Apply suggestions from code review

Co-authored-by: per1234 <accounts@perglass.com>

Author: umbynos

docs/platform-specification.md

@@ -155,13 +155,17 @@ the name of the architecture is set as well.
 
 There are some other **{build.xxx}** properties available, that are explained in the boards.txt section of this guide.
 
+#### Security credential properties
+
 Some of them allow specifying trusted security credentials (signing and encryption keys) that can be used by a
 ["secure boot" system](guides/secure-boot.md):
 
 - `build.keys.keychain`: for the directory containing the keys
 - `build.keys.sign_key`: for the signing key
 - `build.keys.encrypt_key`: for the encryption key
 
+If any of these properties are defined, the others are required.
+
 These properties can be overwritten respectively with `--keys-keychain`, `--sign-key`, `--encrypt-key`
 [compile](commands/arduino-cli_compile.md) flags in the Arduino CLI.
 

legacy/builder/setup_build_properties.go

@@ -131,8 +131,8 @@ func (s *SetupBuildProperties) Run(ctx *types.Context) error {
 	signProp := buildProperties.ContainsKey("build.keys.sign_key")
 	encryptProp := buildProperties.ContainsKey("build.keys.encrypt_key")
 	// we verify that all the properties for the secure boot keys are defined or none of them is defined.
-	if !(keychainProp || signProp || encryptProp) && (keychainProp && signProp && encryptProp) {
-		return errors.Errorf("%s core does not specify correctly default sign and encryption keys", ctx.BuildCore)
+	if (keychainProp || signProp || encryptProp) && !(keychainProp && signProp && encryptProp) {
+		return errors.Errorf("%s platform does not specify correctly default sign and encryption keys", targetPlatform.Platform)
 	}
 
 	ctx.BuildProperties = buildProperties