diff --git a/src/core/ddsc/src/dds_serdata_builtintopic.c b/src/core/ddsc/src/dds_serdata_builtintopic.c
index 4bedb15..a12331b 100644
--- a/src/core/ddsc/src/dds_serdata_builtintopic.c
+++ b/src/core/ddsc/src/dds_serdata_builtintopic.c
@@ -295,5 +295,6 @@ const struct ddsi_serdata_ops ddsi_serdata_ops_builtintopic = {
   .to_ser_unref = serdata_builtin_to_ser_unref,
   .to_topicless = serdata_builtin_to_topicless,
   .topicless_to_sample = serdata_builtin_topicless_to_sample,
-  .print = serdata_builtin_topic_print
+  .print = serdata_builtin_topic_print,
+  .get_keyhash = 0
 };
diff --git a/src/core/ddsc/src/dds_stream.c b/src/core/ddsc/src/dds_stream.c
index e020735..4896120 100644
--- a/src/core/ddsc/src/dds_stream.c
+++ b/src/core/ddsc/src/dds_stream.c
@@ -1579,7 +1579,10 @@ void dds_stream_extract_keyhash (dds_istream_t * __restrict is, dds_keyhash_t *
   const dds_topic_descriptor_t *desc = topic->type;
   kh->m_set = 1;
   if (desc->m_nkeys == 0)
+  {
     kh->m_iskey = 1;
+    kh->m_keysize = 0;
+  }
   else if (desc->m_flagset & DDS_TOPIC_FIXED_KEY)
   {
     dds_ostreamBE_t os;
@@ -1592,12 +1595,14 @@ void dds_stream_extract_keyhash (dds_istream_t * __restrict is, dds_keyhash_t *
     else
       dds_stream_extract_keyBE_from_data (is, &os, topic);
     assert (os.x.m_index <= 16);
+    kh->m_keysize = (unsigned)os.x.m_index & 0x1f;
   }
   else
   {
     dds_ostreamBE_t os;
     ddsrt_md5_state_t md5st;
     kh->m_iskey = 0;
+    kh->m_keysize = 16;
     dds_ostreamBE_init (&os, 0);
     if (just_key)
       dds_stream_extract_keyBE_from_key (is, &os, topic);
diff --git a/src/core/ddsi/include/dds/ddsi/ddsi_serdata.h b/src/core/ddsi/include/dds/ddsi/ddsi_serdata.h
index 4d9b64b..61a57e8 100644
--- a/src/core/ddsi/include/dds/ddsi/ddsi_serdata.h
+++ b/src/core/ddsi/include/dds/ddsi/ddsi_serdata.h
@@ -132,6 +132,11 @@ typedef bool (*ddsi_serdata_eqkey_t) (const struct ddsi_serdata *a, const struct
      returning bufsize-1) if it had to truncate) */
 typedef size_t (*ddsi_serdata_print_t) (const struct ddsi_sertopic *topic, const struct ddsi_serdata *d, char *buf, size_t size);
 
+/* Add keyhash (from serdata) to buffer (forcing md5 when necessary).
+   - key needs to be set within serdata (can already be md5)
+   - buf needs to be at least 16 bytes large */
+typedef void (*ddsi_serdata_get_keyhash_t) (const struct ddsi_serdata *d, struct nn_keyhash *buf, bool force_md5);
+
 struct ddsi_serdata_ops {
   ddsi_serdata_eqkey_t eqkey;
   ddsi_serdata_size_t get_size;
@@ -146,9 +151,11 @@ struct ddsi_serdata_ops {
   ddsi_serdata_topicless_to_sample_t topicless_to_sample;
   ddsi_serdata_free_t free;
   ddsi_serdata_print_t print;
+  ddsi_serdata_get_keyhash_t get_keyhash;
 };
 
 #define DDSI_SERDATA_HAS_PRINT 1
+#define DDSI_SERDATA_HAS_ADD_KEYHASH 1
 
 DDS_EXPORT void ddsi_serdata_init (struct ddsi_serdata *d, const struct ddsi_sertopic *tp, enum ddsi_serdata_kind kind);
 
