Amazon's AWS can be accessed via simple HTTP commands. The commands must be signed, using a protocol similar to OAuth. Aws_sign generates a signature header to use when making an AWS request. The advantage of aws_sign over other AWS signing software is that aws_sign is very simple and is written in plain old C.
To use it, you supply the four cryptographic cookies and the method & URL of the request. If it's a POST request with extra parameters, you have to give those too. Aws_sign puts all this together and makes the signature string. The signature is generated using HMAC-SHA1 as specified in RFC section 3.4.2, and is returned as an Authorization header value as specified in RFC section 3.5.1. This header can then be used in an HTTP request via, for example, the -h flag in http_get(1) and http_post(1) or the -H flag in curl(1).
The signature may also be generated as query parameters, as specified in RFC section 3.5.3, by using the -q flag.
The signature generation code is available as either a command-line program or as a C function if you want to link it into your code directly.
See the manual entry for more details.