make s3 objects private.rb

require 'aws/s3'

bucket_name = 'yourbucket'
marker = ""

AWS::S3::Base.establish_connection!(
  :access_key_id => "abc",
  :secret_access_key => "abc"
)

#create the read-only by me policy
owner_grant = AWS::S3::ACL::Grant.new
grantee = AWS::S3::ACL::Grantee.new
grantee.type = "CanonicalUser"
grantee.id = 'abc' # get this from your security page https://console.aws.amazon.com/iam/home?#security_credential
grantee.display_name = "your name"
owner_grant.grantee = grantee
owner_grant.permission = 'READ'

# Iterate over all files inside bucket and apply the policy to each files
loop do
  objects = AWS::S3::Bucket.objects(bucket_name, :marker=>marker, :max_keys=>1000)

  marker = objects.last.key
  puts "new marker is \"#{marker}\""

  objects.each do |obj|
      policy = AWS::S3::S3Object.acl(obj.key, bucket_name)
      policy.grants = [owner_grant]
      AWS::S3::S3Object.acl(obj.key, bucket_name, policy)
  end
end