diff --git a/toxcore/Messenger.c b/toxcore/Messenger.c
index 07504988..3308b1ed 100644
--- a/toxcore/Messenger.c
+++ b/toxcore/Messenger.c
@@ -186,6 +186,10 @@ int32_t m_addfriend(Messenger *m, uint8_t *address, uint8_t *data, uint16_t leng
 
     uint8_t client_id[crypto_box_PUBLICKEYBYTES];
     id_copy(client_id, address);
+
+    if (!public_key_valid(client_id))
+        return FAERR_BADCHECKSUM;
+
     uint16_t check, checksum = address_checksum(address, FRIEND_ADDRESS_SIZE - sizeof(checksum));
     memcpy(&check, address + crypto_box_PUBLICKEYBYTES + sizeof(uint32_t), sizeof(check));
 
@@ -261,6 +265,9 @@ int32_t m_addfriend_norequest(Messenger *m, const uint8_t *client_id)
     if (getfriend_id(m, client_id) != -1)
         return -1;
 
+    if (!public_key_valid(client_id))
+        return -1;
+
     /* Resize the friend list if necessary. */
     if (realloc_friendlist(m, m->numfriends + 1) != 0)
         return FAERR_NOMEM;
diff --git a/toxcore/crypto_core.c b/toxcore/crypto_core.c
index 87e276e7..1799b600 100644
--- a/toxcore/crypto_core.c
+++ b/toxcore/crypto_core.c
@@ -66,6 +66,20 @@ uint64_t random_64b(void)
     return randnum;
 }
 
+/* Check if a Tox public key crypto_box_PUBLICKEYBYTES is valid or not.
+ * This should only be used for input validation.
+ *
+ * return 0 if it isn't.
+ * return 1 if it is.
+ */
+int public_key_valid(const uint8_t *public_key)
+{
+    if (public_key[31] >= 128) /* Last bit of key is always zero. */
+        return 0;
+
+    return 1;
+}
+
 /* Precomputes the shared key from their public_key and our secret_key.
  * This way we can avoid an expensive elliptic curve scalar multiply for each
  * encrypt/decrypt operation.
diff --git a/toxcore/crypto_core.h b/toxcore/crypto_core.h
index 814c4362..7362d49e 100644
--- a/toxcore/crypto_core.h
+++ b/toxcore/crypto_core.h
@@ -53,6 +53,13 @@ int crypto_cmp(const uint8_t *mem1, const uint8_t *mem2, uint32_t length);
 uint32_t random_int(void);
 uint64_t random_64b(void);
 
+/* Check if a Tox public key crypto_box_PUBLICKEYBYTES is valid or not.
+ * This should only be used for input validation.
+ *
+ * return 0 if it isn't.
+ * return 1 if it is.
+ */
+int public_key_valid(const uint8_t *public_key);
 
 /* Encrypts plain of length length to encrypted of length + 16 using the
  * public key(32 bytes) of the receiver and the secret key of the sender and a 24 byte nonce.