@@ -221,6 +228,10 @@ DDS_EXPORT inline bool ddsi_serdata_print_topicless (const struct ddsi_sertopic
   }
 }
 
+DDS_EXPORT inline void ddsi_serdata_get_keyhash (const struct ddsi_serdata *d, struct nn_keyhash *buf, bool force_md5) {
+  d->ops->get_keyhash (d, buf, force_md5);
+}
+
 #if defined (__cplusplus)
 }
 #endif
diff --git a/src/core/ddsi/include/dds/ddsi/ddsi_serdata_default.h b/src/core/ddsi/include/dds/ddsi/ddsi_serdata_default.h
index ae582ff..daa8a7b 100644
--- a/src/core/ddsi/include/dds/ddsi/ddsi_serdata_default.h
+++ b/src/core/ddsi/include/dds/ddsi/ddsi_serdata_default.h
@@ -47,6 +47,7 @@ typedef struct dds_keyhash {
   unsigned char m_hash [16]; /* Key hash value. Also possibly key. Suitably aligned for accessing as uint32_t's */
   unsigned m_set : 1;        /* has it been initialised? */
   unsigned m_iskey : 1;      /* m_hash is key value */
+  unsigned m_keysize : 5;    /* size of the key within the hash buffer */
 } dds_keyhash_t;
 
 /* Debug builds may want to keep some additional state */
diff --git a/src/core/ddsi/include/dds/ddsi/q_entity.h b/src/core/ddsi/include/dds/ddsi/q_entity.h
index 77545d4..b7225fc 100644
--- a/src/core/ddsi/include/dds/ddsi/q_entity.h
+++ b/src/core/ddsi/include/dds/ddsi/q_entity.h
@@ -251,6 +251,7 @@ struct writer
   unsigned reliable: 1; /* iff 1, writer is reliable <=> heartbeat_xevent != NULL */
   unsigned handle_as_transient_local: 1; /* controls whether data is retained in WHC */
   unsigned include_keyhash: 1; /* iff 1, this writer includes a keyhash; keyless topics => include_keyhash = 0 */
+  unsigned force_md5_keyhash: 1; /* iff 1, when keyhash has to be hashed, no matter the size */
   unsigned retransmitting: 1; /* iff 1, this writer is currently retransmitting */
 #ifdef DDSI_INCLUDE_SSM
   unsigned supports_ssm: 1;
diff --git a/src/core/ddsi/include/dds/ddsi/q_xmsg.h b/src/core/ddsi/include/dds/ddsi/q_xmsg.h
index 24f32b0..eb99196 100644
--- a/src/core/ddsi/include/dds/ddsi/q_xmsg.h
+++ b/src/core/ddsi/include/dds/ddsi/q_xmsg.h
@@ -129,7 +129,7 @@ void nn_xmsg_submsg_init (struct nn_xmsg *msg, struct nn_xmsg_marker marker, Sub
 void nn_xmsg_add_timestamp (struct nn_xmsg *m, nn_wctime_t t);
 void nn_xmsg_add_entityid (struct nn_xmsg * m);
 void *nn_xmsg_addpar (struct nn_xmsg *m, nn_parameterid_t pid, size_t len);
-void nn_xmsg_addpar_keyhash (struct nn_xmsg *m, const struct ddsi_serdata *serdata);
+void nn_xmsg_addpar_keyhash (struct nn_xmsg *m, const struct ddsi_serdata *serdata, bool force_md5);
 void nn_xmsg_addpar_statusinfo (struct nn_xmsg *m, unsigned statusinfo);
 void nn_xmsg_addpar_sentinel (struct nn_xmsg *m);
 int nn_xmsg_addpar_sentinel_ifparam (struct nn_xmsg *m);
diff --git a/src/core/ddsi/src/ddsi_serdata.c b/src/core/ddsi/src/ddsi_serdata.c
index 68caae0..33d9584 100644
--- a/src/core/ddsi/src/ddsi_serdata.c
+++ b/src/core/ddsi/src/ddsi_serdata.c
@@ -47,3 +47,4 @@ extern inline bool ddsi_serdata_topicless_to_sample (const struct ddsi_sertopic
 extern inline bool ddsi_serdata_eqkey (const struct ddsi_serdata *a, const struct ddsi_serdata *b);
 extern inline bool ddsi_serdata_print (const struct ddsi_serdata *d, char *buf, size_t size);
 extern inline bool ddsi_serdata_print_topicless (const struct ddsi_sertopic *topic, const struct ddsi_serdata *d, char *buf, size_t size);
+extern inline void ddsi_serdata_get_keyhash (const struct ddsi_serdata *d, struct nn_keyhash *buf, bool force_md5);
diff --git a/src/core/ddsi/src/ddsi_serdata_default.c b/src/core/ddsi/src/ddsi_serdata_default.c
index 666e0d3..34f88e9 100644
--- a/src/core/ddsi/src/ddsi_serdata_default.c
+++ b/src/core/ddsi/src/ddsi_serdata_default.c
@@ -231,6 +231,7 @@ static void serdata_default_init(struct ddsi_serdata_default *d, const struct dd
   memset (d->keyhash.m_hash, 0, sizeof (d->keyhash.m_hash));
   d->keyhash.m_set = 0;
   d->keyhash.m_iskey = 0;
+  d->keyhash.m_keysize = 0;
 }
 
