diff --git a/src/wp_ecx_sig.c b/src/wp_ecx_sig.c
index 04e6de13..80ed0d35 100644
--- a/src/wp_ecx_sig.c
+++ b/src/wp_ecx_sig.c
@@ -370,21 +370,25 @@ static int wp_ed25519_digest_sign(wp_EcxSigCtx *ctx, unsigned char *sig,
     else if (sig == NULL) {
         *sigLen = ED25519_SIG_SIZE;
     }
-    else if (*sigLen != ED25519_SIG_SIZE) {
+    else if (*sigLen < ED25519_SIG_SIZE) {
         ok = 0;
     }
     else {
         int rc;
-        word32 len;
+        word32 len = ED25519_SIG_SIZE;
         ed25519_key* ed25519 = (ed25519_key*)wp_ecx_get_key(ctx->ecx);
 
         if (sigSize == (size_t)-1) {
             sigSize = *sigLen;
         }
-        len = (word32)sigSize;
 
         /* EdDSA signs with the public half; make sure it is derived first. */
         ok = wp_ecx_ensure_pub(ctx->ecx);
+        /* wolfCrypt is given the signature size rather than the buffer size,
+         * which may be too large to hold in a word32. */
+        if (ok && (sigSize < ED25519_SIG_SIZE)) {
+            ok = 0;
+        }
         if (ok && (!WP_FITS_WORD32(tbsLen))) {
             ok = 0;
         }
@@ -583,16 +587,20 @@ static int wp_ed448_digest_sign(wp_EcxSigCtx *ctx, unsigned char *sig,
     }
     else {
         int rc;
-        word32 len;
+        word32 len = ED448_SIG_SIZE;
         ed448_key* ed448 = (ed448_key*)wp_ecx_get_key(ctx->ecx);
 
         if (sigSize == (size_t)-1) {
             sigSize = *sigLen;
         }
-        len = (word32)sigSize;
 
         /* EdDSA signs with the public half; make sure it is derived first. */
         ok = wp_ecx_ensure_pub(ctx->ecx);
+        /* wolfCrypt is given the signature size rather than the buffer size,
+         * which may be too large to hold in a word32. */
+        if (ok && (sigSize < ED448_SIG_SIZE)) {
+            ok = 0;
+        }
         if (ok && (!WP_FITS_WORD32(tbsLen))) {
             ok = 0;
         }
diff --git a/test/test_ecx.c b/test/test_ecx.c
index 5551b9c3..84176a4b 100644
--- a/test/test_ecx.c
+++ b/test/test_ecx.c
@@ -197,6 +197,151 @@ int test_ecx_encode_epki(void *data)
 }
 #endif /* WP_HAVE_ED25519 && WP_HAVE_EPKI_TEST */
 
