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+# httpsig
+
+`go get github.com/go-fed/httpsig`
+
+Implementation of [HTTP Signatures](https://tools.ietf.org/html/draft-cavage-http-signatures).
+
+Supports many different combinations of MAC, HMAC signing of hash, or RSA
+signing of hash schemes. Its goals are:
+
+* Have a very simple interface for signing and validating
+* Support a variety of signing algorithms and combinations
+* Support setting either headers (`Authorization` or `Signature`)
+* Remaining flexible with headers included in the signing string
+* Support both HTTP requests and responses
+* Explicitly not support known-cryptographically weak algorithms
+* Support automatic signing and validating Digest headers
+
+## How to use
+
+`import "github.com/go-fed/httpsig"`
+
+### Signing
+
+Signing a request or response requires creating a new `Signer` and using it:
+
+```
+func sign(privateKey crypto.PrivateKey, pubKeyId string, r *http.Request) error {
+	prefs := []httpsig.Algorithm{httpsig.RSA_SHA512, httpsig.RSA_SHA256}
+	digestAlgorithm := DigestSha256
+	// The "Date" and "Digest" headers must already be set on r, as well as r.URL.
+	headersToSign := []string{httpsig.RequestTarget, "date", "digest"}
+	signer, chosenAlgo, err := httpsig.NewSigner(prefs, digestAlgorithm, headersToSign, httpsig.Signature)
+	if err != nil {
+		return err
+	}
+	// To sign the digest, we need to give the signer a copy of the body...
+	// ...but it is optional, no digest will be signed if given "nil"
+	body := ...
+	// If r were a http.ResponseWriter, call SignResponse instead.
+	return signer.SignRequest(privateKey, pubKeyId, r, body)
+}
+```
+
+`Signer`s are not safe for concurrent use by goroutines, so be sure to guard
+access:
+
+```
+type server struct {
+	signer httpsig.Signer
+	mu *sync.Mutex
+}
+
+func (s *server) handlerFunc(w http.ResponseWriter, r *http.Request) {
+	privateKey := ...
+	pubKeyId := ...
+	// Set headers and such on w
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	// To sign the digest, we need to give the signer a copy of the response body...
+	// ...but it is optional, no digest will be signed if given "nil"
+	body := ...
+	err := s.signer.SignResponse(privateKey, pubKeyId, w, body)
+	if err != nil {
+		...
+	}
+	...
+}
+```
+
+The `pubKeyId` will be used at verification time.
+
+### Verifying
+
+Verifying requires an application to use the `pubKeyId` to both retrieve the key
+needed for verification as well as determine the algorithm to use. Use a
+`Verifier`:
+
+```
+func verify(r *http.Request) error {
+	verifier, err := httpsig.NewVerifier(r)
+	if err != nil {
+		return err
+	}
+	pubKeyId := verifier.KeyId()
+	var algo httpsig.Algorithm = ...
+	var pubKey crypto.PublicKey = ...
+	// The verifier will verify the Digest in addition to the HTTP signature
+	return verifier.Verify(pubKey, algo)
+}
+```
+
+`Verifier`s are not safe for concurrent use by goroutines, but since they are
+constructed on a per-request or per-response basis it should not be a common
+restriction.