Hi Martin,
So I would suggest that to use SHA-1 functin to reduce any key longer than 64 bytes to 64 bytes and then to pass a new key to SET_HMAC_KEY.
Unfortunatelly I cannot cut the key.
The definition of the [HMAC-SHA1 OAuth key|http://tools.ietf.org/html/rfc5849#page-25] according RFC 5849:
key is set to the concatenated values of:
1. The client shared-secret, after being encoded
(Section 3.6).
2. An "&" character (ASCII code 38), which MUST be included
even when either secret is empty.
3. The token shared-secret, after being encoded
(Section 3.6).
And the result in my case is a string of 83 characters.
Regards, Uwe