+/* Size the scratch buffers from the largest EdDSA type that is built, so
+ * an Ed25519-only build compiles. */
+#ifdef WP_HAVE_ED448
+#define WP_ECX_SIG_SIZE_MAX     ED448_SIG_SIZE
+#else
+#define WP_ECX_SIG_SIZE_MAX     ED25519_SIG_SIZE
+#endif
+
+/* Fill byte for the undersized buffer, so a stray write is visible. */
+#define WP_ECX_SIG_GUARD        0x5a
+
+/**
+ * Sign with an output buffer larger than the signature.
+ *
+ * The length passed in is the size of the buffer, an upper bound, so a
+ * caller with a fixed maximum signature buffer must be accepted.
+ *
+ * @param [in] libCtx    Library context to sign with.
+ * @param [in] type      Key type, ED25519 or ED448.
+ * @param [in] keyDer    DER encoded private key.
+ * @param [in] keyDerSz  Length of the DER key in bytes.
+ * @param [in] sigSize   Size of the signature for the key type.
+ * @return  0 on success, non-zero on failure.
+ */
+static int test_ecx_sign_buffer_size(OSSL_LIB_CTX *libCtx, int type,
+    const unsigned char *keyDer, size_t keyDerSz, size_t sigSize)
+{
+    int err = 0;
+    const unsigned char *p = keyDer;
+    EVP_PKEY *pkey = NULL;
+    EVP_MD_CTX *mdCtx = NULL;
+    unsigned char sigExact[WP_ECX_SIG_SIZE_MAX];
+    unsigned char sigLarge[WP_ECX_SIG_SIZE_MAX * 2];
+    unsigned char sigSmall[WP_ECX_SIG_SIZE_MAX];
+    size_t exactLen = sigSize;
+    size_t largeLen = sizeof(sigLarge);
+    size_t smallLen = 0;
+    size_t i;
+    static const unsigned char msg[] = "ECX signature buffer size message";
+
+    pkey = d2i_PrivateKey_ex(type, NULL, &p, (long)keyDerSz, libCtx, NULL);
+    err = (pkey == NULL);
+
+    /* An exactly sized buffer has always worked; keep it covered. */
+    if (err == 0) {
+        mdCtx = EVP_MD_CTX_new();
+        err = (mdCtx == NULL);
+    }
+    if (err == 0) {
+        err = EVP_DigestSignInit_ex(mdCtx, NULL, NULL, libCtx, NULL, pkey,
+            NULL) != 1;
+    }
+    if (err == 0) {
+        err = EVP_DigestSign(mdCtx, sigExact, &exactLen, msg,
+            sizeof(msg) - 1) != 1;
+        if (err) {
+            PRINT_ERR_MSG("Sign with an exactly sized buffer failed");
+        }
+    }
+    EVP_MD_CTX_free(mdCtx);
+    mdCtx = NULL;
+
+    if (err == 0) {
+        mdCtx = EVP_MD_CTX_new();
+        err = (mdCtx == NULL);
+    }
+    if (err == 0) {
+        err = EVP_DigestSignInit_ex(mdCtx, NULL, NULL, libCtx, NULL, pkey,
+            NULL) != 1;
+    }
+    if (err == 0) {
+        err = EVP_DigestSign(mdCtx, sigLarge, &largeLen, msg,
+            sizeof(msg) - 1) != 1;
+        if (err) {
+            PRINT_ERR_MSG("Sign with an oversized buffer failed");
+        }
+    }
+    if (err == 0) {
+        err = (largeLen != sigSize);
+        if (err) {
+            PRINT_ERR_MSG("Oversized buffer returned the wrong length");
+        }
+    }
+    if (err == 0) {
+        err = (exactLen != largeLen) ||
+            (memcmp(sigExact, sigLarge, exactLen) != 0);
+        if (err) {
+            PRINT_ERR_MSG("Buffer size changed the signature");
+        }
+    }
+    EVP_MD_CTX_free(mdCtx);
+    mdCtx = NULL;
+
+    /* A buffer smaller than the signature must be refused and left alone. */
+    if (err == 0) {
+        mdCtx = EVP_MD_CTX_new();
+        err = (mdCtx == NULL);
+    }
+    if (err == 0) {
+        err = EVP_DigestSignInit_ex(mdCtx, NULL, NULL, libCtx, NULL, pkey,
+            NULL) != 1;
+    }
+    if (err == 0) {
+        memset(sigSmall, WP_ECX_SIG_GUARD, sizeof(sigSmall));
+        smallLen = sigSize - 1;
+        err = EVP_DigestSign(mdCtx, sigSmall, &smallLen, msg,
+            sizeof(msg) - 1) == 1;
+        if (err) {
+            PRINT_ERR_MSG("Sign with an undersized buffer succeeded");
+        }
+    }
+    for (i = 0; (err == 0) && (i < sizeof(sigSmall)); i++) {
+        if (sigSmall[i] != WP_ECX_SIG_GUARD) {
+            PRINT_ERR_MSG("Undersized buffer was written to");
+            err = 1;
+        }
+    }
+
+    EVP_MD_CTX_free(mdCtx);
+    EVP_PKEY_free(pkey);
+    return err;
+}
+
+int test_ecx_sign_buffer_sizes(void *data)
+{
+    int err = 0;
+
+    (void)data;
+
+    #ifdef WP_HAVE_ED25519
+    PRINT_MSG("Ed25519 signature output buffer sizes");
+    err = test_ecx_sign_buffer_size(wpLibCtx, EVP_PKEY_ED25519,
+        ed25519_key_der, sizeof(ed25519_key_der), ED25519_SIG_SIZE);
+    #endif
+    #ifdef WP_HAVE_ED448
+    if (err == 0) {
+        PRINT_MSG("Ed448 signature output buffer sizes");
+        err = test_ecx_sign_buffer_size(wpLibCtx, EVP_PKEY_ED448,
+            ed448_key_der, sizeof(ed448_key_der), ED448_SIG_SIZE);
+    }
+    #endif
+
+    return err;
+}
+
 int test_ecx_sign_verify(void *data)
 {
     int err = 0;
diff --git a/test/unit.c b/test/unit.c
index da5d820d..8906d545 100644
--- a/test/unit.c
+++ b/test/unit.c
@@ -562,6 +562,7 @@ TEST_CASE test_case[] = {
     TEST_DECL(test_ecx_encode_epki, NULL),
 #endif
     TEST_DECL(test_ecx_sign_verify, NULL),
+    TEST_DECL(test_ecx_sign_buffer_sizes, NULL),
     TEST_DECL(test_ecx_sign_verify_raw_priv, NULL),
     TEST_DECL(test_ecx_sign_verify_raw_pub, NULL),
     TEST_DECL(test_ecx_misc, NULL),
diff --git a/test/unit.h b/test/unit.h
index 86c255ca..8384c73e 100644
--- a/test/unit.h
+++ b/test/unit.h
@@ -595,6 +595,7 @@ int test_pbkdf2(void *data);
 int test_ecx_encode_epki(void *data);
 #endif
 int test_ecx_sign_verify(void *data);
+int test_ecx_sign_buffer_sizes(void *data);
 int test_ecx_sign_verify_raw_priv(void *data);
 int test_ecx_sign_verify_raw_pub(void *data);
 int test_ecx_misc(void *data);