src/drivers/sw_crypto: Late initialize tomcypt

This saves a lot of flash space, in case functions from libtomcrypt
are not used (currently only RSA related).

When RSA is not used, the linker can now drop all libtomcrypt related things.
This is especially relevant for bootloaders using the SW crypto.

Signed-off-by: Jukka Laitinen <jukkax@ssrc.tii.ae>

src/drivers/sw_crypto/crypto.c

@@ -64,6 +64,12 @@ extern void libtomcrypt_init(void);
  */
 static int crypto_open_count = 0;
 
+/*
+ * Status of libtomcrypt initialization. This is a large library, which
+ * is initialized & pulled in by linker only when it is actually used
+ */
+static bool tomcrypt_initialized = false;
+
 typedef struct {
 	size_t key_size;
 	uint8_t *key;
@@ -76,6 +82,14 @@ typedef struct {
 	uint64_t ctr;
 } chacha20_context_t;
 
+static inline void initialize_tomcrypt(void)
+{
+	if (!tomcrypt_initialized) {
+		libtomcrypt_init();
+		tomcrypt_initialized = true;
+	}
+}
+
 /* Clear key cache */
 static void clear_key_cache(void)
 {
@@ -135,7 +149,6 @@ void crypto_init()
 {
 	keystore_init();
 	clear_key_cache();
-	libtomcrypt_init();
 }
 
 crypto_session_handle_t crypto_open(px4_crypto_algorithm_t algorithm)
@@ -269,6 +282,8 @@ bool crypto_encrypt_data(crypto_session_handle_t handle,
 			uint8_t *public_key = (uint8_t *)crypto_get_key_ptr(handle.keystore_handle, key_idx, &key_sz);
 			*cipher_size = 0;
 
+			initialize_tomcrypt();
+
 			if (public_key &&
 			    rsa_import(public_key, key_sz, &key) == CRYPT_OK) {
 				if (outlen >= ltc_mp.unsigned_size(key.N) &&
@@ -413,6 +428,8 @@ size_t crypto_get_min_blocksize(crypto_session_handle_t handle, uint8_t key_idx)
 			size_t pub_key_sz;
 			uint8_t *pub_key = (uint8_t *)crypto_get_key_ptr(handle.keystore_handle, key_idx, &pub_key_sz);
 
+			initialize_tomcrypt();
+
 			if (pub_key &&
 			    rsa_import(pub_key, pub_key_sz, &enc_key) == CRYPT_OK) {
 				ret = ltc_mp.unsigned_size(enc_key.N);