diff --git a/src/arith_uint256.cpp b/src/arith_uint256.cpp
index d8a0f7596..b3a305ba9 100644
--- a/src/arith_uint256.cpp
+++ b/src/arith_uint256.cpp
@@ -1,236 +1,239 @@
 // Copyright (c) 2009-2010 Satoshi Nakamoto
 // Copyright (c) 2009-2016 The Bitcoin Core developers
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
 #include <arith_uint256.h>
 
 #include <crypto/common.h>
 #include <uint256.h>
 #include <utilstrencodings.h>
 
 #include <cstdio>
 #include <cstring>
 
 template <unsigned int BITS>
 base_uint<BITS>::base_uint(const std::string &str) {
+    static_assert(BITS / 32 > 0 && BITS % 32 == 0,
+                  "Template parameter BITS must be a positive multiple of 32.");
+
     SetHex(str);
 }
 
 template <unsigned int BITS>
 base_uint<BITS> &base_uint<BITS>::operator<<=(unsigned int shift) {
     base_uint<BITS> a(*this);
     for (int i = 0; i < WIDTH; i++)
         pn[i] = 0;
     int k = shift / 32;
     shift = shift % 32;
     for (int i = 0; i < WIDTH; i++) {
         if (i + k + 1 < WIDTH && shift != 0)
             pn[i + k + 1] |= (a.pn[i] >> (32 - shift));
         if (i + k < WIDTH) pn[i + k] |= (a.pn[i] << shift);
     }
     return *this;
 }
 
 template <unsigned int BITS>
 base_uint<BITS> &base_uint<BITS>::operator>>=(unsigned int shift) {
     base_uint<BITS> a(*this);
     for (int i = 0; i < WIDTH; i++)
         pn[i] = 0;
     int k = shift / 32;
     shift = shift % 32;
     for (int i = 0; i < WIDTH; i++) {
         if (i - k - 1 >= 0 && shift != 0)
             pn[i - k - 1] |= (a.pn[i] << (32 - shift));
         if (i - k >= 0) pn[i - k] |= (a.pn[i] >> shift);
     }
     return *this;
 }
 
 template <unsigned int BITS>
 base_uint<BITS> &base_uint<BITS>::operator*=(uint32_t b32) {
     uint64_t carry = 0;
     for (int i = 0; i < WIDTH; i++) {
         uint64_t n = carry + (uint64_t)b32 * pn[i];
         pn[i] = n & 0xffffffff;
         carry = n >> 32;
     }
     return *this;
 }
 
 template <unsigned int BITS>
 base_uint<BITS> &base_uint<BITS>::operator*=(const base_uint &b) {
     base_uint<BITS> a = *this;
     *this = 0;
     for (int j = 0; j < WIDTH; j++) {
         uint64_t carry = 0;
         for (int i = 0; i + j < WIDTH; i++) {
             uint64_t n = carry + pn[i + j] + (uint64_t)a.pn[j] * b.pn[i];
             pn[i + j] = n & 0xffffffff;
             carry = n >> 32;
         }
     }
     return *this;
 }
 
 template <unsigned int BITS>
 base_uint<BITS> &base_uint<BITS>::operator/=(const base_uint &b) {
     // make a copy, so we can shift.
     base_uint<BITS> div = b;
     // make a copy, so we can subtract.
     base_uint<BITS> num = *this;
     // the quotient.
     *this = 0;
     int num_bits = num.bits();
     int div_bits = div.bits();
     if (div_bits == 0) throw uint_error("Division by zero");
     // the result is certainly 0.
     if (div_bits > num_bits) return *this;
     int shift = num_bits - div_bits;
     // shift so that div and num align.
     div <<= shift;
     while (shift >= 0) {
         if (num >= div) {
             num -= div;
             // set a bit of the result.
             pn[shift / 32] |= (1 << (shift & 31));
         }
         // shift back.
         div >>= 1;
         shift--;
     }
     // num now contains the remainder of the division.
     return *this;
 }
 
