[sw] Change cert template engine to pre-generated method.

The current certificate builder library consumes ~7KB.
Additionally, the immutable ROM extension duplicates the ASN.1/CBOR library,
adding another ~1.5KB.

Given the proximity to the 64KB size limit, this change aims to provide a
more lightweight certificate construction approach.

Change-Id: I11a10ca1c833d238cae67ef30daf9ffcac40e9ae
Signed-off-by: Yi-Hsuan Deng <[email protected]>

rules/certificates.bzl

@@ -117,14 +117,21 @@ def certificate_template(name, template, cert_format = "x509"):
         output_group = "unittest",
     )
 
+    if cert_format == "x509":
+        runtime_deps = [
+            "@//sw/device/silicon_creator/lib/cert:asn1",
+            "@//sw/device/silicon_creator/lib/cert:template",
+        ]
+    else:
+        runtime_deps = [
+            "@//sw/device/silicon_creator/lib/cert:cbor",
+        ]
+
     native.cc_library(
         name = "{}_library".format(name),
         srcs = [":{}_srcs".format(name)],
         hdrs = [":{}_hdrs".format(name)],
-        deps = [
-            "@//sw/device/silicon_creator/lib/cert:asn1",
-            "@//sw/device/silicon_creator/lib/cert:cbor",
-        ],
+        deps = runtime_deps,
     )
 
     native.cc_test(

sw/device/silicon_creator/lib/cert/BUILD

@@ -83,6 +83,17 @@ cc_test(
     ],
 )
 
+cc_library(
+    name = "template",
+    srcs = ["template.c"],
+    hdrs = ["template.h"],
+    deps = [
+        "//sw/device/lib/base:macros",
+        "//sw/device/lib/base:memory",
+        "//sw/device/lib/base:status",
+    ],
+)
+
 cc_library(
     name = "cert",
     srcs = ["cert.c"],

sw/device/silicon_creator/lib/cert/cdi_0.hjson

@@ -10,43 +10,46 @@
         // (x and y) which are 32-bytes integers.
         owner_intermediate_pub_key_ec_x: {
             type: "integer",
-            size: 32,
+            exact-size: 32,
         },
         owner_intermediate_pub_key_ec_y: {
             type: "integer",
-            size: 32,
+            exact-size: 32,
         },
         // Owner intermediate public key ID: this is a 20-byte hash
         // derived from the owner intermediate public key.
         owner_intermediate_pub_key_id: {
             type: "byte-array",
-            size: 20,
+            exact-size: 20,
+            tweak-msb: true,
         },
         // This is the identifier of the public key used to
         // sign this certificate (creator key).
         creator_pub_key_id: {
             type: "byte-array",
-            size: 20,
+            exact-size: 20,
+            tweak-msb: true,
         },
         // Hash of the ROM_EXT (SHA256).
         rom_ext_hash: {
             type: "byte-array",
-            size: 32,
+            exact-size: 32,
         },
         // ROM_EXT security version, used to prevent rollback.
         rom_ext_security_version: {
-            type: "integer",
-            size: 4,
+            type: "byte-array",
+            exact-size: 4,
+            tweak-msb: true,
         }
         // Certificate signature: the result of signing with ECDSA
         // are two integers named "r" and "s"
         cert_signature_r: {
             type: "integer",
-            size: 32,
+            range-size: [24, 32],
         },
         cert_signature_s: {
             type: "integer",
-            size: 32,
+            range-size: [24, 32],
         },
     },
 
@@ -85,7 +88,7 @@
                 type: "dice_tcb_info",
                 vendor: "OpenTitan",
                 model: "ROM_EXT",
-                svn: { var: "rom_ext_security_version" },
+                svn: { var: "rom_ext_security_version", convert: "big-endian" },
                 layer: 1,
                 fw_ids: [
                     { hash_algorithm: "sha256", digest: { var: "rom_ext_hash" } },

sw/device/silicon_creator/lib/cert/cdi_1.hjson

@@ -10,48 +10,51 @@
         // (x and y) which are 32-bytes integers.
         owner_pub_key_ec_x: {
             type: "integer",
-            size: 32,
+            exact-size: 32,
         },
         owner_pub_key_ec_y: {
             type: "integer",
-            size: 32,
+            exact-size: 32,
         },
         // Owner public key ID: this is a 20-byte hash
         // derived from the owner public key.
         owner_pub_key_id: {
             type: "byte-array",
-            size: 20,
+            exact-size: 20,
+            tweak-msb: true,
         },
         // This is the identifier of the public key used to
         // sign this certificate (owner intermediate key).
         owner_intermediate_pub_key_id: {
             type: "byte-array",
-            size: 20,
+            exact-size: 20,
+            tweak-msb: true,
         },
         // Hash of the owner stage firmware (SHA256).
         owner_hash: {
             type: "byte-array",
-            size: 32,
+            exact-size: 32,
         },
         // Hash of the owner manifest (SHA256).
         owner_manifest_hash: {
             type: "byte-array",
-            size: 32,
+            exact-size: 32,
         },
         // Owner security version, used to prevent rollback.
         owner_security_version: {
-            type: "integer",
-            size: 4,
+            type: "byte-array",
+            exact-size: 4,
+            tweak-msb: true,
         }
         // Certificate signature: the result of signing with ECDSA
         // are two integers named "r" and "s"
         cert_signature_r: {
             type: "integer",
-            size: 32,
+            range-size: [24, 32],
         },
         cert_signature_s: {
             type: "integer",
-            size: 32,
+            range-size: [24, 32],
         },
     },
 
@@ -90,7 +93,7 @@
                 type: "dice_tcb_info",
                 vendor: "OpenTitan",
                 model: "Owner",
-                svn: { var: "owner_security_version" },
+                svn: { var: "owner_security_version", convert: "big-endian" },
                 layer: 2,
                 fw_ids: [
                     { hash_algorithm: "sha256", digest: { var: "owner_hash" } },

sw/device/silicon_creator/lib/cert/cert.c

@@ -102,8 +102,10 @@ rom_error_t cert_x509_asn1_check_serial_number(const uint8_t *cert_page_buffer,
          &cert_page_buffer[sn_bytes_offset], asn1_integer_length);
 
   // Check the serial number in the certificate matches what was expected.
+  // The first byte is skipped since the MSb might be modified for achieving
+  // fixed-length certificate.
   *matches = kHardenedBoolFalse;
-  for (size_t i = 0; i < kCertX509Asn1SerialNumberSizeInBytes; ++i) {
+  for (size_t i = 1; i < kCertX509Asn1SerialNumberSizeInBytes; ++i) {
     if (launder32(actual_serial_number[i]) != expected_sn_bytes[i]) {
       HARDENED_CHECK_NE(actual_serial_number[i], expected_sn_bytes[i]);
       return kErrorOk;