Class: Zwiebel::Utilities

Inherits:
Object
  • Object
show all
Defined in:
lib/zwiebel/utilities.rb

Constant Summary collapse

SALT_LENGTH = 
16
MAC_LENGTH = 
32
S_KEY_LENGTH = 
32
S_IV_LENGTH = 
16

Class Method Summary collapse

Class Method Details

.current_time_period ⇒ Object 



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# File 'lib/zwiebel/utilities.rb', line 25

def self.current_time_period
  # tor rend-spec-v3
  # 2.2.1 [TIME-PERIODS]
  current_time = Time.now.utc.to_i
  (current_time / 60 - 1440) / 1440
end

.decrypt_layer(encrypted_data:, constant:, revision_counter:, subcredential:, blinded_key:) ⇒ Object 



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# File 'lib/zwiebel/utilities.rb', line 32

def self.decrypt_layer(encrypted_data:, constant:, revision_counter:, subcredential:, blinded_key:)
  cleaned_data = encrypted_data.gsub("-----BEGIN MESSAGE-----\n", "").gsub("\n-----END MESSAGE-----", "")
  encrypted = Base64.decode64(cleaned_data)

  if encrypted.length < SALT_LENGTH + MAC_LENGTH
    raise ContentLengthError, "encrypted data should be at least #{SALT_LENGTH + MAC_LENGTH} bytes"
  end

  salt = encrypted.byteslice(0, SALT_LENGTH)
  ciphertext = encrypted.byteslice(SALT_LENGTH..-(MAC_LENGTH + 1))
  expected_mac = encrypted.byteslice(-MAC_LENGTH..-1)

  key_digest = blinded_key + subcredential + [revision_counter].pack("Q>") + salt + constant
  bit_length = (S_KEY_LENGTH + S_IV_LENGTH + MAC_LENGTH) * 8

  keys = Shake256.new(bit_length: bit_length, message_type: "hex").hexdigest(key_digest.unpack1("H*"))
  keys = [keys].pack("H*")

  secret_key = keys.byteslice(0, S_KEY_LENGTH)
  secret_iv = keys.byteslice(S_KEY_LENGTH, S_IV_LENGTH)
  mac_key = keys.byteslice(S_KEY_LENGTH + S_IV_LENGTH, MAC_LENGTH)

  mac_prefix = [mac_key.length].pack("Q>") + mac_key + [salt.length].pack("Q>") + salt
  mac_for = OpenSSL::Digest.digest("SHA3-256", mac_prefix + ciphertext)

  if expected_mac != mac_for
    raise DataError, "incorrect message authentication code"
  end

  decipher = OpenSSL::Cipher.new("aes-256-ctr")
  decipher.decrypt
  decipher.key = secret_key
  decipher.iv = secret_iv
  decipher.update(ciphertext) + decipher.final
end