 static struct ddsi_serdata_default *serdata_default_allocnew (struct serdatapool *serpool, uint32_t init_size)
@@ -355,6 +356,7 @@ static struct ddsi_serdata *ddsi_serdata_from_keyhash_cdr (const struct ddsi_ser
     memcpy (d->keyhash.m_hash, keyhash->value, sizeof (d->keyhash.m_hash));
     d->keyhash.m_set = 1;
     d->keyhash.m_iskey = 1;
+    d->keyhash.m_keysize = sizeof (d->keyhash.m_hash);
     return fix_serdata_default(d, tp->c.serdata_basehash);
   }
 }
@@ -368,6 +370,7 @@ static struct ddsi_serdata *ddsi_serdata_from_keyhash_cdr_nokey (const struct dd
   (void)keyhash;
   d->keyhash.m_set = 1;
   d->keyhash.m_iskey = 1;
+  d->keyhash.m_keysize = 0;
   return fix_serdata_default_nokey(d, tp->c.serdata_basehash);
 }
 
@@ -376,11 +379,15 @@ static void gen_keyhash_from_sample (const struct ddsi_sertopic_default *topic,
   const struct dds_topic_descriptor *desc = (const struct dds_topic_descriptor *) topic->type;
   kh->m_set = 1;
   if (desc->m_nkeys == 0)
+  {
     kh->m_iskey = 1;
+    kh->m_keysize = 0;
+  }
   else if (desc->m_flagset & DDS_TOPIC_FIXED_KEY)
   {
     dds_ostreamBE_t os;
     kh->m_iskey = 1;
+    kh->m_keysize = sizeof(kh->m_hash);
     dds_ostreamBE_init (&os, 0);
     os.x.m_buffer = kh->m_hash;
     os.x.m_size = 16;
@@ -391,6 +398,7 @@ static void gen_keyhash_from_sample (const struct ddsi_sertopic_default *topic,
     dds_ostreamBE_t os;
     ddsrt_md5_state_t md5st;
     kh->m_iskey = 0;
+    kh->m_keysize = sizeof(kh->m_hash);
     dds_ostreamBE_init (&os, 64);
     dds_stream_write_keyBE (&os, sample, topic);
     ddsrt_md5_init (&md5st);
@@ -461,6 +469,7 @@ static struct ddsi_serdata *serdata_default_from_sample_plist (const struct ddsi
     case PID_GROUP_GUID:
       d->keyhash.m_set = 1;
       d->keyhash.m_iskey = 1;
+      d->keyhash.m_keysize = sizeof(d->keyhash.m_hash);
       memcpy (d->keyhash.m_hash, rawkey, 16);
 #ifndef NDEBUG
       keysize = 16;
@@ -477,6 +486,7 @@ static struct ddsi_serdata *serdata_default_from_sample_plist (const struct ddsi
       topic_name_sz_BE = ddsrt_toBE4u (topic_name_sz);
       d->keyhash.m_set = 1;
       d->keyhash.m_iskey = 0;
+      d->keyhash.m_keysize = sizeof(d->keyhash.m_hash);
       ddsrt_md5_init (&md5st);
       ddsrt_md5_append (&md5st, (const ddsrt_md5_byte_t *) &topic_name_sz_BE, sizeof (topic_name_sz_BE));
