The CB-ACCESS-SIGN header is generated by creating a sha256 HMAC using the secret key on the prehash string timestamp + method + requestPath + body (where + represents string concatenation). The timestamp value is the same as the CB-ACCESS-TIMESTAMP header.
1 2  | access_signature(timestamp, request_path, request_body = "",
  method = c("get", "post", "put", "delete"))
 | 
timestamp | 
 The CB-ACCESS-TIMESTAMP header MUST be number of seconds since Unix Epoch.  | 
request_path | 
 path requesting  | 
request_body | 
 The body is the request body string or omitted if there is no request body (typically for GET requests).  | 
method | 
 The method should be UPPER CASE.  | 
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