 template <unsigned int BITS>
 int base_uint<BITS>::CompareTo(const base_uint<BITS> &b) const {
     for (int i = WIDTH - 1; i >= 0; i--) {
         if (pn[i] < b.pn[i]) return -1;
         if (pn[i] > b.pn[i]) return 1;
     }
     return 0;
 }
 
 template <unsigned int BITS> bool base_uint<BITS>::EqualTo(uint64_t b) const {
     for (int i = WIDTH - 1; i >= 2; i--) {
         if (pn[i]) return false;
     }
     if (pn[1] != (b >> 32)) return false;
     if (pn[0] != (b & 0xfffffffful)) return false;
     return true;
 }
 
 template <unsigned int BITS> double base_uint<BITS>::getdouble() const {
     double ret = 0.0;
     double fact = 1.0;
     for (int i = 0; i < WIDTH; i++) {
         ret += fact * pn[i];
         fact *= 4294967296.0;
     }
     return ret;
 }
 
 template <unsigned int BITS> std::string base_uint<BITS>::GetHex() const {
     return ArithToUint256(*this).GetHex();
 }
 
 template <unsigned int BITS> void base_uint<BITS>::SetHex(const char *psz) {
     *this = UintToArith256(uint256S(psz));
 }
 
 template <unsigned int BITS>
 void base_uint<BITS>::SetHex(const std::string &str) {
     SetHex(str.c_str());
 }
 
 template <unsigned int BITS> std::string base_uint<BITS>::ToString() const {
     return (GetHex());
 }
 
 template <unsigned int BITS> unsigned int base_uint<BITS>::bits() const {
     for (int pos = WIDTH - 1; pos >= 0; pos--) {
         if (pn[pos]) {
             for (int nbits = 31; nbits > 0; nbits--) {
                 if (pn[pos] & 1U << nbits) {
                     return 32 * pos + nbits + 1;
                 }
             }
             return 32 * pos + 1;
         }
     }
     return 0;
 }
 
 // Explicit instantiations for base_uint<256>
 template base_uint<256>::base_uint(const std::string &);
 template base_uint<256> &base_uint<256>::operator<<=(unsigned int);
 template base_uint<256> &base_uint<256>::operator>>=(unsigned int);
 template base_uint<256> &base_uint<256>::operator*=(uint32_t b32);
 template base_uint<256> &base_uint<256>::operator*=(const base_uint<256> &b);
 template base_uint<256> &base_uint<256>::operator/=(const base_uint<256> &b);
 template int base_uint<256>::CompareTo(const base_uint<256> &) const;
 template bool base_uint<256>::EqualTo(uint64_t) const;
 template double base_uint<256>::getdouble() const;
 template std::string base_uint<256>::GetHex() const;
 template std::string base_uint<256>::ToString() const;
 template void base_uint<256>::SetHex(const char *);
 template void base_uint<256>::SetHex(const std::string &);
 template unsigned int base_uint<256>::bits() const;
 
 // This implementation directly uses shifts instead of going through an
 // intermediate MPI representation.
 arith_uint256 &arith_uint256::SetCompact(uint32_t nCompact, bool *pfNegative,
                                          bool *pfOverflow) {
     int nSize = nCompact >> 24;
     uint32_t nWord = nCompact & 0x007fffff;
     if (nSize <= 3) {
         nWord >>= 8 * (3 - nSize);
         *this = nWord;
     } else {
         *this = nWord;
         *this <<= 8 * (nSize - 3);
     }
     if (pfNegative) *pfNegative = nWord != 0 && (nCompact & 0x00800000) != 0;
     if (pfOverflow)
         *pfOverflow =
             nWord != 0 && ((nSize > 34) || (nWord > 0xff && nSize > 33) ||
                            (nWord > 0xffff && nSize > 32));
     return *this;
 }
 
