Grabbed the encoder module from dev.

Only minor changes to make it IDF friendly.

components/modules/Kconfig

@@ -12,4 +12,11 @@ config LUA_MODULE_FILE
   help
       Includes the file module (recommended).
 
+config LUA_MODULE_ENCODER
+  bool "Encoder module"
+  default "n"
+  help
+      Includes the encoder module. This provides hex and base64 encoding and
+      decoding functionality.
+
 endmenu

components/modules/encoder.c

@@ -0,0 +1,162 @@
+// Module encoders
+
+#include "module.h"
+#include "lauxlib.h"
+#include "lmem.h"
+#include <stdint.h>
+#include <string.h>
+
+#define BASE64_INVALID '\xff'
+#define BASE64_PADDING '='
+#define ISBASE64(c) (unbytes64[c] != BASE64_INVALID)
+
+static const uint8_t b64[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
+
+static uint8_t *toBase64 ( lua_State* L, const uint8_t *msg, size_t *len){
+  size_t i, n = *len;
+
+  if (!n)  // handle empty string case 
+    return NULL;
+
+  uint8_t * q, *out = (uint8_t *)luaM_malloc(L, (n + 2) / 3 * 4);
+  uint8_t bytes64[sizeof(b64)];
+  memcpy(bytes64, b64, sizeof(b64));   //Avoid lots of flash unaligned fetches
+  
+  for (i = 0, q = out; i < n; i += 3) {
+    int a = msg[i];
+    int b = (i + 1 < n) ? msg[i + 1] : 0;
+    int c = (i + 2 < n) ? msg[i + 2] : 0;
+    *q++ = bytes64[a >> 2];
+    *q++ = bytes64[((a & 3) << 4) | (b >> 4)];
+    *q++ = (i + 1 < n) ? bytes64[((b & 15) << 2) | (c >> 6)] : BASE64_PADDING;
+    *q++ = (i + 2 < n) ? bytes64[(c & 63)] : BASE64_PADDING;
+  }
+  *len = q - out;
+  return out;
+}
+
+static uint8_t *fromBase64 ( lua_State* L, const uint8_t *enc_msg, size_t *len){
+  int i, n = *len, blocks = (n>>2), pad = 0;
+  const uint8_t *p;
+  uint8_t unbytes64[UCHAR_MAX+1], *msg, *q;
+
+  if (!n)  // handle empty string case 
+    return NULL;
+ 
+  if (n & 3)
+    luaL_error (L, "Invalid base64 string"); 
+   
+  memset(unbytes64, BASE64_INVALID, sizeof(unbytes64));
+  for (i = 0; i < sizeof(b64)-1; i++) unbytes64[b64[i]] = i;  // sequential so no exceptions 
+  
+  if (enc_msg[n-1] == BASE64_PADDING) {
+    pad =  (enc_msg[n-2] != BASE64_PADDING) ? 1 : 2;
+    blocks--;  //exclude padding block
+  }    
+
+ for (i = 0; i < n - pad; i++) if (!ISBASE64(enc_msg[i])) luaL_error (L, "Invalid base64 string");
+  unbytes64[BASE64_PADDING] = 0;
+
+  msg = q = (uint8_t *) luaM_malloc(L, 1+ (3 * n / 4)); 
+  for (i = 0, p = enc_msg; i<blocks; i++)  {
+    uint8_t a = unbytes64[*p++]; 
+    uint8_t b = unbytes64[*p++]; 
+    uint8_t c = unbytes64[*p++]; 
+    uint8_t d = unbytes64[*p++];
+    *q++ = (a << 2) | (b >> 4);
+    *q++ = (b << 4) | (c >> 2);
+    *q++ = (c << 6) | d;
+  }
+
+  if (pad) { //now process padding block bytes
+    uint8_t a = unbytes64[*p++];
+    uint8_t b = unbytes64[*p++];
+    *q++ = (a << 2) | (b >> 4);
+    if (pad == 1) *q++ = (b << 4) | (unbytes64[*p] >> 2);
+  }
+  *len = q - msg;
+  return msg;
+}
+
+static inline uint8_t to_hex_nibble(uint8_t b) {
+  return b + ( b < 10 ? '0' : 'a' - 10 );
+  }
+    
+static uint8_t *toHex ( lua_State* L, const uint8_t *msg, size_t *len){
+  int i, n = *len;
+  uint8_t *q, *out = (uint8_t *)luaM_malloc(L, n * 2);
+  for (i = 0, q = out; i < n; i++) {
+    *q++ = to_hex_nibble(msg[i] >> 4);
+    *q++ = to_hex_nibble(msg[i] & 0xf);
+  }
+  *len = 2*n; 
+  return out;
+}
+
+static uint8_t *fromHex ( lua_State* L, const uint8_t *msg, size_t *len){
+  int i, n = *len;
+  const uint8_t *p;
+  uint8_t b, *q, *out = (uint8_t *)luaM_malloc(L, n * 2);
+  uint8_t c = 0;
+  
+  if (n &1)
+    luaL_error (L, "Invalid hex string");
+ 
+  for (i = 0, p = msg, q = out; i < n; i++) {
+     if (*p >= '0' && *p <= '9') {
+       b = *p++ - '0';
+     } else if (*p >= 'a' && *p <= 'f') {
+       b = *p++ - ('a' - 10);
+     } else if (*p >= 'A' && *p <= 'F') {
+       b = *p++ - ('A' - 10);
+     } else {
+       luaL_error (L, "Invalid hex string");
+       __builtin_unreachable ();
+     }
+     if ((i&1) == 0) {
+       c = b<<4;
+     } else {
+       *q++ = c+ b;
+     }    
+  }
+  *len = n>>1; 
+  return out;
+}
+
+// All encoder functions are of the form:
+// Lua:  output_string = encoder.function(input_string) 
+// Where input string maybe empty, but not nil
+// Hence these all call the do_func wrapper
+static int do_func (lua_State *L, uint8_t * (*conv_func)(lua_State *, const uint8_t *, size_t *)) {
+  size_t l;
+  const uint8_t *input = (const uint8_t *)luaL_checklstring(L, 1, &l);
+//  luaL_argcheck(L, l>0, 1, "input string empty");
+  uint8_t *output = conv_func(L, input, &l);
+  
+  if (output) {
+    lua_pushlstring(L, (char *)output, l);
+    luaM_free(L, output);
+  } else {
+    lua_pushstring(L, "");
+  }
+  return 1;
+}
+
+#define DECLARE_FUNCTION(f) static int encoder_ ## f (lua_State *L) \
+{ return do_func(L, f); }
+
+  DECLARE_FUNCTION(fromBase64);
+  DECLARE_FUNCTION(toBase64);
+  DECLARE_FUNCTION(fromHex);
+  DECLARE_FUNCTION(toHex);
+
+// Module function map
+static const LUA_REG_TYPE encoder_map[] = {
+  { LSTRKEY("fromBase64"), LFUNCVAL(encoder_fromBase64)  },
+  { LSTRKEY("toBase64"),   LFUNCVAL(encoder_toBase64) },
+  { LSTRKEY("fromHex"),    LFUNCVAL(encoder_fromHex)  },
+  { LSTRKEY("toHex"),      LFUNCVAL(encoder_toHex) },
+  { LNILKEY, LNILVAL }
+};
+
+NODEMCU_MODULE(ENCODER, "encoder", encoder_map, NULL);