diff --git a/acme/client.go b/acme/client.go
index 6897ed4b..598f5692 100644
--- a/acme/client.go
+++ b/acme/client.go
@@ -670,3 +670,82 @@ func parseLinks(links []string) map[string]string {
 
 	return linkMap
 }
+
+// validate makes the ACME server start validating a
+// challenge response, only returning once it is done.
+func validate(j *jws, uri string, chlng challenge) error {
+	var challengeResponse challenge
+
+	if err := postJSON(j, uri, chlng, &challengeResponse); err != nil {
+		return err
+	}
+
+	interval := 1 * time.Second
+	maxInterval := 15 * time.Minute
+
+	// After the path is sent, the ACME server will access our server.
+	// Repeatedly check the server for an updated status on our request.
+	for {
+		switch challengeResponse.Status {
+		case "valid":
+			logf("The server validated our request")
+			return nil
+		case "pending":
+			break
+		case "invalid":
+			return errors.New("The server could not validate our request.")
+		default:
+			return errors.New("The server returned an unexpected state.")
+		}
+
+		// Poll with exponential back-off.
+		time.Sleep(interval)
+		interval *= 2
+		if interval > maxInterval {
+			interval = maxInterval
+		}
+
+		if err := getJSON(uri, &challengeResponse); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
+
+// getJSON performs an HTTP GET request and parses the response body
+// as JSON, into the provided respBody object.
+func getJSON(uri string, respBody interface{}) error {
+	resp, err := http.Get(uri)
+	if err != nil {
+		return fmt.Errorf("failed to get %q: %v", uri, err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode >= http.StatusBadRequest {
+		return handleHTTPError(resp)
+	}
+
+	return json.NewDecoder(resp.Body).Decode(respBody)
+}
+
+// postJSON performs an HTTP POST request and parses the response body
+// as JSON, into the provided respBody object.
+func postJSON(j *jws, uri string, reqBody, respBody interface{}) error {
+	jsonBytes, err := json.Marshal(reqBody)
+	if err != nil {
+		return errors.New("Failed to marshal network message...")
+	}
+
+	resp, err := j.post(uri, jsonBytes)
+	if err != nil {
+		return fmt.Errorf("Failed to post JWS message. -> %v", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode >= http.StatusBadRequest {
+		return handleHTTPError(resp)
+	}
+
+	return json.NewDecoder(resp.Body).Decode(respBody)
+}
diff --git a/acme/http_challenge.go b/acme/http_challenge.go
index 8ace6795..979ec4b3 100644
--- a/acme/http_challenge.go
+++ b/acme/http_challenge.go
@@ -1,13 +1,10 @@
 package acme
 
 import (
-	"encoding/json"
-	"errors"
 	"fmt"
 	"net"
 	"net/http"
 	"strings"
-	"time"
 )
 
 type httpChallenge struct {
@@ -47,49 +44,7 @@ func (s *httpChallenge) Solve(chlng challenge, domain string) error {
 		close(s.end)
 	}()
 
-	jsonBytes, err := json.Marshal(challenge{Resource: "challenge", Type: chlng.Type, Token: chlng.Token, KeyAuthorization: keyAuth})
-	if err != nil {
-		return errors.New("Failed to marshal network message...")
-	}
-
-	// Tell the server we handle HTTP-01
-	resp, err := s.jws.post(chlng.URI, jsonBytes)
-	if err != nil {
-		return fmt.Errorf("Failed to post JWS message. -> %v", err)
-	}
-
-	// After the path is sent, the ACME server will access our server.
-	// Repeatedly check the server for an updated status on our request.
-	var challengeResponse challenge
-Loop:
-	for {
-		if resp.StatusCode >= http.StatusBadRequest {
-			return handleHTTPError(resp)
-		}
-
-		err = json.NewDecoder(resp.Body).Decode(&challengeResponse)
-		resp.Body.Close()
-		if err != nil {
-			return err
-		}
-
-		switch challengeResponse.Status {
-		case "valid":
-			logf("The server validated our request")
-			break Loop
-		case "pending":
-			break
-		case "invalid":
-			return errors.New("The server could not validate our request.")
-		default:
-			return errors.New("The server returned an unexpected state.")
-		}
-
-		time.Sleep(1 * time.Second)
-		resp, err = http.Get(chlng.URI)
-	}
-
-	return nil
+	return validate(s.jws, chlng.URI, challenge{Resource: "challenge", Type: chlng.Type, Token: chlng.Token, KeyAuthorization: keyAuth})
 }
 
 func (s *httpChallenge) startHTTPServer(domain string, token string, keyAuth string) {
diff --git a/acme/tls_sni_challenge.go b/acme/tls_sni_challenge.go
index d5f249bd..634bf899 100644
--- a/acme/tls_sni_challenge.go
+++ b/acme/tls_sni_challenge.go
@@ -5,12 +5,9 @@ import (
 	"crypto/sha256"
 	"crypto/tls"
 	"encoding/hex"
-	"encoding/json"
-	"errors"
 	"fmt"
 	"net"
 	"net/http"
-	"time"
 )
 
 type tlsSNIChallenge struct {
@@ -57,49 +54,7 @@ func (t *tlsSNIChallenge) Solve(chlng challenge, domain string) error {
 		close(t.end)
 	}()
 
-	jsonBytes, err := json.Marshal(challenge{Resource: "challenge", Type: chlng.Type, Token: chlng.Token, KeyAuthorization: keyAuth})
-	if err != nil {
-		return errors.New("Failed to marshal network message...")
-	}
-
-	// Tell the server we handle TLS-SNI-01
-	resp, err := t.jws.post(chlng.URI, jsonBytes)
-	if err != nil {
-		return fmt.Errorf("Failed to post JWS message. -> %v", err)
-	}
-
-	// After the path is sent, the ACME server will access our server.
-	// Repeatedly check the server for an updated status on our request.
-	var challengeResponse challenge
-Loop:
-	for {
-		if resp.StatusCode >= http.StatusBadRequest {
-			return handleHTTPError(resp)
-		}
-
-		err = json.NewDecoder(resp.Body).Decode(&challengeResponse)
-		resp.Body.Close()
-		if err != nil {
-			return err
-		}
-
-		switch challengeResponse.Status {
-		case "valid":
-			logf("The server validated our request")
-			break Loop
-		case "pending":
-			break
-		case "invalid":
-			return errors.New("The server could not validate our request.")
-		default:
-			return errors.New("The server returned an unexpected state.")
-		}
-
-		time.Sleep(1 * time.Second)
-		resp, err = http.Get(chlng.URI)
-	}
-
-	return nil
+	return validate(t.jws, chlng.URI, challenge{Resource: "challenge", Type: chlng.Type, Token: chlng.Token, KeyAuthorization: keyAuth})
 }
 
 func (t *tlsSNIChallenge) generateCertificate(keyAuth string) (tls.Certificate, error) {