diff --git a/xwords4/common/config.mk b/xwords4/common/config.mk
index 413139618..1516adaef 100644
--- a/xwords4/common/config.mk
+++ b/xwords4/common/config.mk
@@ -52,6 +52,7 @@ COMMONSRC = \
 	$(COMMONDIR)/dutil.c \
 	$(COMMONDIR)/device.c \
 	$(COMMONDIR)/knownplyr.c \
+	$(COMMONDIR)/md5.c \
 
 # PENDING: define this in terms of above!!!
 
@@ -93,5 +94,6 @@ COMMON5 = \
 	$(COMMONOBJDIR)/dutil.o \
 	$(COMMONOBJDIR)/device.o \
 	$(COMMONOBJDIR)/knownplyr.o \
+	$(COMMONOBJDIR)/md5.o \
 
 COMMONOBJ = $(COMMON1) $(COMMON2) $(COMMON3) $(COMMON4) $(COMMON5)
diff --git a/xwords4/common/md5.c b/xwords4/common/md5.c
index f79a427b7..cfded5b07 100644
--- a/xwords4/common/md5.c
+++ b/xwords4/common/md5.c
@@ -1,9 +1,14 @@
+// as copied from https://github.com/Zunawe/md5-c
 /*
  * Derived from the RSA Data Security, Inc. MD5 Message-Digest Algorithm
  * and modified slightly to be functionally identical but condensed into control structures.
  */
+#ifdef NO_NATIVE_MD5SDUM
 
 #include "md5.h"
+#include "comtypes.h"
+
+static void md5Step(uint32_t *buffer, uint32_t *input);
 
 /*
  * Constants defined by the MD5 algorithm
@@ -62,11 +67,17 @@ uint32_t rotateLeft(uint32_t x, uint32_t n){
     return (x << n) | (x >> (32 - n));
 }
 
+typedef struct {
+    uint64_t size;        // Size of input in bytes
+    uint32_t buffer[4];   // Current accumulation of hash
+    uint8_t input[64];    // Input to be used in the next step
+    uint8_t digest[16];   // Result of algorithm
+} MD5Context;
 
 /*
  * Initialize a context
  */
-void md5Init(MD5Context *ctx){
+static void md5Init(MD5Context *ctx){
     ctx->size = (uint64_t)0;
 
     ctx->buffer[0] = (uint32_t)A;
@@ -81,7 +92,7 @@ void md5Init(MD5Context *ctx){
  * If the input fills out a block of 512 bits, apply the algorithm (md5Step)
  * and save the result in the buffer. Also updates the overall size.
  */
-void md5Update(MD5Context *ctx, uint8_t *input_buffer, size_t input_len){
+static void md5Update(MD5Context *ctx, uint8_t *input_buffer, size_t input_len){
     uint32_t input[16];
     unsigned int offset = ctx->size % 64;
     ctx->size += (uint64_t)input_len;
@@ -114,7 +125,7 @@ void md5Update(MD5Context *ctx, uint8_t *input_buffer, size_t input_len){
  * Pad the current input to get to 448 bytes, append the size in bits to the very end,
  * and save the result of the final iteration into digest.
  */
-void md5Finalize(MD5Context *ctx){
+static void md5Finalize(MD5Context *ctx){
     uint32_t input[16];
     unsigned int offset = ctx->size % 64;
     unsigned int padding_length = offset < 56 ? 56 - offset : (56 + 64) - offset;
@@ -148,7 +159,8 @@ void md5Finalize(MD5Context *ctx){
 /*
  * Step on 512 bits of input with the main MD5 algorithm.
  */
-void md5Step(uint32_t *buffer, uint32_t *input){
+static void
+md5Step(uint32_t *buffer, uint32_t *input){
     uint32_t AA = buffer[0];
     uint32_t BB = buffer[1];
     uint32_t CC = buffer[2];
@@ -191,33 +203,18 @@ void md5Step(uint32_t *buffer, uint32_t *input){
     buffer[3] += DD;
 }
 
-/*
- * Functions that run the algorithm on the provided input and put the digest into result.
- * result should be able to store 16 bytes.
- */
-void md5String(char *input, uint8_t *result){
-    MD5Context ctx;
-    md5Init(&ctx);
-    md5Update(&ctx, (uint8_t *)input, strlen(input));
-    md5Finalize(&ctx);
-
-    memcpy(result, ctx.digest, 16);
-}
-
-void md5File(FILE *file, uint8_t *result){
-    char *input_buffer = malloc(1024);
-    size_t input_size = 0;
-
-    MD5Context ctx;
-    md5Init(&ctx);
-
-    while((input_size = fread(input_buffer, 1, 1024, file)) > 0){
-        md5Update(&ctx, (uint8_t *)input_buffer, input_size);
+void
+calcMD5Sum( MD5Result* out, uint8_t* data, size_t len )
+{
+    MD5Context ctx = {0};
+    md5Init( &ctx );
+    md5Update( &ctx, data, len );
+    md5Finalize( &ctx );
+    char* output = out->output;
+    for ( unsigned int ii = 0; ii < VSIZE(ctx.digest); ++ii ) {
+        uint8_t byt = ctx.digest[ii];
+        sprintf( output, "%02x", byt );
+        output += 2;
     }
-
-    md5Finalize(&ctx);
-
-    free(input_buffer);
-
-    memcpy(result, ctx.digest, 16);
 }
+#endif
diff --git a/xwords4/common/md5.h b/xwords4/common/md5.h
index ca172f193..c11b63e1f 100644
--- a/xwords4/common/md5.h
+++ b/xwords4/common/md5.h
@@ -1,24 +1,18 @@
+// as copied from https://github.com/Zunawe/md5-c
+
 #ifndef MD5_H
 #define MD5_H
 
