Publish the /dims5/ signing algorithm as a library, with Go and Java clients

[beetlebugorg]

What

/dims5/ requires an HMAC-SHA256 signature over the commands, the image URL, and the canonical query. A caller in any language has to reproduce that message byte for byte, and today nothing in this repository lets one do so without reading C.

Three pieces:

1. A C library that exposes the signing rules, so a caller links it rather than reimplementing it.
2. A Go client.
3. A Java client.

Why

src/signature.c holds the rules and src/signature.h declares them, but every function takes an apr_pool_t and the header includes mod_dims.h, which pulls in httpd. A caller outside the module cannot use it.

test/endurance/sign.c reimplements the same rules against OpenSSL for the soak client. Its header says so: "Every function here reproduces what the module checks. A change to the module's rules that this file does not follow appears in a run as a signed request the service refuses." That is a second copy, and a third copy exists in go-dims.

The message format is easy to get wrong. dims_signed_query orders the parameters by name, percent encodes each one with uppercase hex, writes a space as a plus, and leaves out sig, url, eurl, _keys, and download. A client that sorts differently or escapes a tilde produces a signature the module refuses, and the failure says only "Key mismatch".

Scope

The C library. Split the pool and httpd types out of signature.h so the rules take plain C strings and a caller-supplied buffer. The module keeps its pool-based wrapper. Ship a header and a static library, and a dims-sign command that signs a URL from the shell.

Go. A package with one exported function that takes the key, the base URL, the commands, the image URL, and the query, and returns the signed URL. go-dims already has an implementation to compare against.

Java. The same shape, using javax.crypto.Mac with HmacSHA256.

Verify

The signature is portable between mod_dims and go-dims, and a test asserts it. Extend that: one fixture file of message and signature pairs, read by the C, Go, and Java test suites. A client that disagrees fails its own suite rather than a soak run.

Cover the cases that differ between naive implementations:

• a query parameter with several values
• a parameter name that sorts after another only when compared byte by byte
• a space, a plus, and a tilde in a value
• a value holding a percent sign
• _keys naming a parameter that is absent

Notes

docs/docs/endpoints/dims5.md documents the URL shape. The signing rules belong beside it once a caller can follow them.