diff --git a/BTP.markdown b/BTP.markdown
index 10af7ff..5b73526 100644
--- a/BTP.markdown
+++ b/BTP.markdown
@@ -38,10 +38,10 @@ The key derivation function takes an input key *k*, a label *p*, and zero or mor
 
 Each device derives four initial keys from *S*:
 
-* *ack = KDF(*S*, "ALICE_CIPHER_KEY")*
-* *bck = KDF(*S*, "BOB_CIPHER_KEY")*
-* *atk = KDF(*S*, "ALICE_TAG_KEY")*
-* *btk = KDF(*S*, "BOB_TAG_KEY")*
+* *ack = KDF(S, "ALICE_CIPHER_KEY")*
+* *bck = KDF(S, "BOB_CIPHER_KEY")*
+* *atk = KDF(S, "ALICE_TAG_KEY")*
+* *btk = KDF(S, "BOB_TAG_KEY")*
 
 Alice sets *ock = ack*, *ick = bck*, *otk = atk*, and *itk = btk*. Bob sets *ock = bck*, *ick = ack*, *otk = btk*, and *itk = atk*. Thus Alice's outgoing keys (*ock* and *otk*) are the same as Bob's incoming keys (*ick* and *itk*) and vice versa.