-#include <stdio.h>
+#ifdef NO_NATIVE_MD5SDUM
+
 #include <stdint.h>
-#include <string.h>
 #include <stdlib.h>
 
-typedef struct{
-    uint64_t size;        // Size of input in bytes
-    uint32_t buffer[4];   // Current accumulation of hash
-    uint8_t input[64];    // Input to be used in the next step
-    uint8_t digest[16];   // Result of algorithm
-}MD5Context;
+typedef struct _MD5Result {
+    char output[33];
+} MD5Result;
 
-void md5Init(MD5Context *ctx);
-void md5Update(MD5Context *ctx, uint8_t *input, size_t input_len);
-void md5Finalize(MD5Context *ctx);
-void md5Step(uint32_t *buffer, uint32_t *input);
-
-void md5String(char *input, uint8_t *result);
-void md5File(FILE *file, uint8_t *result);
+void calcMD5Sum( MD5Result* out, uint8_t* data, size_t len );
+#endif
 
 #endif
diff --git a/xwords4/wasm/Makefile b/xwords4/wasm/Makefile
index 1984a33a6..0a27f441e 100644
--- a/xwords4/wasm/Makefile
+++ b/xwords4/wasm/Makefile
@@ -52,6 +52,8 @@ DEFINES += -DXWFEATURE_COMMS_INVITE
 DEFINES += -DMQTT_DEV_TOPICS
 DEFINES += -DMQTT_GAMEID_TOPICS
 DEFINES += -DXWFEATURE_NLI_FROM_ARGV
+DEFINES += -DNO_NATIVE_MD5SDUM
+
 
 DEFINES += -Wno-switch
 DEFINES += -DGITREV=\"$(shell git describe --tags --dirty)\"
diff --git a/xwords4/wasm/main.c b/xwords4/wasm/main.c
index d26ac3b9d..3252339d1 100644
--- a/xwords4/wasm/main.c
+++ b/xwords4/wasm/main.c
@@ -42,6 +42,7 @@
 #include "movestak.h"
 #include "knownplyr.h"
 #include "dbgutil.h"
+#include "md5.h"
 
 #include "main.h"
 #include "wasmdraw.h"
@@ -307,6 +308,11 @@ EM_JS( void, js_notify, (const char* msg), {
                          );
     });
 
+EM_JS( void, js_ackMsg, (const char* sum, int gameID), {
+        const jsum = UTF8ToString(sum);
+        postAck(jsum, gameID);
+    })
+
 static void
 loadPrefs( Globals* globals )
 {
@@ -880,6 +886,14 @@ main_getLocalName( Globals* globals, char* playerName, size_t buflen )
     return haveName;
 }
 
+void
+main_ackMQTTMsg( Globals* globals, const XP_U8* msg, XP_U16 len, int gameID )
+{
+    MD5Result out;
+    calcMD5Sum( &out, (uint8_t*)msg, len );
+    js_ackMsg( out.output, gameID );
+}
+
 static void
 onPlayerNamed( void* closure, const char* name )
 {
diff --git a/xwords4/wasm/main.h b/xwords4/wasm/main.h
index 8e85ea96f..8602e1ce9 100644
--- a/xwords4/wasm/main.h
+++ b/xwords4/wasm/main.h
@@ -121,5 +121,5 @@ void main_pickBlank( GameState* gs, int playerNum, int col, int row,
 void main_updateScreen( GameState* gs );
 void main_needDictForGame(GameState* gs, const char* lc, const XP_UCHAR* dictName);
 bool main_getLocalName( Globals* globals, char* playerName, size_t buflen );
-
+void main_ackMQTTMsg( Globals* globals, const XP_U8* msg, XP_U16 len, int gameID );
 #endif
diff --git a/xwords4/wasm/wasmdutil.c b/xwords4/wasm/wasmdutil.c
index 06e295b45..9a1d94e25 100644
--- a/xwords4/wasm/wasmdutil.c
+++ b/xwords4/wasm/wasmdutil.c
@@ -509,9 +509,8 @@ wasm_dutil_ackMQTTMsg( XW_DUtilCtxt* duc, XWEnv xwe, XP_U32 gameID,
                        const MQTTDevID* senderID, const XP_U8* msg,
                        XP_U16 len )
 {
-    /* There's no native md5 hash in js, so not using this right now. Messages
-     * will go unack'd. */
-    XP_LOGFF("unimplemented");
+    Globals* globals = (Globals*)duc->closure;
+    main_ackMQTTMsg( globals, msg, len, gameID );
 }
 
 static void
diff --git a/xwords4/wasm/xwutils.js b/xwords4/wasm/xwutils.js
index d2e87c6a2..86686e22f 100644
--- a/xwords4/wasm/xwutils.js
+++ b/xwords4/wasm/xwutils.js
@@ -59,6 +59,24 @@ function registerOnce(devid, gitrev, now, dbg) {
 	}
 }
 
+function postAck(sum, gameID) {
+	fetch('/xw4/api/v1/ack', {
+		method: 'post',
+		body: JSON.stringify({ "sum": sum, "gid": gameID, }),
+		headers: {
+			'Content-Type': 'application/json'
+		},
+	})
+		.then(handleFetchErrors)
+		.then(res => {
+			return res.json();
+		}).then(data => {
+			console.log('data from ack: ' + JSON.stringify(data));
+		}).catch(ex => {
+			console.error('postAck(): fetch()=>' + ex);
+		});
+}
+
 function handleFetchErrors(response) {
 	if ( response.ok ) {
 		return response;