 uint32_t arith_uint256::GetCompact(bool fNegative) const {
     int nSize = (bits() + 7) / 8;
     uint32_t nCompact = 0;
     if (nSize <= 3) {
         nCompact = GetLow64() << 8 * (3 - nSize);
     } else {
         arith_uint256 bn = *this >> 8 * (nSize - 3);
         nCompact = bn.GetLow64();
     }
     // The 0x00800000 bit denotes the sign.
     // Thus, if it is already set, divide the mantissa by 256 and increase the
     // exponent.
     if (nCompact & 0x00800000) {
         nCompact >>= 8;
         nSize++;
     }
     assert((nCompact & ~0x007fffff) == 0);
     assert(nSize < 256);
     nCompact |= nSize << 24;
     nCompact |= (fNegative && (nCompact & 0x007fffff) ? 0x00800000 : 0);
     return nCompact;
 }
 
 uint256 ArithToUint256(const arith_uint256 &a) {
     uint256 b;
     for (int x = 0; x < a.WIDTH; ++x)
         WriteLE32(b.begin() + x * 4, a.pn[x]);
     return b;
 }
 arith_uint256 UintToArith256(const uint256 &a) {
     arith_uint256 b;
     for (int x = 0; x < b.WIDTH; ++x)
         b.pn[x] = ReadLE32(a.begin() + x * 4);
     return b;
 }
diff --git a/src/arith_uint256.h b/src/arith_uint256.h
index de3c34f24..8ef40c9c5 100644
--- a/src/arith_uint256.h
+++ b/src/arith_uint256.h
@@ -1,298 +1,311 @@
 // Copyright (c) 2009-2010 Satoshi Nakamoto
 // Copyright (c) 2009-2016 The Bitcoin Core developers
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
 #ifndef BITCOIN_ARITH_UINT256_H
 #define BITCOIN_ARITH_UINT256_H
 
 #include <cassert>
 #include <cstdint>
 #include <cstring>
+#include <limits>
 #include <stdexcept>
 #include <string>
 #include <vector>
 
 class uint256;
 
 class uint_error : public std::runtime_error {
 public:
     explicit uint_error(const std::string &str) : std::runtime_error(str) {}
 };
 
 /** Template base class for unsigned big integers. */
 template <unsigned int BITS> class base_uint {
 protected:
     static constexpr int WIDTH = BITS / 32;
     uint32_t pn[WIDTH];
 
 public:
     base_uint() {
+        static_assert(
+            BITS / 32 > 0 && BITS % 32 == 0,
+            "Template parameter BITS must be a positive multiple of 32.");
+
         for (int i = 0; i < WIDTH; i++)
             pn[i] = 0;
     }
 
     base_uint(const base_uint &b) {
+        static_assert(
+            BITS / 32 > 0 && BITS % 32 == 0,
+            "Template parameter BITS must be a positive multiple of 32.");
+
         for (int i = 0; i < WIDTH; i++)
             pn[i] = b.pn[i];
     }
 
     base_uint &operator=(const base_uint &b) {
         for (int i = 0; i < WIDTH; i++)
             pn[i] = b.pn[i];
         return *this;
     }
 
     base_uint(uint64_t b) {
+        static_assert(
+            BITS / 32 > 0 && BITS % 32 == 0,
+            "Template parameter BITS must be a positive multiple of 32.");
+
         pn[0] = (unsigned int)b;
         pn[1] = (unsigned int)(b >> 32);
         for (int i = 2; i < WIDTH; i++)
             pn[i] = 0;
     }
 
     explicit base_uint(const std::string &str);
 
     const base_uint operator~() const {
         base_uint ret;
         for (int i = 0; i < WIDTH; i++)
             ret.pn[i] = ~pn[i];
         return ret;
     }
 
     const base_uint operator-() const {
         base_uint ret;
         for (int i = 0; i < WIDTH; i++)
             ret.pn[i] = ~pn[i];
         ++ret;
         return ret;
     }
 
