feat(crypto): add ARC4 implementation

storm/Crypto.cpp

@@ -0,0 +1,57 @@
+#include "storm/Crypto.hpp"
+#include "storm/Error.hpp"
+#include <cstring>
+
+void SARC4PrepareKey(const void* data, uint32_t len, SARC4Key* key) {
+    STORM_ASSERT(data);
+    STORM_VALIDATE(data, ERROR_INVALID_PARAMETER);
+
+    key->x = 0;
+    key->y = 0;
+
+    // ARC4 key-scheduling algorithm
+
+    for (uint32_t i = 0; i < 256; i++) {
+        key->state[i] = i;
+    }
+
+    uint32_t j = 0;
+    for (uint32_t i = 0; i < 256; i++) {
+        j = (j + key->state[i] + static_cast<const uint8_t*>(data)[i % len]) % 256;
+
+        // Swap values
+        auto si = key->state[i];
+        auto sj = key->state[j];
+        key->state[i] = sj;
+        key->state[j] = si;
+    }
+}
+
+void SARC4ProcessBuffer(void* data, uint32_t len, const SARC4Key* inKey, SARC4Key* outKey) {
+    if (inKey != outKey) {
+        memcpy(outKey, inKey, sizeof(SARC4Key));
+    }
+
+    // Note: The original implementation uses two loops. The first loop decrypts/encrypts data
+    // 4-bytes-at-a-time. The second loop decrypts/encrypts all bytes outside of that alignment
+    // 1-byte-at-a-time. For simplicity's sake, our implementation handles 1-byte-at-a-time.
+
+    auto x = outKey->x;
+    auto y = outKey->y;
+
+    for (uint32_t i = 0; i < len; i++) {
+        x += 1;
+        auto sx = outKey->state[x];
+
+        y += sx;
+        auto sy = outKey->state[y];
+
+        outKey->state[x] = sy;
+        outKey->state[y] = sx;
+
+        static_cast<uint8_t*>(data)[i] ^= outKey->state[static_cast<uint8_t>(sx + sy)];
+    }
+
+    outKey->x = x;
+    outKey->y = y;
+}