diff --git a/src/InternalDHKeyExchange.cc b/src/InternalDHKeyExchange.cc
index 83058f5e..ccd69415 100644
--- a/src/InternalDHKeyExchange.cc
+++ b/src/InternalDHKeyExchange.cc
@@ -55,7 +55,7 @@ void DHKeyExchange::init(const unsigned char* prime, size_t primeBits,
   if (pr.empty()) {
     throw DL_ABORT_EX("No valid prime supplied");
   }
-  prime_ = n(pr.c_str(), pr.length());
+  prime_ = n(reinterpret_cast<const unsigned char*>(pr.c_str()), pr.length());
 
   std::string gen = reinterpret_cast<const char*>(generator);
   if (gen.length() % 2) {
@@ -65,12 +65,13 @@ void DHKeyExchange::init(const unsigned char* prime, size_t primeBits,
   if (gen.empty()) {
     throw DL_ABORT_EX("No valid generator supplied");
   }
-  generator_ = n(gen.c_str(), gen.length());
+  generator_ =
+      n(reinterpret_cast<const unsigned char*>(gen.c_str()), gen.length());
 
   size_t pbytes = (privateKeyBits + 7) / 8;
   unsigned char buf[pbytes];
   util::generateRandomData(buf, pbytes);
-  privateKey_ = n(reinterpret_cast<char*>(buf), pbytes);
+  privateKey_ = n(buf, pbytes);
 
   keyLength_ = (primeBits + 7) / 8;
 }
@@ -88,7 +89,7 @@ size_t DHKeyExchange::getPublicKey(unsigned char* out, size_t outLength) const
             static_cast<unsigned long>(keyLength_),
             static_cast<unsigned long>(outLength)));
   }
-  publicKey_.binary(reinterpret_cast<char*>(out), outLength);
+  publicKey_.binary(out, outLength);
   return keyLength_;
 }
 
@@ -114,10 +115,9 @@ size_t DHKeyExchange::computeSecret(unsigned char* out, size_t outLength,
             static_cast<unsigned long>(peerPublicKeyLength)));
   }
 
-  n peerKey(reinterpret_cast<const char*>(peerPublicKeyData),
-            peerPublicKeyLength);
+  n peerKey(peerPublicKeyData, peerPublicKeyLength);
   n secret = peerKey.mul_mod(privateKey_, prime_);
-  secret.binary(reinterpret_cast<char*>(out), outLength);
+  secret.binary(out, outLength);
 
   return outLength;
 }
diff --git a/src/bignum.h b/src/bignum.h
index 5fb4402b..94a84a56 100644
--- a/src/bignum.h
+++ b/src/bignum.h
@@ -30,20 +30,20 @@ public:
   typedef std::make_unsigned<char_t>::type uchar_t;
 
 private:
-  std::unique_ptr<char_t[]> buf_;
+  std::unique_ptr<uchar_t[]> buf_;
 
