EAP-pwd: Fix zero-padding of input to H()

Another niceness of OpenSSL is that if the high-order bit of a 521-bit
big num is not set then BN_bn2bin() will just return 65 bytes instead of
66 bytes with the 1st (big endian, after all) being all zero. When this
happens the wrong number of octets are mixed into function H(). So
there's a whole bunch of "offset" computations and BN_bn2bin() dumps the
big number into a buffer + offset. That should be obvious in the patch
too.

1 files changed, 33 insertions, 14 deletions

diff --git a/src/eap_peer/eap_pwd.c b/src/eap_peer/eap_pwd.c
index e4705b7..6511a66 100644
--- a/src/eap_peer/eap_pwd.c
+++ b/src/eap_peer/eap_pwd.c
@@ -465,6 +465,7 @@ eap_pwd_perform_confirm_exchange(struct eap_sm *sm, struct eap_pwd_data *data,
 	u32 cs;
 	u16 grp;
 	u8 conf[SHA256_DIGEST_LENGTH], *cruft = NULL, *ptr;
+	int offset;
 
 	/*
 	 * first build up the ciphersuite which is group | random_function |
@@ -497,7 +498,8 @@ eap_pwd_perform_confirm_exchange(struct eap_sm *sm, struct eap_pwd_data *data,
 	 * value may start with a few zeros and the previous one did not.
 	 */
 	os_memset(cruft, 0, BN_num_bytes(data->grp->prime));
-	BN_bn2bin(data->k, cruft);
+	offset = BN_num_bytes(data->grp->prime) - BN_num_bytes(data->k);
+	BN_bn2bin(data->k, cruft + offset);
 	H_Update(&ctx, cruft, BN_num_bytes(data->grp->prime));
 
 	/* server element: x, y */
@@ -509,15 +511,19 @@ eap_pwd_perform_confirm_exchange(struct eap_sm *sm, struct eap_pwd_data *data,
 		goto fin;
 	}
 	os_memset(cruft, 0, BN_num_bytes(data->grp->prime));
-	BN_bn2bin(x, cruft);
+	offset = BN_num_bytes(data->grp->prime) - BN_num_bytes(x);
+	BN_bn2bin(x, cruft + offset);
 	H_Update(&ctx, cruft, BN_num_bytes(data->grp->prime));
 	os_memset(cruft, 0, BN_num_bytes(data->grp->prime));
-	BN_bn2bin(y, cruft);
+	offset = BN_num_bytes(data->grp->prime) - BN_num_bytes(y);
+	BN_bn2bin(y, cruft + offset);
 	H_Update(&ctx, cruft, BN_num_bytes(data->grp->prime));
 
 	/* server scalar */
 	os_memset(cruft, 0, BN_num_bytes(data->grp->prime));
-	BN_bn2bin(data->server_scalar, cruft);
+	offset = BN_num_bytes(data->grp->order) -
+		BN_num_bytes(data->server_scalar);
+	BN_bn2bin(data->server_scalar, cruft + offset);
 	H_Update(&ctx, cruft, BN_num_bytes(data->grp->order));
 
 	/* my element: x, y */
@@ -530,15 +536,19 @@ eap_pwd_perform_confirm_exchange(struct eap_sm *sm, struct eap_pwd_data *data,
 	}
 
 	os_memset(cruft, 0, BN_num_bytes(data->grp->prime));
-	BN_bn2bin(x, cruft);
+	offset = BN_num_bytes(data->grp->prime) - BN_num_bytes(x);
+	BN_bn2bin(x, cruft + offset);
 	H_Update(&ctx, cruft, BN_num_bytes(data->grp->prime));
 	os_memset(cruft, 0, BN_num_bytes(data->grp->prime));
-	BN_bn2bin(y, cruft);
+	offset = BN_num_bytes(data->grp->prime) - BN_num_bytes(y);
+	BN_bn2bin(y, cruft + offset);
 	H_Update(&ctx, cruft, BN_num_bytes(data->grp->prime));
 
 	/* my scalar */
 	os_memset(cruft, 0, BN_num_bytes(data->grp->prime));
-	BN_bn2bin(data->my_scalar, cruft);
+	offset = BN_num_bytes(data->grp->order) -
+		BN_num_bytes(data->my_scalar);
+	BN_bn2bin(data->my_scalar, cruft + offset);
 	H_Update(&ctx, cruft, BN_num_bytes(data->grp->order));
 
 	/* the ciphersuite */
@@ -564,7 +574,8 @@ eap_pwd_perform_confirm_exchange(struct eap_sm *sm, struct eap_pwd_data *data,
 
 	/* k */
 	os_memset(cruft, 0, BN_num_bytes(data->grp->prime));
-	BN_bn2bin(data->k, cruft);
+	offset = BN_num_bytes(data->grp->prime) - BN_num_bytes(data->k);
+	BN_bn2bin(data->k, cruft + offset);
 	H_Update(&ctx, cruft, BN_num_bytes(data->grp->prime));
 
 	/* my element */
@@ -576,15 +587,19 @@ eap_pwd_perform_confirm_exchange(struct eap_sm *sm, struct eap_pwd_data *data,
 		goto fin;
 	}
 	os_memset(cruft, 0, BN_num_bytes(data->grp->prime));
-	BN_bn2bin(x, cruft);
+	offset = BN_num_bytes(data->grp->prime) - BN_num_bytes(x);
+	BN_bn2bin(x, cruft + offset);
 	H_Update(&ctx, cruft, BN_num_bytes(data->grp->prime));
 	os_memset(cruft, 0, BN_num_bytes(data->grp->prime));
-	BN_bn2bin(y, cruft);
+	offset = BN_num_bytes(data->grp->prime) - BN_num_bytes(y);
+	BN_bn2bin(y, cruft + offset);
 	H_Update(&ctx, cruft, BN_num_bytes(data->grp->prime));
 
 	/* my scalar */
 	os_memset(cruft, 0, BN_num_bytes(data->grp->prime));
-	BN_bn2bin(data->my_scalar, cruft);
+	offset = BN_num_bytes(data->grp->order) -
+		BN_num_bytes(data->my_scalar);
+	BN_bn2bin(data->my_scalar, cruft + offset);
 	H_Update(&ctx, cruft, BN_num_bytes(data->grp->order));
 
 	/* server element: x, y */
@@ -596,15 +611,19 @@ eap_pwd_perform_confirm_exchange(struct eap_sm *sm, struct eap_pwd_data *data,
 		goto fin;
 	}
 	os_memset(cruft, 0, BN_num_bytes(data->grp->prime));
-	BN_bn2bin(x, cruft);
+	offset = BN_num_bytes(data->grp->prime) - BN_num_bytes(x);
+	BN_bn2bin(x, cruft + offset);
 	H_Update(&ctx, cruft, BN_num_bytes(data->grp->prime));
 	os_memset(cruft, 0, BN_num_bytes(data->grp->prime));
-	BN_bn2bin(y, cruft);
+	offset = BN_num_bytes(data->grp->prime) - BN_num_bytes(y);
+	BN_bn2bin(y, cruft + offset);
 	H_Update(&ctx, cruft, BN_num_bytes(data->grp->prime));
 
 	/* server scalar */
 	os_memset(cruft, 0, BN_num_bytes(data->grp->prime));
-	BN_bn2bin(data->server_scalar, cruft);
+	offset = BN_num_bytes(data->grp->order) -
+		BN_num_bytes(data->server_scalar);
+	BN_bn2bin(data->server_scalar, cruft + offset);
 	H_Update(&ctx, cruft, BN_num_bytes(data->grp->order));
 
 	/* the ciphersuite */