diff --git a/bn_mp_fread.c b/bn_mp_fread.c
index d0de595..85e5ee3 100644
--- a/bn_mp_fread.c
+++ b/bn_mp_fread.c
@@ -20,6 +20,7 @@
 int mp_fread(mp_int *a, int radix, FILE *stream)
 {
    int err, ch, neg, y;
+   unsigned pos;
 
    /* clear a */
    mp_zero(a);
@@ -34,13 +35,14 @@ int mp_fread(mp_int *a, int radix, FILE *stream)
    }
 
    for (;;) {
-      /* find y in the radix map */
-      for (y = 0; y < radix; y++) {
-         if (mp_s_rmap[y] == ch) {
-            break;
-         }
+      pos = ch - '(';
+      if (mp_s_rmap_reverse_sz < pos) {
+         break;
       }
-      if (y == radix) {
+
+      y = mp_s_rmap_reverse[pos];
+
+      if (y == 0xff || y >= radix) {
          break;
       }
 
diff --git a/bn_mp_radix_smap.c b/bn_mp_radix_smap.c
index 4c6e57c..262775c 100644
--- a/bn_mp_radix_smap.c
+++ b/bn_mp_radix_smap.c
@@ -17,6 +17,20 @@
 
 /* chars used in radix conversions */
 const char *mp_s_rmap = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz+/";
+const uint8_t mp_s_rmap_reverse[] = {
+      0xff, 0xff, 0xff, 0x3e, 0xff, 0xff, 0xff, 0x3f, /* ()*+,-./ */
+      0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, /* 01234567 */
+      0x08, 0x09, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, /* 89:;<=>? */
+      0xff, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f, 0x10, /* @ABCDEFG */
+      0x11, 0x12, 0x13, 0x14, 0x15, 0x16, 0x17, 0x18, /* HIJKLMNO */
+      0x19, 0x1a, 0x1b, 0x1c, 0x1d, 0x1e, 0x1f, 0x20, /* PQRSTUVW */
+      0x21, 0x22, 0x23, 0xff, 0xff, 0xff, 0xff, 0xff, /* XYZ[\]^_ */
+      0xff, 0x24, 0x25, 0x26, 0x27, 0x28, 0x29, 0x2a, /* `abcdefg */
+      0x2b, 0x2c, 0x2d, 0x2e, 0x2f, 0x30, 0x31, 0x32, /* hijklmno */
+      0x33, 0x34, 0x35, 0x36, 0x37, 0x38, 0x39, 0x3a, /* pqrstuvw */
+      0x3b, 0x3c, 0x3d, 0xff, 0xff, 0xff, 0xff, 0xff, /* xyz{|}~. */
+};
+const size_t mp_s_rmap_reverse_sz = sizeof(mp_s_rmap_reverse);
 #endif
 
 /* ref:         $Format:%D$ */
diff --git a/bn_mp_read_radix.c b/bn_mp_read_radix.c
index bc31cc5..c82f85d 100644
--- a/bn_mp_read_radix.c
+++ b/bn_mp_read_radix.c
@@ -19,6 +19,7 @@
 int mp_read_radix(mp_int *a, const char *str, int radix)
 {
    int     y, res, neg;
+   unsigned pos;
    char    ch;
 
    /* zero the digit bignum */
@@ -49,26 +50,25 @@ int mp_read_radix(mp_int *a, const char *str, int radix)
        * [e.g. in hex]
        */
       ch = (radix <= 36) ? (char)toupper((int)*str) : *str;
-      for (y = 0; y < 64; y++) {
-         if (ch == mp_s_rmap[y]) {
-            break;
-         }
+      pos = ch - '(';
+      if (mp_s_rmap_reverse_sz < pos) {
+         break;
       }
+      y = mp_s_rmap_reverse[pos];
 
       /* if the char was found in the map
        * and is less than the given radix add it
        * to the number, otherwise exit the loop.
        */
-      if (y < radix) {
-         if ((res = mp_mul_d(a, (mp_digit)radix, a)) != MP_OKAY) {
-            return res;
-         }
-         if ((res = mp_add_d(a, (mp_digit)y, a)) != MP_OKAY) {
-            return res;
-         }
-      } else {
+      if (y == 0xff || y >= radix) {
          break;
       }
+      if ((res = mp_mul_d(a, (mp_digit)radix, a)) != MP_OKAY) {
+         return res;
+      }
+      if ((res = mp_add_d(a, (mp_digit)y, a)) != MP_OKAY) {
+         return res;
+      }
       ++str;
    }
 
diff --git a/tommath_private.h b/tommath_private.h
index 087ddcd..7e47f18 100644
--- a/tommath_private.h
+++ b/tommath_private.h
@@ -76,6 +76,8 @@ int s_mp_exptmod(const mp_int *G, const mp_int *X, const mp_int *P, mp_int *Y, i
 void bn_reverse(unsigned char *s, int len);
 
 extern const char *mp_s_rmap;
+extern const uint8_t mp_s_rmap_reverse[];
+extern const size_t mp_s_rmap_reverse_sz;
 
 /* Fancy macro to set an MPI from another type.
  * There are several things assumed: