diff --git a/docs/url-signing.md b/docs/url-signing.md
index 19bcaa1..fcbbdf5 100644
--- a/docs/url-signing.md
+++ b/docs/url-signing.md
@@ -1,6 +1,71 @@
 # How to generate signed requests
 
-## Go
+Signing requests allows an imageproxy instance to proxy images from arbitrary
+remote hosts, but without opening the service up for potential abuse.  When
+appropriately configured, the imageproxy instance will only serve requests that
+are for allowed hosts, or which have a valid signature.
+
+Signatures can be calculated in two ways:
+
+1. they can be calculated solely on the remote image URL, in which case any
+   transformations of the image can be requested without changes to the
+   signature value.  This used to be the only way to sign requests, but is no
+   longer recommended since it still leaves the imageproxy instance open to
+   potential abuse.
+
+2. they can be calculated based on the combination of the remote image URL and
+   the requested transformation options.
+
+In both cases, the signature is calculated using HMAC-SHA256 and a secret key
+which is provided to imageproxy on startup.  The message to be signed is the
+remote URL, with the transformation options optionally set as the URL fragment,
+[as documented below](#Signing-options).  The signature is url-safe base64
+encoded, and [provided as an option][s-option] in the imageproxy request.
+
+imageproxy will accept signatures for URLs with or without options
+transparently.  It's up to the publisher of the signed URLs to decide which
+method they use to generate the URL.
+
+[s-option]: https://godoc.org/willnorris.com/go/imageproxy#hdr-Signature
+
+## Signing options
+
+Transformation options for a proxied URL are [specified as a comma separated
+string][ParseOptions] of individual options, which can be supplied in any
+order.  When calculating a signature, options should be put in their canonical
+form, sorted in lexigraphical order (omitting the signature option itself), and
+appended to the remote URL as the URL fragment.
+
+Currently, only [size option][] has a canonical form, which is
+`{width}x{height}` with the number `0` used when no value is specified.  For
+example, a request that does not request any size option would still have a
+canonical size value of `0x0`, indicating that no size transformation is being
+performed.  If only a height of 500px is requested, the canonical form would be
+`0x500`.
+
+For example, requesting the remote URL of `http://example.com/image.jpg`,
+resized to 100 pixels square, rotated 90 degrees, and converted to 75% quality
+might produce an imageproxy URL similar to:
+
+    http://localhost:8080/100,r90,q75/http://example.com/image.jpg
+
+When calculating a signature for this request including transformation options,
+the signed value would be:
+
+    http://example.com/image.jpg#100x100,q75,r90
+
+The `100` size option was put in its canonical form of `100x100`, and the
+options are sorted, moving `q75` before `r90`.
+
+[ParseOptions]: https://godoc.org/willnorris.com/go/imageproxy#ParseOptions
+[size option]: https://godoc.org/willnorris.com/go/imageproxy#hdr-Size_and_Cropping
+
+## Language Examples
+
+Here are examples of calculating signatures in a variety of languages.  These
+demonstrate the HMAC-SHA256 bits, but not the option canonicalization.
+
+### Go
 
 main.go:
 ```go
@@ -27,14 +92,14 @@ $ go run main.go "test" "https://www.google.fr/images/srpr/logo11w.png"
 result:  RYifAJRfbhsitJeOrDNxWURCCkPsVR4ihCPXNv-ePbA=
 ```
 
-## OpenSSL
+### OpenSSL
 
 ```shell
 $ echo -n "https://www.google.fr/images/srpr/logo11w.png" | openssl dgst -sha256 -hmac "test" -binary|base64| tr '/+' '_-'
 RYifAJRfbhsitJeOrDNxWURCCkPsVR4ihCPXNv-ePbA=
 ```
 
-## Java
+### Java
 
 ```java
 import org.apache.commons.codec.binary.Base64;
@@ -63,7 +128,7 @@ $ java -cp commons-codec-1.10.jar:. EncodeUrl test https://www.google.fr/images/
 RYifAJRfbhsitJeOrDNxWURCCkPsVR4ihCPXNv-ePbA
 ```
 
-## Ruby
+### Ruby
 
 ```ruby
 require 'openssl'
@@ -79,7 +144,7 @@ puts Base64.urlsafe_encode64(OpenSSL::HMAC.digest(OpenSSL::Digest.new('sha256'),
 RYifAJRfbhsitJeOrDNxWURCCkPsVR4ihCPXNv-ePbA=
 ```
 
