refactored sodium_crypto_secretbox() into separate source file.

src/libexim-encrypt-dlfunc.c

@@ -0,0 +1,69 @@
+#include <stdlib.h>
+#include <string.h>
+#include <strings.h>
+#include <sodium.h>
+
+/* Local encryption key */
+#include "recipient_pk.h"
+
+/* Exim4 dlfunc API header */
+#include <local_scan.h>
+
+/*
+ * Encrypt the second argument with password from the first argument
+ */
+int sodium_crypto_secretbox(uschar ** yield, int argc, uschar * argv[])
+{
+	if (sodium_init() == -1) {
+		*yield = string_copy(US "Unable to initialize libsodium");
+		return ERROR;
+	}
+	if (argc != 2) {
+		*yield =
+		    string_sprintf
+		    ("Wrong number of arguments (got %i, expected 2)", argc);
+		return ERROR;
+	}
+	// get password
+	unsigned char *password = argv[0];
+	size_t passwordlen = strlen((const char *)password);
+
+	// get cleartext message
+	unsigned char *message = argv[1];
+	size_t messagelen = strlen((const char *)message);
+
+	/*
+	 * Derive a key from the password.
+	 * This operations needs to be fast (exim holds no state, this might be called once or mutliple times per email).
+	 * Collisions avoidance or brute force arracks are not a concern here.
+	 */
+	unsigned char keybytes[crypto_secretbox_KEYBYTES];
+	sodium_memzero(keybytes, crypto_secretbox_KEYBYTES);
+	crypto_generichash(keybytes, crypto_secretbox_KEYBYTES,
+			   password, passwordlen, NULL, 0);
+
+	// prepare buffer for ciphertext
+	unsigned int cipherlen = messagelen + crypto_secretbox_MACBYTES;
+	unsigned char *ciphertext = malloc(cipherlen);
+	sodium_memzero(ciphertext, cipherlen);
+
+	// encrypt message
+	unsigned char nonce[crypto_secretbox_NONCEBYTES];
+	randombytes_buf(nonce, sizeof nonce);
+	crypto_secretbox_easy(ciphertext, message, messagelen, nonce, keybytes);
+
+	// base64-encode the ciphertext
+	unsigned int outputsize = sodium_base64_ENCODED_LEN(cipherlen,
+							    sodium_base64_VARIANT_ORIGINAL);
+	unsigned char *outstring = malloc(outputsize);
+	sodium_memzero(outstring, outputsize);
+	sodium_bin2base64((char *const)outstring, outputsize,
+			  ciphertext, cipherlen,
+			  sodium_base64_VARIANT_ORIGINAL);
+	free(ciphertext);
+
+	// return base64-encoded ciphertext
+	*yield = string_copy(outstring);
+	free(outstring);
+	return OK;
+}