     double getdouble() const;
 
     base_uint &operator=(uint64_t b) {
         pn[0] = (unsigned int)b;
         pn[1] = (unsigned int)(b >> 32);
         for (int i = 2; i < WIDTH; i++)
             pn[i] = 0;
         return *this;
     }
 
     base_uint &operator^=(const base_uint &b) {
         for (int i = 0; i < WIDTH; i++)
             pn[i] ^= b.pn[i];
         return *this;
     }
 
     base_uint &operator&=(const base_uint &b) {
         for (int i = 0; i < WIDTH; i++)
             pn[i] &= b.pn[i];
         return *this;
     }
 
     base_uint &operator|=(const base_uint &b) {
         for (int i = 0; i < WIDTH; i++)
             pn[i] |= b.pn[i];
         return *this;
     }
 
     base_uint &operator^=(uint64_t b) {
         pn[0] ^= (unsigned int)b;
         pn[1] ^= (unsigned int)(b >> 32);
         return *this;
     }
 
     base_uint &operator|=(uint64_t b) {
         pn[0] |= (unsigned int)b;
         pn[1] |= (unsigned int)(b >> 32);
         return *this;
     }
 
     base_uint &operator<<=(unsigned int shift);
     base_uint &operator>>=(unsigned int shift);
 
     base_uint &operator+=(const base_uint &b) {
         uint64_t carry = 0;
         for (int i = 0; i < WIDTH; i++) {
             uint64_t n = carry + pn[i] + b.pn[i];
             pn[i] = n & 0xffffffff;
             carry = n >> 32;
         }
         return *this;
     }
 
     base_uint &operator-=(const base_uint &b) {
         *this += -b;
         return *this;
     }
 
     base_uint &operator+=(uint64_t b64) {
         base_uint b;
         b = b64;
         *this += b;
         return *this;
     }
 
     base_uint &operator-=(uint64_t b64) {
         base_uint b;
         b = b64;
         *this += -b;
         return *this;
     }
 
     base_uint &operator*=(uint32_t b32);
     base_uint &operator*=(const base_uint &b);
     base_uint &operator/=(const base_uint &b);
 
     base_uint &operator++() {
         // prefix operator
         int i = 0;
-        while (++pn[i] == 0 && i < WIDTH - 1)
+        while (i < WIDTH && ++pn[i] == 0)
             i++;
         return *this;
     }
 
     const base_uint operator++(int) {
         // postfix operator
         const base_uint ret = *this;
         ++(*this);
         return ret;
     }
 
     base_uint &operator--() {
         // prefix operator
         int i = 0;
-        while (--pn[i] == (uint32_t)-1 && i < WIDTH - 1)
+        while (i < WIDTH && --pn[i] == std::numeric_limits<uint32_t>::max())
             i++;
         return *this;
     }
 
     const base_uint operator--(int) {
         // postfix operator
         const base_uint ret = *this;
         --(*this);
         return ret;
     }
 
     int CompareTo(const base_uint &b) const;
     bool EqualTo(uint64_t b) const;
 
