Remediate SAF-01-004 Crypto: Key Management/Encryption with 1-Byte Key (Critical)

Also, add linter error for FIXME comments in order to prevent shipping unfinished code.

.golangci.yml

@@ -7,5 +7,10 @@ linters:
     - funlen
     - whitespace
     - wsl
-    - godox
 
+linters-settings:
+  godox:
+    # report any comments starting with keywords, this is useful for TODO or FIXME comments that
+    # might be left in the code accidentally and should be resolved before merging
+    keywords:
+      - FIXME

session.go

@@ -314,8 +314,7 @@ func newSession(e *Envelope) (*Session, error) { //nolint:gocognit,gocyclo
 	err := e.LoopSecrets(SignetSchemeKey, func(signet *Signet) error {
 		totalSignetsSeen++
 		keySourceAvailable = true
-		// FIXME
-		return nil
+		return s.calcAndCheckSecurityLevel(nil, signet)
 	})
 	if err != nil {
 		return nil, err