aes: always require full inputs

I.e. no 31 digit for 128-bit blocks, reuse code, etc. Also, generalize
hex string parsing.

1 files changed, 42 insertions, 0 deletions

diff --git a/aes/src/hex.c b/aes/src/hex.c
new file mode 100644
index 0000000..62b6e5a
--- /dev/null
+++ b/aes/src/hex.c
@@ -0,0 +1,42 @@
+/*
+ * Copyright (c) 2026 Egor Tensin <Egor.Tensin@gmail.com>
+ * This file is part of the "AES tools" project.
+ * For details, see https://github.com/egor-tensin/aes-tools.
+ * Distributed under the MIT License.
+ */
+
+#include <aes/all.h>
+
+#include <stdio.h>
+#include <stdlib.h>
+
+AES_StatusCode aes_parse_hex_string(
+    unsigned char* dest,
+    const char* src,
+    size_t numof_bytes,
+    AES_ErrorDetails* err_details
+) {
+    if (dest == NULL)
+        return aes_error_null_argument(err_details, "dest");
+    if (src == NULL)
+        return aes_error_null_argument(err_details, "src");
+
+    char what[30];
+    snprintf(what, sizeof(what), "a %zu-byte hex string", numof_bytes);
+
+    const char* cursor = src;
+
+    for (size_t i = 0; i < numof_bytes; ++i) {
+        int n;
+        unsigned int byte;
+        if (sscanf(cursor, "%2x%n", &byte, &n) != 1 || n != 2)
+            return aes_error_parse(err_details, src, what);
+        dest[i] = (unsigned char)byte;
+        cursor += n;
+    }
+
+    if (*cursor != '\0')
+        return aes_error_parse(err_details, src, what);
+
+    return AES_SUCCESS;
+}