 public:
-  inline ulong() : buf_(aria2::make_unique<char_t[]>(dim)) {}
-  inline ulong(size_t t) : buf_(aria2::make_unique<char_t[]>(dim))
+  inline ulong() : buf_(aria2::make_unique<uchar_t[]>(dim)) {}
+  inline ulong(size_t t) : buf_(aria2::make_unique<uchar_t[]>(dim))
   {
-    memcpy(buf_.get(), (char_t*)&t, sizeof(t));
+    memcpy(buf_.get(), (uchar_t*)&t, sizeof(t));
   }
-  inline ulong(const ulong<dim>& rhs) : buf_(aria2::make_unique<char_t[]>(dim))
+  inline ulong(const ulong<dim>& rhs) : buf_(aria2::make_unique<uchar_t[]>(dim))
   {
     memcpy(buf_.get(), rhs.buf_.get(), dim);
   }
-  explicit inline ulong(const char_t* data, size_t size)
-      : buf_(aria2::make_unique<char_t[]>(dim))
+  explicit inline ulong(const uchar_t* data, size_t size)
+      : buf_(aria2::make_unique<uchar_t[]>(dim))
   {
     if (size > dim) {
       throw std::bad_alloc();
@@ -73,8 +73,8 @@ public:
     const auto b2 = rhs.buf_.get();
     for (ssize_t i = dim - 1; i >= 0; --i) {
       for (ssize_t j = 1; j >= 0; --j) {
-        char_t t = ((uchar_t)(b1[i] << 4 * (1 - j))) >> 4;
-        char_t r = ((uchar_t)(b2[i] << 4 * (1 - j))) >> 4;
+        uchar_t t = ((uchar_t)(b1[i] << 4 * (1 - j))) >> 4;
+        uchar_t r = ((uchar_t)(b2[i] << 4 * (1 - j))) >> 4;
         if (t != r) {
           return t > r;
         }
@@ -101,8 +101,8 @@ public:
     bool base = false;
     for (size_t i = 0; i < dim; ++i) {
       for (ssize_t j = 0; j < 2; ++j) {
-        char_t t = ((uchar_t)(b1[i] << 4 * (1 - j))) >> 4;
-        char_t r = ((uchar_t)(b2[i] << 4 * (1 - j))) >> 4;
+        uchar_t t = ((uchar_t)(b1[i] << 4 * (1 - j))) >> 4;
+        uchar_t r = ((uchar_t)(b2[i] << 4 * (1 - j))) >> 4;
         if (base) {
           t++;
         }
@@ -144,8 +144,8 @@ public:
     bool base = false;
     for (size_t i = 0; i < dim; ++i) {
       for (ssize_t j = 0; j < 2; ++j) {
-        char_t t = ((uchar_t)(b1[i] << 4 * (1 - j))) >> 4;
-        char_t r = ((uchar_t)(b2[i] << 4 * (1 - j))) >> 4;
+        uchar_t t = ((uchar_t)(b1[i] << 4 * (1 - j))) >> 4;
+        uchar_t r = ((uchar_t)(b2[i] << 4 * (1 - j))) >> 4;
         if (base) {
           t--;
         }
@@ -238,14 +238,14 @@ public:
     return rv;
   }
 
-  std::unique_ptr<char_t[]> binary() const
+  std::unique_ptr<uchar_t[]> binary() const
   {
     ulong<dim> c = *this;
-    std::unique_ptr<char_t[]> rv;
+    std::unique_ptr<uchar_t[]> rv;
     rv.swap(c.buf_);
     return rv;
   }
-  void binary(char_t* buf, size_t len) const
+  void binary(uchar_t* buf, size_t len) const
   {
     memcpy(buf, buf_.get(), std::min(dim, len));
   }
@@ -258,8 +258,8 @@ private:
     size_t rv = dim * 2;
     const auto b = buf_.get();
     for (ssize_t i = dim - 1; i >= 0; --i) {
-      char_t f = b[i] >> 4;
-      char_t s = (b[i] << 4) >> 4;
+      uchar_t f = b[i] >> 4;
+      uchar_t s = (b[i] << 4) >> 4;
       if (!f && !s) {
         rv -= 2;
         continue;
@@ -282,8 +282,8 @@ private:
     const size_t d2 = digits / 2;
     for (size_t i = d2; i < dim; ++i) {
       for (size_t j = 0; j < 2; ++j) {
-        char_t c = ((uchar_t)(bt[(dim - 1) - i] << 4 * (1 - j))) >> 4;
-        char_t r = c << (npar * (1 - j) * 4 + (1 - npar) * j * 4);
+        uchar_t c = ((uchar_t)(bt[(dim - 1) - i] << 4 * (1 - j))) >> 4;
+        uchar_t r = c << (npar * (1 - j) * 4 + (1 - npar) * j * 4);
         ssize_t idx = i - d2 - npar * j;
         if (idx >= 0) {
           b[(dim - 1) - idx] += r;