Before diving too deep into this, can you try
token = Base.url_encode64(message)
EDIT
I took the provided ruby code on the Shopify documentation, replaced dynamic values with static values (the json string and iv) and used the resulting token to set up a simple test.
I think you already had it:
multipass_secret = "1234567890abcdef1234567890abcdef"
customer_data = %{
email: "test@test.shopify.com",
created_at: "2019-03-07T11:34:51.153+09:00"
}
key_material = :crypto.hash(:sha256, multipass_secret)
# Split the key into 2 binaries each containing exactly 16 bytes
<< encryption_key::binary-size(16), signature_key::binary-size(16) >> = key_material
customer_data_as_string = Jason.encode!(customer_data)
# Initialization Vector
ivec = "testieivphrasefo" # :crypto.strong_rand_bytes(16)
to_add = 16 - rem(byte_size(customer_data_as_string), 16)
padded = customer_data_as_string <> :binary.copy(<<to_add>>, to_add)
cipher_text = ivec <> :crypto.block_encrypt(:aes_cbc128, encryption_key, ivec, padded)
signature = :crypto.hmac(:sha256, signature_key, cipher_text)
message = cipher_text <> signature
Base.url_encode64(message)
# Should give you:
# "dGVzdGllaXZwaHJhc2VmbxNXNKV-epmKd9gZPxrsFbqUbYcYVFjtPSleoB-61FpmhCtibtr8cxudaPBvk8QBqdggqsOofjr21PdwM4qvHNNJh8jyMzH9emUfjxNLj2M6dZ5BGyBlFhF7OWQHWljxQywhqP7RCC0klKG_zYwlNm8="
Just change Base.encode16/2 to Base.url_encode64/1
