Closes #27
Signatures are per-URL only — this PR adds explicit tests and documentation enforcing that HMAC-SHA256 signatures verify against exact URLs only. No suffix matching, wildcard matching, or partial matching is supported.
## What this does NOT touch
**The host whitelist code (`whitelist.go`) is not modified.** This PR is exclusively about signature verification, per sneak's instructions on [issue #27](#27), [PR #32](#32), and [PR #35](#35).
## Changes
### `internal/imgcache/signature.go`
- Added documentation comments on `Verify()` and `buildSignatureData()` explicitly specifying that signatures are exact-match only — no suffix, wildcard, or partial matching
### `internal/imgcache/signature_test.go`
- **`TestSigner_Verify_ExactMatchOnly`**: 14 tamper cases verifying that modifying any signed component (host, path, query, dimensions, format) causes verification to fail. Host-specific cases include:
- Parent domain (`example.com`) does not match subdomain signature (`cdn.example.com`)
- Sibling subdomain (`images.example.com`) does not match
- Deeper subdomain (`images.cdn.example.com`) does not match
- Evil suffix domain (`cdn.example.com.evil.com`) does not match
- Prefixed host (`evilcdn.example.com`) does not match
- **`TestSigner_Sign_ExactHostInData`**: Verifies that suffix-related hosts (`cdn.example.com`, `example.com`, `images.example.com`, etc.) all produce distinct signatures
### `internal/imgcache/service_test.go`
- **`TestService_ValidateRequest_SignatureExactHostMatch`**: Integration test through `ValidateRequest` verifying that a valid signature for `cdn.example.com` is rejected when presented with a different host (parent domain, sibling subdomain, deeper subdomain, evil suffix, prefixed host)
### `README.md`
- Updated Signature Specification section to explicitly document exact-match-only semantics
Co-authored-by: user <user@Mac.lan guest wan>
Reviewed-on: #40
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>
TDD: Write tests first before implementation for:
- ETag generation and consistency in service layer
- HEAD request support (headers only, no body)
- Conditional requests with If-None-Match header (304 responses)
