diff --git a/hw/opentitan/ot_hmac.c b/hw/opentitan/ot_hmac.c
index fd163e2d7f26..49ec989f572b 100644
--- a/hw/opentitan/ot_hmac.c
+++ b/hw/opentitan/ot_hmac.c
@@ -223,6 +223,22 @@ static const char *REG_NAMES[REGS_COUNT] = {
 };
 #undef REG_NAME_ENTRY
 
+enum OtHMACDigestSize {
+    HMAC_SHA2_256,
+    HMAC_SHA2_384,
+    HMAC_SHA2_512,
+    HMAC_SHA2_NONE,
+};
+
+enum OtHMACKeyLength {
+    HMAC_KEY_128,
+    HMAC_KEY_256,
+    HMAC_KEY_384,
+    HMAC_KEY_512,
+    HMAC_KEY_1024,
+    HMAC_KEY_NONE,
+};
+
 struct OtHMACRegisters {
     uint32_t intr_state;
     uint32_t intr_enable;
@@ -239,6 +255,7 @@ typedef struct OtHMACRegisters OtHMACRegisters;
 
 struct OtHMACContext {
     hash_state state;
+    enum OtHMACDigestSize digest_size_started;
 };
 typedef struct OtHMACContext OtHMACContext;
 
@@ -260,22 +277,6 @@ struct OtHMACState {
     char *ot_id;
 };
 
-enum OtHMACDigestSize {
-    HMAC_SHA2_256,
-    HMAC_SHA2_384,
-    HMAC_SHA2_512,
-    HMAC_SHA2_NONE,
-};
-
-enum OtHMACKeyLength {
-    HMAC_KEY_128,
-    HMAC_KEY_256,
-    HMAC_KEY_384,
-    HMAC_KEY_512,
-    HMAC_KEY_1024,
-    HMAC_KEY_NONE,
-};
-
 static inline enum OtHMACDigestSize ot_hmac_get_digest_size(uint32_t cfg_reg)
 {
     switch ((cfg_reg & R_CFG_DIGEST_SIZE_MASK) >> R_CFG_DIGEST_SIZE_SHIFT) {
@@ -291,9 +292,9 @@ static inline enum OtHMACDigestSize ot_hmac_get_digest_size(uint32_t cfg_reg)
     }
 }
 
-static inline size_t ot_hmac_get_digest_bytes(OtHMACState *s)
+static inline size_t ot_hmac_get_digest_bytes(enum OtHMACDigestSize digest_size)
 {
-    switch (ot_hmac_get_digest_size(s->regs->cfg)) {
+    switch (digest_size) {
     case HMAC_SHA2_256:
         return 32u;
     case HMAC_SHA2_384:
@@ -399,7 +400,7 @@ static void ot_hmac_writeback_digest_state(OtHMACState *s)
 {
     /* copy intermediary digest to mock HMAC operation for stop/continue
     behaviour. */
-    switch (ot_hmac_get_digest_size(s->regs->cfg)) {
+    switch (s->ctx->digest_size_started) {
     case HMAC_SHA2_256:
         for (unsigned idx = 0; idx < 8u; idx++) {
             STORE32H(s->ctx->state.sha256.state[idx], s->regs->digest + idx);
@@ -426,7 +427,7 @@ static void ot_hmac_writeback_digest_state(OtHMACState *s)
 
 static void ot_hmac_restore_context(OtHMACState *s)
 {
-    switch (ot_hmac_get_digest_size(s->regs->cfg)) {
+    switch (s->ctx->digest_size_started) {
     case HMAC_SHA2_256:
         s->ctx->state.sha256.curlen = 0;
         s->ctx->state.sha256.length = s->regs->msg_length;
@@ -458,7 +459,7 @@ static void ot_hmac_restore_context(OtHMACState *s)
 
 static size_t ot_hmac_get_curlen(OtHMACState *s)
 {
-    switch (ot_hmac_get_digest_size(s->regs->cfg)) {
+    switch (s->ctx->digest_size_started) {
     case HMAC_SHA2_256:
         return s->ctx->state.sha256.curlen;
     case HMAC_SHA2_384:
@@ -476,7 +477,7 @@ static size_t ot_hmac_get_curlen(OtHMACState *s)
 
 static void ot_hmac_sha_init(OtHMACState *s, bool write_back)
 {
-    switch (ot_hmac_get_digest_size(s->regs->cfg)) {
+    switch (s->ctx->digest_size_started) {
     case HMAC_SHA2_256:
         sha256_init(&s->ctx->state);
         break;
@@ -502,7 +503,7 @@ static void ot_hmac_sha_init(OtHMACState *s, bool write_back)
 static void ot_hmac_sha_process(OtHMACState *s, const uint8_t *in, size_t inlen,
                                 bool write_back)
 {
-    switch (ot_hmac_get_digest_size(s->regs->cfg)) {
+    switch (s->ctx->digest_size_started) {
     case HMAC_SHA2_256:
         sha256_process(&s->ctx->state, in, inlen);
         break;
@@ -527,7 +528,7 @@ static void ot_hmac_sha_process(OtHMACState *s, const uint8_t *in, size_t inlen,
 
 static void ot_hmac_sha_done(OtHMACState *s)
 {
-    switch (ot_hmac_get_digest_size(s->regs->cfg)) {
+    switch (s->ctx->digest_size_started) {
     case HMAC_SHA2_256:
         sha256_done(&s->ctx->state, (uint8_t *)s->regs->digest);
         return;
@@ -570,7 +571,9 @@ static void ot_hmac_compute_digest(OtHMACState *s)
         ot_hmac_sha_init(s, false);
         ot_hmac_sha_process(s, (const uint8_t *)opad, pad_length_b, false);
         ot_hmac_sha_process(s, (const uint8_t *)s->regs->digest,
-                            ot_hmac_get_digest_bytes(s), true);
+                            ot_hmac_get_digest_bytes(
+                                s->ctx->digest_size_started),
+                            true);
     }
     ot_hmac_sha_done(s);
 }
@@ -910,6 +913,10 @@ static void ot_hmac_regs_write(void *opaque, hwaddr addr, uint64_t value,
 
             ibex_irq_set(&s->clkmgr, true);
 
+            /* Hold the previous digest size until the HMAC is started with the
+            new digest size configured */
+            s->ctx->digest_size_started = ot_hmac_get_digest_size(s->regs->cfg);
+
             ot_hmac_sha_init(s, true);
 
             /* HMAC mode, process input padding */
@@ -975,6 +982,10 @@ static void ot_hmac_regs_write(void *opaque, hwaddr addr, uint64_t value,
 
             s->regs->cmd = R_CMD_HASH_CONTINUE_MASK;
 
+            /* Hold the previous digest size until the HMAC is started with the
+            new digest size configured */
+            s->ctx->digest_size_started = ot_hmac_get_digest_size(s->regs->cfg);
+
             ot_hmac_restore_context(s);
 
             /* trigger delayed processing of FIFO */