The server's API key (HMAC secret).
The URL path to scope the key to. Normalized by
lower-casing and stripping leading/trailing slashes before hashing, so
/Foo/Bar/, foo/bar and /foo/bar all produce the same key.
A 32-character lower-case hex key.
Compute a path-specific link key:
HMAC-SHA256(apiKey, normalizedPath), truncated to 32 hex characters.