diff --git a/bin/confgen/keygen.c b/bin/confgen/keygen.c
index c8832b5c4e..c5cce21a57 100644
--- a/bin/confgen/keygen.c
+++ b/bin/confgen/keygen.c
@@ -20,6 +20,7 @@
 #include <isc/base64.h>
 #include <isc/buffer.h>
 #include <isc/file.h>
+#include <isc/md.h>
 #include <isc/mem.h>
 #include <isc/print.h>
 #include <isc/result.h>
@@ -97,7 +98,7 @@ generate_key(isc_mem_t *mctx, dns_secalg_t alg, int keysize,
 	isc_result_t result = ISC_R_SUCCESS;
 	isc_buffer_t key_rawbuffer;
 	isc_region_t key_rawregion;
-	char key_rawsecret[64];
+	char key_rawsecret[ISC_MAX_BLOCK_SIZE];
 	dst_key_t *key = NULL;
 
 	switch (alg) {
diff --git a/bin/tests/system/rndc_confgen/tests_rndc_confgen.py b/bin/tests/system/rndc_confgen/tests_rndc_confgen.py
new file mode 100644
index 0000000000..585e933e79
--- /dev/null
+++ b/bin/tests/system/rndc_confgen/tests_rndc_confgen.py
@@ -0,0 +1,48 @@
+# Copyright (C) Internet Systems Consortium, Inc. ("ISC")
+#
+# SPDX-License-Identifier: MPL-2.0
+#
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0.  If a copy of the MPL was not distributed with this
+# file, you can obtain one at https://mozilla.org/MPL/2.0/.
+#
+# See the COPYRIGHT file distributed with this work for additional
+# information regarding copyright ownership.
+
+import base64
+import os
+import re
+
+import pytest
+
+import isctest
+
+
+def _extract_secret(stdout: bytes) -> bytes:
+    match = re.search(rb'secret\s+"([^"]+)"', stdout)
+    assert match is not None, f"no secret in output: {stdout!r}"
+    return base64.b64decode(match.group(1))
+
+
+@pytest.mark.parametrize(
+    "algorithm,bits",
+    [
+        ("hmac-sha256", 1),
+        ("hmac-sha256", 256),
+        ("hmac-sha256", 512),
+        ("hmac-sha384", 1),
+        ("hmac-sha384", 384),
+        ("hmac-sha384", 513),
+        ("hmac-sha384", 768),
+        ("hmac-sha384", 1024),
+        ("hmac-sha512", 1),
+        ("hmac-sha512", 512),
+        ("hmac-sha512", 513),
+        ("hmac-sha512", 1024),
+    ],
+)
+def test_rndc_confgen_hmac_keysize(algorithm, bits):
+    cmd = isctest.run.cmd([os.environ["RNDCCONFGEN"], "-A", algorithm, "-b", str(bits)])
+    secret = _extract_secret(cmd.proc.stdout)
+    assert len(secret) == (bits + 7) // 8
+    assert f"algorithm {algorithm};".encode() in cmd.proc.stdout
diff --git a/lib/dns/hmac_link.c b/lib/dns/hmac_link.c
index 07c3090de9..60ad0de632 100644
--- a/lib/dns/hmac_link.c
+++ b/lib/dns/hmac_link.c
@@ -280,7 +280,7 @@ hmac_generate(const isc_md_type_t *type, dst_key_t *key) {
 	isc_buffer_t b;
 	isc_result_t ret;
 	unsigned int bytes, len;
-	unsigned char data[ISC_MAX_MD_SIZE] = { 0 };
+	unsigned char data[ISC_MAX_BLOCK_SIZE] = { 0 };
 
 	len = isc_md_type_get_block_size(type);