cryptenroll: allow to use a public key on a token

This patch allows systemd-cryptenroll to enroll directly with a public key if a certificate is missing on a token.

Fixes: #30675

src/basic/memory-util.c

@@ -39,3 +39,19 @@ bool memeqbyte(uint8_t byte, const void *data, size_t length) {
         /* Now we know first 16 bytes match, memcmp() with self.  */
         return memcmp(data, p + 16, length) == 0;
 }
+
+void *memdup_reverse(const void *mem, size_t size) {
+        assert(mem);
+        assert(size != 0);
+
+        void *p = malloc(size);
+        if (!p)
+                return NULL;
+
+        uint8_t *p_dst = p;
+        const uint8_t *p_src = mem;
+        for (size_t i = 0, k = size; i < size; i++, k--)
+                p_dst[i] = p_src[k-1];
+
+        return p;
+}

src/basic/memory-util.h

@@ -107,3 +107,6 @@ static inline void erase_and_freep(void *p) {
 static inline void erase_char(char *p) {
         explicit_bzero_safe(p, sizeof(char));
 }
+
+/* Makes a copy of the buffer with reversed order of bytes */
+void *memdup_reverse(const void *mem, size_t size);