String key = XXXXXXXXXXXXXXXXXXXX
String secret = XXXXXXXXXXXXXXXXXXXXXXXXXXXXX
 
String stringToSign = 'PUT\n\n' + attach.ContentType + '\n' + '/' + bucketname + '/' + filename;
 
Blob mac = Crypto.generateMac('HMACSHA1', blob.valueOf(stringToSign), blob.valueof(secret));
String signed = EncodingUtil.base64Encode(mac);
String authHeader = 'AWS' + ' ' + key + ':' + signed;