def hmac_sha1_signature
      # hmac secret is same as plaintext signature 
      secret = plaintext_signature
      Base64.encode64(OpenSSL::HMAC.digest(OpenSSL::Digest::SHA1.new, secret, signature_base)).gsub(/\n/, '')
    end
def hmac_sha1_signature
      # hmac secret is same as plaintext signature 
      secret = plaintext_signature
      Base64.encode64(OpenSSL::HMAC.digest(OpenSSL::Digest::SHA1.new, secret, signature_base)).gsub(/\n/, '')
    end
signature, with method HMAC-SHA1. section 3.4.2

 @return [String]