You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: kms_cipher_provider.rb
    path of file: ./repos/aws-sdk-ruby/gems/aws-sdk-s3/lib/aws-sdk-s3/encryptionV2
    
    the code of the file until where you have to start completion: # frozen_string_literal: true

require 'base64'

module Aws
  module S3
    module EncryptionV2
      # @api private
      class KmsCipherProvider

        def initialize(options = {})
          @kms_key_id = validate_kms_key(options[:kms_key_
    
        def build_encryption_context(cek_alg, options = {})
          kms_context = (options[:kms_encryption_context] || {})
            .each_with_object({}) { |(k, v), h| h[k.to_s] = v }
          if kms_context.include? 'aws:x-amz-cek-alg'
            raise ArgumentError, 'Conflict in reserved KMS Encryption Context ' \
              'key aws:x-amz-cek-alg. This value is reserved for the S3 ' \
              'Encryption Client and cannot be set by the user.'
          end