diff --git a/common.go b/common.go
index 432216a..aa58c85 100644
--- a/common.go
+++ b/common.go
@@ -1,6 +1,7 @@
 package awsauth
 
 import (
+	"bytes"
 	"crypto/hmac"
 	"crypto/md5"
 	"crypto/sha1"
@@ -73,9 +74,9 @@ func hmacSHA1(key []byte, content string) []byte {
 	return mac.Sum(nil)
 }
 
-func hashSHA256(content string) string {
+func hashSHA256(content []byte) string {
 	h := sha256.New()
-	h.Write([]byte(content))
+	h.Write(content)
 	return fmt.Sprintf("%x", h.Sum(nil))
 }
 
@@ -85,13 +86,12 @@ func hashMD5(content []byte) string {
 	return base64.StdEncoding.EncodeToString(h.Sum(nil))
 }
 
-func readAndReplaceBody(req *http.Request) string {
+func readAndReplaceBody(req *http.Request) []byte {
 	if req.Body == nil {
-		return ""
+		return []byte{}
 	}
-	rawPayload, _ := ioutil.ReadAll(req.Body)
-	payload := string(rawPayload)
-	req.Body = ioutil.NopCloser(strings.NewReader(payload))
+	payload, _ := ioutil.ReadAll(req.Body)
+	req.Body = ioutil.NopCloser(bytes.NewReader(payload))
 	return payload
 }
 
diff --git a/common_test.go b/common_test.go
index 87e9fb9..ac8482c 100644
--- a/common_test.go
+++ b/common_test.go
@@ -36,7 +36,7 @@ func TestCommonFunctions(t *testing.T) {
 	})
 
 	Convey("SHA-256 hashes should be properly hex-encoded (base 16)", t, func() {
-		input := "This is... Sparta!!"
+		input := []byte("This is... Sparta!!")
 		actual := hashSHA256(input)
 
 		So(actual, ShouldEqual, "5c81a4ef1172e89b1a9d575f4cd82f4ed20ea9137e61aa7f1ab936291d24e79a")
diff --git a/s3.go b/s3.go
index 34b7a20..363e869 100644
--- a/s3.go
+++ b/s3.go
@@ -14,10 +14,12 @@ func signatureS3(stringToSign string) string {
 }
 
 func stringToSignS3(req *http.Request) string {
-
 	str := req.Method + "\n"
 
-	// TODO: Content-MD5 here...
+	body := readAndReplaceBody(req)
+	if len(body) > 0 {
+		str += hashMD5(body)
+	}
 	str += "\n"
 
 	str += req.Header.Get("Content-Type") + "\n"
diff --git a/sign4.go b/sign4.go
index 5aa4463..9d8361b 100644
--- a/sign4.go
+++ b/sign4.go
@@ -17,7 +17,7 @@ func hashedCanonicalRequestV4(req *http.Request, meta *metadata) string {
 	meta.signedHeaders = "content-type;host;x-amz-date"
 
 	canonicalRequest := concat("\n", req.Method, req.URL.Path, req.URL.RawQuery, headersToSign, meta.signedHeaders, hashedPayload)
-	return hashSHA256(canonicalRequest)
+	return hashSHA256([]byte(canonicalRequest))
 }
 
 func stringToSignV4(req *http.Request, hashedCanonReq string, meta *metadata) string {
diff --git a/sign4_test.go b/sign4_test.go
index 394f35a..2504c6a 100644
--- a/sign4_test.go
+++ b/sign4_test.go
@@ -123,13 +123,13 @@ func TestSignature4Helpers(t *testing.T) {
 		req := test_plainRequestV4(false)
 
 		Convey("Its body should be read and replaced without differences", func() {
-			expected := requestValuesV4.Encode()
+			expected := []byte(requestValuesV4.Encode())
 
 			actual1 := readAndReplaceBody(req)
-			So(actual1, ShouldEqual, expected)
+			So(actual1, ShouldResemble, expected)
 
 			actual2 := readAndReplaceBody(req)
-			So(actual2, ShouldEqual, expected)
+			So(actual2, ShouldResemble, expected)
 		})
 	})
 }