Feature: HttpGetter: support MD5 digest authentication

the MD5 scheme should still be widely deployed, even though it is
deprecated. it is also still the default if not specific algorithm
is requested by the server.

include/HttpGetter.h

@@ -69,7 +69,7 @@ class HttpGetter {
     char const* getErrorText() const { return _errBuffer; }
 
 private:
-    String getAuthDigest(String const& authReq, unsigned int counter);
+    std::pair<bool, String> getAuthDigest(String const& authReq, unsigned int counter);
     HttpRequestConfig const& _config;
 
     template<typename... Args>

src/HttpGetter.cpp

@@ -2,6 +2,7 @@
 #include "HttpGetter.h"
 #include <WiFiClientSecure.h>
 #include "mbedtls/sha256.h"
+#include "mbedtls/md5.h"
 #include <base64.h>
 #include <ESPmDNS.h>
 
@@ -155,8 +156,12 @@ HttpRequestResult HttpGetter::performGetRequest()
             return { false };
         }
         String authReq = upTmpHttpClient->header("WWW-Authenticate");
-        String authorization = getAuthDigest(authReq, 1);
-        upTmpHttpClient->addHeader("Authorization", authorization);
+        auto authorization = getAuthDigest(authReq, 1);
+        if (!authorization.first) {
+            logError("Digest Error: %s", authorization.second.c_str());
+            return { false };
+        }
+        upTmpHttpClient->addHeader("Authorization", authorization.second);
 
         // use a new TCP connection if the server sent "Connection: close".
         bool restart = true;
@@ -183,6 +188,29 @@ HttpRequestResult HttpGetter::performGetRequest()
     return { true, std::move(upTmpHttpClient), _spWiFiClient };
 }
 
+template<size_t binLen>
+static String bin2hex(uint8_t* hash) {
+    size_t constexpr kOutLen = binLen * 2 + 1;
+    char res[kOutLen];
+    for (int i = 0; i < binLen; i++) {
+        snprintf(res + (i*2), sizeof(res) - (i*2), "%02x", hash[i]);
+    }
+    return res;
+}
+
+static String md5(const String& data) {
+    uint8_t hash[16];
+
+    mbedtls_md5_context ctx;
+    mbedtls_md5_init(&ctx);
+    mbedtls_md5_starts_ret(&ctx);
+    mbedtls_md5_update_ret(&ctx, reinterpret_cast<const unsigned char*>(data.c_str()), data.length());
+    mbedtls_md5_finish_ret(&ctx, hash);
+    mbedtls_md5_free(&ctx);
+
+    return bin2hex<sizeof(hash)>(hash);
+}
+
 static String sha256(const String& data) {
     uint8_t hash[32];
 
@@ -193,12 +221,7 @@ static String sha256(const String& data) {
     mbedtls_sha256_finish(&ctx, hash);
     mbedtls_sha256_free(&ctx);
 
-    char res[sizeof(hash) * 2 + 1];
-    for (int i = 0; i < sizeof(hash); i++) {
-        snprintf(res + (i*2), sizeof(res) - (i*2), "%02x", hash[i]);
-    }
-
-    return res;
+    return bin2hex<sizeof(hash)>(hash);
 }
 
 static String extractParam(String const& authReq, String const& param, char delimiter) {
@@ -219,7 +242,22 @@ static String getcNonce(int len) {
     return s;
 }
 
-String HttpGetter::getAuthDigest(String const& authReq, unsigned int counter) {
+static std::pair<bool, String> getAlgo(String const& authReq) {
+    // the algorithm is NOT enclosed in double quotes, so we can't use extractParam
+    auto paramBegin = authReq.indexOf("algorithm=");
+    if (paramBegin == -1) { return { true, "MD5" }; } // default as per RFC2617
+    auto valueBegin = paramBegin + 10;
+
+    String algo = authReq.substring(valueBegin, valueBegin + 3);
+    if (algo == "MD5") { return { true, algo }; }
+
+    algo = authReq.substring(valueBegin, valueBegin + 7);
+    if (algo == "SHA-256") { return { true, algo }; }
+
+    return { false, "unsupported digest algorithm" };
+}
+
+std::pair<bool, String> HttpGetter::getAuthDigest(String const& authReq, unsigned int counter) {
     // extracting required parameters for RFC 2617 Digest
     String realm = extractParam(authReq, "realm=\"", '"');
     String nonce = extractParam(authReq, "nonce=\"", '"');
@@ -228,21 +266,19 @@ String HttpGetter::getAuthDigest(String const& authReq, unsigned int counter) {
     char nc[9];
     snprintf(nc, sizeof(nc), "%08x", counter);
 
-    // sha256 of the user:realm:password
-    String ha1 = sha256(String(_config.Username) + ":" + realm + ":" + _config.Password);
-
-    // sha256 of method:uri
-    String ha2 = sha256("GET:" + _uri);
+    auto algo = getAlgo(authReq);
+    if (!algo.first) { return { false, algo.second }; }
 
-    // sha256 of h1:nonce:nc:cNonce:auth:h2
-    String response = sha256(ha1 + ":" + nonce + ":" + String(nc) +
+    auto hash = (algo.second == "SHA-256") ? &sha256 : &md5;
+    String ha1 = hash(String(_config.Username) + ":" + realm + ":" + _config.Password);
+    String ha2 = hash("GET:" + _uri);
+    String response = hash(ha1 + ":" + nonce + ":" + String(nc) +
             ":" + cNonce + ":" + "auth" + ":" + ha2);
 
-    // Final authorization String
-    return String("Digest username=\"") + _config.Username +
+    return { true, String("Digest username=\"") + _config.Username +
         "\", realm=\"" + realm + "\", nonce=\"" + nonce + "\", uri=\"" +
         _uri + "\", cnonce=\"" + cNonce + "\", nc=" + nc +
-        ", qop=auth, response=\"" + response  + "\", algorithm=SHA-256";
+        ", qop=auth, response=\"" + response  + "\", algorithm=" + algo.second };
 }
 
 void HttpGetter::addHeader(char const* key, char const* value)