Trivial fixes

signature/json.go

@@ -50,7 +50,7 @@ func stringField(m map[string]interface{}, fieldName string) (string, error) {
 	}
 	v, ok := untyped.(string)
 	if !ok {
-		return "", jsonFormatError(fmt.Sprintf("Field %s is not a JSON object", fieldName))
+		return "", jsonFormatError(fmt.Sprintf("Field %s is not a string", fieldName))
 	}
 	return v, nil
 }

signature/signature.go

@@ -159,7 +159,7 @@ func (s privateSignature) sign(mech SigningMechanism, keyIdentity string) ([]byt
 // signatureAcceptanceRules specifies how to decide whether an untrusted signature is acceptable.
 // We centralize the actual parsing and data extraction in verifyAndExtractSignature; this supplies
 // the policy.  We use an object instead of supplying func parameters to verifyAndExtractSignature
-// because all of the functions have the same type, so there is a risk of exchanging the functions;
+// because the functions have the same or similar types, so there is a risk of exchanging the functions;
 // named members of this struct are more explicit.
 type signatureAcceptanceRules struct {
 	validateKeyIdentity                func(string) error