class ECE

# File lib/ece/ece.rb, line 79
def self.decrypt(data, params)
  key = extract_key(params)
  rs = params[:rs] ? params [:rs] : 4096
  raise "The rs parameter must be greater than 1." if rs <= 1
  rs += TAG_LENGTH
  raise "Message is truncated" if data.length % rs == 0
  result = ""
  counter = 0
  (0..data.length).step(rs) do |i|
    block = decrypt_record(key, counter, data[i..i+rs-1])
    result += block
    counter +=1
  end
  result
end

# File lib/ece/ece.rb, line 95
def self.decrypt_record(params, counter, buffer, pad=0)
  gcm = OpenSSL::Cipher.new('aes-128-gcm')
  gcm.decrypt
  gcm.key = params[:key]
  gcm.iv = generate_nonce(params[:nonce], counter)
  pad_bytes = 1
  if params[:auth] # old spec used 1 byte for padding, latest one always uses 2 bytes
    pad_bytes = 2
  end
  raise "Block is too small" if buffer.length <= TAG_LENGTH+pad_bytes
  gcm.auth_tag = buffer[-TAG_LENGTH..-1]
  decrypted = gcm.update(buffer[0..-TAG_LENGTH-1]) + gcm.final

  if params[:auth]
    padding_length = decrypted[0..1].unpack("n")[0]
    raise "Padding is too big" if padding_length+2 > decrypted.length
    padding = decrypted[2..padding_length]
    raise "Wrong padding"  unless padding = "\x00"*padding_length
    return decrypted[2+padding_length..-1]
  else
    padding_length = decrypted[0].unpack("C")[0]
    raise "Padding is too big" if padding_length+1 > decrypted.length
    padding = decrypted[1..padding_length]
    raise "Wrong padding"  unless padding = "\x00"*padding_length
    return decrypted[1..-1]
  end
end

# File lib/ece/ece.rb, line 58
def self.encrypt(data, params)
  key = extract_key(params)
  rs = params[:rs] ? params [:rs] : 4096
  padsize = params[:padsize] ? params [:padsize] : 0
  raise "The rs parameter must be greater than 1." if rs <= 1
  rs -=1 #this ensures encrypted data cannot be truncated
  result = ""
  pad_bytes = 1
  if params[:auth] # old spec used 1 byte for padding, latest one always uses 2 bytes
    pad_bytes = 2
  end

  counter = 0
  (0..data.length).step(rs-pad_bytes+1) do |i|
    block = encrypt_record(key, counter, data[i..i+rs-pad_bytes], padsize)
    result += block
    counter +=1
  end
  result
end

# File lib/ece/ece.rb, line 123
def self.encrypt_record(params, counter, buffer, pad=0)
  gcm = OpenSSL::Cipher.new('aes-128-gcm')
  gcm.encrypt
  gcm.key = params[:key]
  gcm.iv = generate_nonce(params[:nonce], counter)
  gcm.auth_data = ""
  padding = ""
  if params[:auth]
    padding = [pad].pack('n') + "\x00"*pad # 2 bytes, big endian, then n zero bytes of padding
    buf = padding+buffer
    record = gcm.update(buf)
  else
    record = gcm.update("\x00"+buffer) # 1 padding byte, not fully implemented
  end
  enc = record + gcm.final + gcm.auth_tag
  enc
end

# File lib/ece/ece.rb, line 29
def self.extract_key(params)
  raise "Salt must be 16-bytes long" unless params[:salt].length==16

  input_key = params[:key]
  auth = false
  if params.has_key?(:auth) # Encrypted Content Encoding, March 11 2016, http://httpwg.org/http-extensions/draft-ietf-httpbis-encryption-encoding.html
    auth = true
    input = HKDF.new(input_key, {salt: params[:auth] , algorithm: 'sha256', info: "Content-Encoding: auth\x00"})
    input_key = input.next_bytes(SHA256_LENGTH)
    secret =  HKDF.new(input_key, {salt: params[:salt], algorithm: 'sha256', info: get_info("aesgcm", params[:user_public_key], params[:server_public_key])})
    nonce =  HKDF.new(input_key, salt: params[:salt], algorithm: 'sha256', info: get_info("nonce", params[:user_public_key], params[:server_public_key]))
  else
    secret =  HKDF.new(input_key, {salt: params[:salt], algorithm: 'sha256', info: "Content-Encoding: aesgcm128"})
    nonce =  HKDF.new(input_key, salt: params[:salt], algorithm: 'sha256', info: "Content-Encoding: nonce")
  end

  {key: secret.next_bytes(KEY_LENGTH), nonce: nonce.next_bytes(NONCE_LENGTH), auth: auth}
end

# File lib/ece/ece.rb, line 48
def self.generate_nonce(nonce, counter)
  raise "Nonce must be #{NONCE_LENGTH} bytes long." unless nonce.length == NONCE_LENGTH
  output = nonce.dup
  integer = nonce[-6..-1].unpack('B*')[0].to_i(2) #taking last 6 bytes, treating as integer
  x = ((integer ^ counter) & 0xffffff) + ((((integer / 0x1000000) ^ (counter / 0x1000000)) & 0xffffff) * 0x1000000)
  bytestring = x.to_s(16).length < 12 ? "0"*(12-x.to_s(16).length)+x.to_s(16) : x.to_s(16) #it's for correct handling of cases when generated integer is less than 6 bytes
  output[-6..-1] = [bytestring].pack('H*')                                                 #without it packing would produce less than 6 bytes
  output                                                                                   #I didn't find pack directive for such usage, so there is a such solution
end

# File lib/ece/ece.rb, line 23
def self.get_info(type, client_public, server_public)
  cl_len_no = [client_public.size].pack('n')
  sv_len_no = [server_public.size].pack('n')
  "Content-Encoding: #{type}\x00P-256\x00#{cl_len_no}#{client_public}#{sv_len_no}#{server_public}"
end

# File lib/ece/ece.rb, line 19
def self.hkdf_extract(salt, ikm) #ikm stays for input keying material
  hmac_hash(salt,ikm)
end

# File lib/ece/ece.rb, line 14
def self.hmac_hash(key, input)
  digest = OpenSSL::Digest.new('sha256')
  OpenSSL::HMAC.digest(digest, key, input)
end