-## Python
+### Python
 
 ```python
 import hmac
@@ -91,7 +156,7 @@ data = 'https://octodex.github.com/images/codercat.jpg'
 print base64.urlsafe_b64encode(hmac.new(key, msg=data, digestmod=hashlib.sha256).digest()) 
 ```
 
-## JavaScript
+### JavaScript
 
 ```javascript
 import crypto from 'crypto';
@@ -102,7 +167,7 @@ let data = 'https://octodex.github.com/images/codercat.jpg';
 console.log(URLSafeBase64.encode(crypto.createHmac('sha256', key).update(data).digest()));
 ```
 
-## PHP
+### PHP
 
 ````php
 <?php
@@ -114,4 +179,4 @@ echo strtr(base64_encode(hash_hmac('sha256', $data, $key, 1)), '/+' , '_-');
 ````shell
 $ php ex.php
 RYifAJRfbhsitJeOrDNxWURCCkPsVR4ihCPXNv-ePbA=
-````
\ No newline at end of file
+````
diff --git a/imageproxy.go b/imageproxy.go
index 61a3961..bdde2bb 100644
--- a/imageproxy.go
+++ b/imageproxy.go
@@ -146,15 +146,15 @@ func (p *Proxy) serveImage(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	// assign static settings from proxy to req.Options
-	req.Options.ScaleUp = p.ScaleUp
-
 	if err := p.allowed(req); err != nil {
 		log.Printf("%s: %v", err, req)
 		http.Error(w, msgNotAllowed, http.StatusForbidden)
 		return
 	}
 
+	// assign static settings from proxy to req.Options
+	req.Options.ScaleUp = p.ScaleUp
+
 	actualReq, _ := http.NewRequest("GET", req.String(), nil)
 	if p.UserAgent != "" {
 		actualReq.Header.Set("User-Agent", p.UserAgent)
@@ -322,10 +322,22 @@ func validSignature(key []byte, r *Request) bool {
 		return false
 	}
 
+	// check signature with URL only
 	mac := hmac.New(sha256.New, key)
 	mac.Write([]byte(r.URL.String()))
 	want := mac.Sum(nil)
+	if hmac.Equal(got, want) {
+		return true
+	}
 
+	// check signature with URL and options
+	u, opt := *r.URL, r.Options // make copies
+	opt.Signature = ""
+	u.Fragment = opt.String()
+
+	mac = hmac.New(sha256.New, key)
+	mac.Write([]byte(u.String()))
+	want = mac.Sum(nil)
 	return hmac.Equal(got, want)
 }
 
diff --git a/imageproxy_test.go b/imageproxy_test.go
index a10a92f..035ae38 100644
--- a/imageproxy_test.go
+++ b/imageproxy_test.go
@@ -228,6 +228,10 @@ func TestValidSignature(t *testing.T) {
 		{"http://test/image", Options{Signature: "NDx5zZHx7QfE8E-ijowRreq6CJJBZjwiRfOVk_mkfQQ="}, true},
 		{"http://test/image", Options{Signature: "NDx5zZHx7QfE8E-ijowRreq6CJJBZjwiRfOVk_mkfQQ"}, true},
 		{"http://test/image", emptyOptions, false},
+		// url-only signature with options
+		{"http://test/image", Options{Signature: "NDx5zZHx7QfE8E-ijowRreq6CJJBZjwiRfOVk_mkfQQ", Rotate: 90}, true},
+		// signature calculated from url plus options
+		{"http://test/image", Options{Signature: "ZGTzEm32o4iZ7qcChls3EVYaWyrDd9u0etySo0-WkF8=", Rotate: 90}, true},
 	}
 
 	for _, tt := range tests {
@@ -237,7 +241,7 @@ func TestValidSignature(t *testing.T) {
 		}
 		req := &Request{u, tt.options, &http.Request{}}
 		if got, want := validSignature(key, req), tt.valid; got != want {
-			t.Errorf("validSignature(%v, %q) returned %v, want %v", key, u, got, want)
+			t.Errorf("validSignature(%v, %v) returned %v, want %v", key, req, got, want)
 		}
 	}
 }