       ddsrt_md5_append (&md5st, (const ddsrt_md5_byte_t *) topic_name, topic_name_sz);
@@ -512,10 +522,14 @@ static struct ddsi_serdata *serdata_default_from_sample_rawcdr (const struct dds
   d->keyhash.m_set = 1;
   d->keyhash.m_iskey = 1;
   if (sample->keysize == 0)
+  {
+    d->keyhash.m_keysize = 0;
     return fix_serdata_default_nokey (d, tp->c.serdata_basehash);
+  }
   else
   {
     memcpy (&d->keyhash.m_hash, sample->key, sample->keysize);
+    d->keyhash.m_keysize = (unsigned)sample->keysize & 0x1f;
     return fix_serdata_default (d, tp->c.serdata_basehash);
   }
 }
@@ -656,6 +670,24 @@ static size_t serdata_default_print_raw (const struct ddsi_sertopic *sertopic_co
   return (size_t) snprintf (buf, size, "(blob)");
 }
 
+static void serdata_default_get_keyhash (const struct ddsi_serdata *serdata_common, struct nn_keyhash *buf, bool force_md5)
+{
+  const struct ddsi_serdata_default *d = (const struct ddsi_serdata_default *)serdata_common;
+  assert(buf);
+  assert(d->keyhash.m_set);
+  if (force_md5 && d->keyhash.m_iskey /* m_iskey == !md5 */)
+  {
+    ddsrt_md5_state_t md5st;
+    ddsrt_md5_init  (&md5st);
+    ddsrt_md5_append(&md5st, (ddsrt_md5_byte_t*)(d->keyhash.m_hash), d->keyhash.m_keysize);
+    ddsrt_md5_finish(&md5st, (ddsrt_md5_byte_t*)(buf->value));
+  }
+  else
+  {
+    memcpy (buf->value, d->keyhash.m_hash, 16);
+  }
+}
+
 const struct ddsi_serdata_ops ddsi_serdata_ops_cdr = {
   .get_size = serdata_default_get_size,
   .eqkey = serdata_default_eqkey,
@@ -669,7 +701,8 @@ const struct ddsi_serdata_ops ddsi_serdata_ops_cdr = {
   .to_ser_unref = serdata_default_to_ser_unref,
   .to_topicless = serdata_default_to_topicless,
   .topicless_to_sample = serdata_default_topicless_to_sample_cdr,
-  .print = serdata_default_print_cdr
+  .print = serdata_default_print_cdr,
+  .get_keyhash = serdata_default_get_keyhash
 };
 
 const struct ddsi_serdata_ops ddsi_serdata_ops_cdr_nokey = {
@@ -685,7 +718,8 @@ const struct ddsi_serdata_ops ddsi_serdata_ops_cdr_nokey = {
   .to_ser_unref = serdata_default_to_ser_unref,
   .to_topicless = serdata_default_to_topicless,
   .topicless_to_sample = serdata_default_topicless_to_sample_cdr_nokey,
-  .print = serdata_default_print_cdr
+  .print = serdata_default_print_cdr,
+  .get_keyhash = serdata_default_get_keyhash
 };
 
