorithm
           when 'SHA-512' then 'SHA512'
           when 'SHA-384' then 'SHA384'
           else 'SHA256'
           end
  lines = fields.map { |f| form_data[f].to_s }
  OpenSSL::Digest.hexdigest(digest, lines.join("\n")) == fields_hash
end

.verify_server_signature(payload:, hmac_secret:) ⇒ VerifyServerSignatureResult

Verifies a server signature payload from the ALTCHA backend.

Parameters:

Returns:



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# File 'lib/altcha/v2.rb', line 549

def self.verify_server_signature(payload:, hmac_secret:)
  start_time = Time.now

  digest = case payload.algorithm
           when 'SHA-512' then 'SHA512'
           when 'SHA-384' then 'SHA384'
           else 'SHA256'
           end

  hash_bytes        = OpenSSL::Digest.digest(digest, payload.verification_data)
  expected_sig      = hmac_hex(hash_bytes, hmac_secret, payload.algorithm)
  verification_data = parse_verification_data(payload.verification_data)

  expired = !!(verification_data &&
               verification_data['expire'] &&
               verification_data['expire'] < Time.now.to_i)

  invalid_signature = !constant_time_equal?(payload.signature.to_s, expected_sig)

  invalid_solution = verification_data.nil? ||
                     verification_data['verified'] != true ||
                     payload.verified != true

  verified = !expired && !invalid_signature && !invalid_solution

  VerifyServerSignatureResult.new(
    expired:           expired,
    invalid_signature: invalid_signature,
    invalid_solution:  invalid_solution,
    time:              elapsed_ms(start_time),
    verification_data: verification_data,
    verified:          verified
  )
end

.verify_solution(challenge, solution, hmac_signature_secret:, hmac_key_signature_secret: nil, hmac_algorithm: 'SHA-256') ⇒ VerifySolutionResult

Verifies a v2 solution against its challenge.

Parameters:

  • challenge (Challenge)
  • solution (Solution)
  • hmac_signature_secret (String) —

    Must match what was used in create_challenge.

  • hmac_key_signature_secret (String, nil) (defaults to: nil) —

    Required when keySignature is present.

  • hmac_algorithm (String) (defaults to: 'SHA-256') —

    Defaults to 'SHA-256'.

Returns:



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# File 'lib/altcha/v2.rb', line 442

def self.verify_solution(challenge, solution, hmac_signature_secret:,
                         hmac_key_signature_secret: nil,
                         hmac_algorithm: 'SHA-256')
  start_time = Time.now

  # 1. Expiration check.
  if challenge.parameters.expires_at && Time.now.to_i > challenge.parameters.expires_at
    return VerifySolutionResult.new(
      expired: true, invalid_signature: nil, invalid_solution: nil,
      time: elapsed_ms(start_time), verified: false
    )
  end

  # 2. Signature presence check.
  unless challenge.signature
    return VerifySolutionResult.new(
      expired: false, invalid_signature: true, invalid_solution: nil,
      time: elapsed_ms(start_time), verified: false
    )
  end

  # 3. Verify challenge signature (tamper detection).
  expected_sig = hmac_hex(
    canonical_json(challenge.parameters.to_h),
    hmac_signature_secret,
    hmac_algorithm
  )
  unless constant_time_equal?(challenge.signature, expected_sig)
    return VerifySolutionResult.new(
      expired: false, invalid_signature: true, invalid_solution: nil,
      time: elapsed_ms(start_time), verified: false
    )
  end

  # 4a. Fast path: verify via key signature when available.
  if challenge.parameters.key_signature && hmac_key_signature_secret
    derived_key_bytes = [solution.derived_key].pack('H*')
    expected_key_sig  = hmac_hex(derived_key_bytes, hmac_key_signature_secret, hmac_algorithm)
    valid = constant_time_equal?(challenge.parameters.key_signature, expected_key_sig)
    return VerifySolutionResult.new(
      expired: false, invalid_signature: false, invalid_solution: !valid,
      time: elapsed_ms(start_time), verified: valid
    )
  end

  # 4b. Slow path: re-derive key from the submitted counter and compare,
  # and require it to satisfy the signed key prefix.
  nonce_bytes       = [challenge.parameters.nonce].pack('H*')
  salt_bytes        = [challenge.parameters.salt].pack('H*')
  password_bytes    = make_password(nonce_bytes, solution.counter)
  derived_key_bytes = derive_key(challenge.parameters, salt_bytes, password_bytes)
  derived_key_hex   = derived_key_bytes.unpack1('H*')
  key_matches       = constant_time_equal?(derived_key_hex, solution.derived_key)
  prefix_matches    = derived_key_hex.start_with?(challenge.parameters.key_prefix)
  invalid           = !(key_matches && prefix_matches)

  VerifySolutionResult.new(
    expired: false, invalid_signature: false, invalid_solution: invalid,
    time: elapsed_ms(start_time), verified: !invalid
  )
end