Modify keying in vault

We currently derive the dkey from the source key at every open or close
(decrypt or encrypt) operation. However, we want to keep the time that
the internal data is exposed (decrypted) as short as possible. While the
vault is open, there's no problem keeping a copy of the dkey around
(because the data is decrypted anyways, therefore it isn't important).
So we change things around and, at the expense of doubling the time that
decryption takes, we make encryption extremely fast. We do this by
computing the next (rekeyed) key at the start of the decryption routine
(but before the data has been decrypted) and keep the dkey stored in the
vault structure for direct access on the next encryption run.

vault.c

@@ -41,6 +41,14 @@ static bool vault_derive_key(const struct vault_t *vault, uint8_t dkey[static 32
 	return true;
 }
 
+static bool vault_rekey(struct vault_t *vault) {
+	/* Generate a new source key  */
+	if (RAND_bytes(vault->source_key, vault->source_key_length) != 1) {
+		return false;
+	}
+	return vault_derive_key(vault, vault->dkey);
+}
+
 static double vault_measure_key_derivation_time(struct vault_t *vault, unsigned int new_iteration_count) {
 	uint8_t dkey[32];
 	double t0, t1;
@@ -67,7 +75,7 @@ static void vault_calibrate_derivation_time(struct vault_t *vault, double target
 	}
 }
 
-struct vault_t* vault_init(unsigned int data_length, double target_derivation_time) {
+struct vault_t* vault_init(unsigned int data_length, double target_decryption_time) {
 	struct vault_t *vault;
 
 	vault = calloc(1, sizeof(struct vault_t));
@@ -94,7 +102,18 @@ struct vault_t* vault_init(unsigned int data_length, double target_derivation_ti
 	}
 	vault->reference_count = 1;
 	vault->data_length = data_length;
-	vault_calibrate_derivation_time(vault, target_derivation_time);
+
+	/* Decryption takes *two* derivations, one for the current key (to decrypt)
+	 * and another in advance after re-keying, therefore we halve the time
+	 * here. */
+	vault_calibrate_derivation_time(vault, target_decryption_time / 2);
+
+	/* Initially gernerate a full key and derive the dkey already (vault is
+	 * open at this point) */
+	if (!vault_rekey(vault)) {
+		vault_free(vault);
+		return NULL;
+	}
 
 	return vault;
 }
@@ -109,8 +128,18 @@ static void vault_destroy_content(struct vault_t *vault) {
 }
 
 static bool vault_decrypt(struct vault_t *vault) {
+	/* At this point we only have the source key, not the dkey yet. Derive the
+	 * dkey into a local piece of memory first */
 	uint8_t dkey[32];
 	if (!vault_derive_key(vault, dkey)) {
+		OPENSSL_cleanse(dkey, sizeof(dkey));
+		return false;
+	}
+
+	/* Then rekey the vault for the upcoming closing. Do this while the vault
+	 * is still encrypted to minimize window of opportunity. */
+	if (!vault_rekey(vault)) {
+		OPENSSL_cleanse(dkey, sizeof(dkey));
 		return false;
 	}
 
@@ -151,19 +180,16 @@ static bool vault_decrypt(struct vault_t *vault) {
 			success = false;
 			break;
 		}
-
 	} while (false);
 
-	if (success) {
-		OPENSSL_cleanse(vault->source_key, vault->source_key_length);
-		OPENSSL_cleanse(vault->auth_tag, 16);
-	} else {
+	if (!success) {
 		/* Vault may be in an inconsistent state. Destroy contents. */
 		vault_destroy_content(vault);
 	}
 
-	EVP_CIPHER_CTX_free(ctx);
 	OPENSSL_cleanse(dkey, sizeof(dkey));
+	OPENSSL_cleanse(vault->auth_tag, 16);
+	EVP_CIPHER_CTX_free(ctx);
 	return success;
 }
 
@@ -180,16 +206,7 @@ bool vault_open(struct vault_t *vault) {
 }
 
 static bool vault_encrypt(struct vault_t *vault) {
-	/* Generate a new key source */
-	if (RAND_bytes(vault->source_key, vault->source_key_length) != 1) {
-		return false;
-	}
-
-	uint8_t key[32];
-	if (!vault_derive_key(vault, key)) {
-		return false;
-	}
-
+	/* We already have a dkey in the structure, so we can quickly encrypt */
 	EVP_CIPHER_CTX *ctx = EVP_CIPHER_CTX_new();
 	if (!ctx) {
 		return false;
@@ -211,7 +228,7 @@ static bool vault_encrypt(struct vault_t *vault) {
 			break;
 		}
 
-		if (EVP_EncryptInit_ex(ctx, NULL, NULL, key, (unsigned char*)&vault->iv) != 1) {
+		if (EVP_EncryptInit_ex(ctx, NULL, NULL, vault->dkey, (unsigned char*)&vault->iv) != 1) {
 			success = false;
 			break;
 		}
@@ -233,13 +250,16 @@ static bool vault_encrypt(struct vault_t *vault) {
 		}
 	} while (false);
 
+	/* The data is encrypted, erase the dkey, but keep the source key (so we
+	 * can decrypt later) */
+	OPENSSL_cleanse(vault->dkey, sizeof(vault->dkey));
+
 	if (!success) {
 		/* Vault may be in an inconsistent state. Destroy contents. */
 		vault_destroy_content(vault);
 	}
 
 	EVP_CIPHER_CTX_free(ctx);
-	OPENSSL_cleanse(key, sizeof(key));
 	return success;
 }
 
@@ -279,7 +299,7 @@ static void dump(const uint8_t *data, unsigned int length) {
 int main(void) {
 	/* gcc -D__TEST_VAULT__ -Wall -std=c11 -Wmissing-prototypes -Wstrict-prototypes -Werror=implicit-function-declaration -Wimplicit-fallthrough -Wshadow -pie -fPIE -fsanitize=address -fsanitize=undefined -fsanitize=leak -pthread -o vault vault.c util.c log.c -lcrypto
 	 */
-	struct vault_t *vault = vault_init(64, 0.1);
+	struct vault_t *vault = vault_init(64, 1);
 	dump(vault->data, vault->data_length);
 	for (int i = 0; i < 10; i++) {
 		if (!vault_close(vault)) {
@@ -287,6 +307,7 @@ int main(void) {
 			abort();
 		}
 		dump(vault->data, vault->data_length);
+
 		if (!vault_open(vault)) {
 			fprintf(stderr, "vault open failed.\n");
 			abort();

vault.h

@@ -36,6 +36,7 @@ struct vault_t {
 	uint8_t *source_key;
 	unsigned int source_key_length;
 	uint8_t auth_tag[16];
+	uint8_t dkey[32];
 	uint64_t iv;
 	unsigned int iteration_cnt;
 };