diff --git a/files/secret_message.txt b/files/secret_message.txt
index c216c17..15c7636 100644
--- a/files/secret_message.txt
+++ b/files/secret_message.txt
@@ -9,7 +9,7 @@ Consider a toy "QR-encryption" scheme where a message is encoded into
 a QR code and then randomized by permuting its tiles with a secret seed.
 
 (a) Under ideal assumptions, explain why this scheme could satisfy
-Kerckhoffs’libzbar0 principle.  
+Kerckhoffs' principle.  
 (b) Now assume an attacker can test permutations against the QR format
 (find patterns, error correction, alignment markers). Why does this
 reduce the effective security of the scheme?