Describe the bug
Cookie name with slash / character is escaped to %2F
Eg. user/123=foo becomes user%2F123=foo

I believe the forward slash character is allowed in the cookie key.

It's unclear why you're passing key here as the converter expects a single parameter:
value = converter.write(value, key)

And on the next line:

Perhaps it would be better to have:
key = converter.write(key)

Expected behavior
Cookie name preserves the original forward slash character.