 const struct ddsi_serdata_ops ddsi_serdata_ops_plist = {
@@ -701,7 +735,8 @@ const struct ddsi_serdata_ops ddsi_serdata_ops_plist = {
   .to_ser_unref = serdata_default_to_ser_unref,
   .to_topicless = serdata_default_to_topicless,
   .topicless_to_sample = 0,
-  .print = serdata_default_print_plist
+  .print = serdata_default_print_plist,
+  .get_keyhash = serdata_default_get_keyhash
 };
 
 const struct ddsi_serdata_ops ddsi_serdata_ops_rawcdr = {
@@ -717,5 +752,6 @@ const struct ddsi_serdata_ops ddsi_serdata_ops_rawcdr = {
   .to_ser_unref = serdata_default_to_ser_unref,
   .to_topicless = serdata_default_to_topicless,
   .topicless_to_sample = 0,
-  .print = serdata_default_print_raw
+  .print = serdata_default_print_raw,
+  .get_keyhash = serdata_default_get_keyhash
 };
diff --git a/src/core/ddsi/src/q_entity.c b/src/core/ddsi/src/q_entity.c
index 3a5b98a..3806294 100644
--- a/src/core/ddsi/src/q_entity.c
+++ b/src/core/ddsi/src/q_entity.c
@@ -3063,6 +3063,7 @@ static void new_writer_guid_common_init (struct writer *wr, const struct ddsi_se
   wr->throttle_tracing = 0;
   wr->rexmit_count = 0;
   wr->rexmit_lost_count = 0;
+  wr->force_md5_keyhash = 0;
 
   wr->status_cb = status_cb;
   wr->status_cb_entity = status_entity;
diff --git a/src/core/ddsi/src/q_transmit.c b/src/core/ddsi/src/q_transmit.c
index 032b638..7e08483 100644
--- a/src/core/ddsi/src/q_transmit.c
+++ b/src/core/ddsi/src/q_transmit.c
@@ -492,7 +492,7 @@ static dds_return_t create_fragment_message_simple (struct writer *wr, seqno_t s
 
   /* Adding parameters means potential reallocing, so sm, ddcmn now likely become invalid */
   if (wr->include_keyhash)
-    nn_xmsg_addpar_keyhash (*pmsg, serdata);
+    nn_xmsg_addpar_keyhash (*pmsg, serdata, wr->force_md5_keyhash);
   if (serdata->statusinfo)
     nn_xmsg_addpar_statusinfo (*pmsg, serdata->statusinfo);
   if (nn_xmsg_addpar_sentinel_ifparam (*pmsg) > 0)
@@ -659,7 +659,7 @@ dds_return_t create_fragment_message (struct writer *wr, seqno_t seq, const stru
     /* Adding parameters means potential reallocing, so sm, ddcmn now likely become invalid */
     if (wr->include_keyhash)
     {
-      nn_xmsg_addpar_keyhash (*pmsg, serdata);
+      nn_xmsg_addpar_keyhash (*pmsg, serdata, wr->force_md5_keyhash);
     }
     if (serdata->statusinfo)
     {
diff --git a/src/core/ddsi/src/q_xmsg.c b/src/core/ddsi/src/q_xmsg.c
index b8cbbb6..b0aad86 100644
--- a/src/core/ddsi/src/q_xmsg.c
+++ b/src/core/ddsi/src/q_xmsg.c
@@ -938,13 +938,12 @@ void *nn_xmsg_addpar (struct nn_xmsg *m, nn_parameterid_t pid, size_t len)
   return p;
 }
 
-void nn_xmsg_addpar_keyhash (struct nn_xmsg *m, const struct ddsi_serdata *serdata)
+void nn_xmsg_addpar_keyhash (struct nn_xmsg *m, const struct ddsi_serdata *serdata, bool force_md5)
 {
   if (serdata->kind != SDK_EMPTY)
   {
-    const struct ddsi_serdata_default *serdata_def = (const struct ddsi_serdata_default *)serdata;
     char *p = nn_xmsg_addpar (m, PID_KEYHASH, 16);
-    memcpy (p, serdata_def->keyhash.m_hash, 16);
+    ddsi_serdata_get_keyhash(serdata, (struct nn_keyhash*)p, force_md5);
   }
 }