     friend inline const base_uint operator+(const base_uint &a,
                                             const base_uint &b) {
         return base_uint(a) += b;
     }
     friend inline const base_uint operator-(const base_uint &a,
                                             const base_uint &b) {
         return base_uint(a) -= b;
     }
     friend inline const base_uint operator*(const base_uint &a,
                                             const base_uint &b) {
         return base_uint(a) *= b;
     }
     friend inline const base_uint operator/(const base_uint &a,
                                             const base_uint &b) {
         return base_uint(a) /= b;
     }
     friend inline const base_uint operator|(const base_uint &a,
                                             const base_uint &b) {
         return base_uint(a) |= b;
     }
     friend inline const base_uint operator&(const base_uint &a,
                                             const base_uint &b) {
         return base_uint(a) &= b;
     }
     friend inline const base_uint operator^(const base_uint &a,
                                             const base_uint &b) {
         return base_uint(a) ^= b;
     }
     friend inline const base_uint operator>>(const base_uint &a, int shift) {
         return base_uint(a) >>= shift;
     }
     friend inline const base_uint operator<<(const base_uint &a, int shift) {
         return base_uint(a) <<= shift;
     }
     friend inline const base_uint operator*(const base_uint &a, uint32_t b) {
         return base_uint(a) *= b;
     }
     friend inline bool operator==(const base_uint &a, const base_uint &b) {
         return memcmp(a.pn, b.pn, sizeof(a.pn)) == 0;
     }
     friend inline bool operator!=(const base_uint &a, const base_uint &b) {
         return memcmp(a.pn, b.pn, sizeof(a.pn)) != 0;
     }
     friend inline bool operator>(const base_uint &a, const base_uint &b) {
         return a.CompareTo(b) > 0;
     }
     friend inline bool operator<(const base_uint &a, const base_uint &b) {
         return a.CompareTo(b) < 0;
     }
     friend inline bool operator>=(const base_uint &a, const base_uint &b) {
         return a.CompareTo(b) >= 0;
     }
     friend inline bool operator<=(const base_uint &a, const base_uint &b) {
         return a.CompareTo(b) <= 0;
     }
     friend inline bool operator==(const base_uint &a, uint64_t b) {
         return a.EqualTo(b);
     }
     friend inline bool operator!=(const base_uint &a, uint64_t b) {
         return !a.EqualTo(b);
     }
 
     std::string GetHex() const;
     void SetHex(const char *psz);
     void SetHex(const std::string &str);
     std::string ToString() const;
 
     unsigned int size() const { return sizeof(pn); }
 
     /**
      * Returns the position of the highest bit set plus one, or zero if the
      * value is zero.
      */
     unsigned int bits() const;
 
     uint64_t GetLow64() const {
         static_assert(WIDTH >= 2, "Assertion WIDTH >= 2 failed (WIDTH = BITS / "
                                   "32). BITS is a template parameter.");
         return pn[0] | (uint64_t)pn[1] << 32;
     }
 };
 
 /** 256-bit unsigned big integer. */
 class arith_uint256 : public base_uint<256> {
 public:
     arith_uint256() {}
     arith_uint256(const base_uint<256> &b) : base_uint<256>(b) {}
     arith_uint256(uint64_t b) : base_uint<256>(b) {}
     explicit arith_uint256(const std::string &str) : base_uint<256>(str) {}
 
     /**
      * The "compact" format is a representation of a whole number N using an
      * unsigned 32bit number similar to a floating point format.
      * The most significant 8 bits are the unsigned exponent of base 256.
      * This exponent can be thought of as "number of bytes of N".
      * The lower 23 bits are the mantissa.
      * Bit number 24 (0x800000) represents the sign of N.
      * N = (-1^sign) * mantissa * 256^(exponent-3)
      *
      * Satoshi's original implementation used BN_bn2mpi() and BN_mpi2bn().
      * MPI uses the most significant bit of the first byte as sign.
      * Thus 0x1234560000 is compact (0x05123456)
      * and  0xc0de000000 is compact (0x0600c0de)
      *
      * Bitcoin only uses this "compact" format for encoding difficulty targets,
      * which are unsigned 256bit quantities. Thus, all the complexities of the
      * sign bit and using base 256 are probably an implementation accident.
      */
     arith_uint256 &SetCompact(uint32_t nCompact, bool *pfNegative = nullptr,
                               bool *pfOverflow = nullptr);
     uint32_t GetCompact(bool fNegative = false) const;
 
     friend uint256 ArithToUint256(const arith_uint256 &);
     friend arith_uint256 UintToArith256(const uint256 &);
 };
 
 uint256 ArithToUint256(const arith_uint256 &);
 arith_uint256 UintToArith256(const uint256 &);
 
 #endif // BITCOIN_ARITH_UINT256_H