Hi,

We need to make a few callouts to a service that is using OAuth 1.0 and requires each request to be signed with HMAC-SHA1.

The service doesn't have any APEX client API. Thus, we have to do it manually.

Unfortunately, 

EncodingUtil.base64Encode(Crypto.generateMac('hmacSHA1', Blob.valueOf(data), Blob.valueOf(key)));

 returns a different string from what we expect. We have compared the output for the same input with libraries for other languages. And the output was different.