dersig constexpr evaluation

dersig.cpp

// Compile with: g++ -std=c++11 dersig.cpp -o dersig

#include <cstddef>
#include <vector>
#include <array>

inline constexpr char getHex(const char in)
{
  return in >= '0' && in <= '9'  ? in - '0' :
         in >= 'A' && in <= 'F'  ? in - 'A' + 10 :
         in >= 'a' && in <= 'f'  ? in - 'a' + 10 : -1;
}

template <size_t Length>
inline constexpr unsigned char getElem(const std::array<char, Length>& sig, unsigned int elem)
{
  return static_cast<unsigned char>(getHex(sig.at((elem*2)+2)) << 4 | ( (elem*2)+3 < sig.size() ? getHex(sig.at((elem*2)+3)) : 0));
}

template <size_t Length>
inline constexpr unsigned int lenR(const std::array<char, Length>& sig)
{
  return getElem(sig, 3);
}

template <size_t Length>
inline constexpr unsigned int lenS(const std::array<char, Length>& sig)
{
  return getElem(sig, 5 + lenR(sig));
}

template <size_t Length>
inline constexpr size_t sigSize(const std::array<char, Length>& sig)
{
  return (sig.size() - 1) / 2;
}

template <size_t Length>
inline constexpr bool IsDERSignature(const std::array<char, Length> sig)
{
    // Format: 0x30 [total-length] 0x02 [R-length] [R] 0x02 [S-length] [S] [sighash]
    // * total-length: 1-byte length descriptor of everything that follows,
    //   excluding the sighash byte.
    // * R-length: 1-byte length descriptor of the R value that follows.
    // * R: arbitrary-length big-endian encoded R value. It cannot start with any
    //   null bytes, unless the first byte that follows is 0x80 or higher, in which
    //   case a single null byte is required.
    // * S-length: 1-byte length descriptor of the S value that follows.
    // * S: arbitrary-length big-endian encoded S value. The same rules apply.
    // * sighash: 1-byte value indicating what data is hashed.

    // Accept empty signature as correctly encoded (but invalid) signature,
    // even though it is not strictly DER.
    return (sigSize(sig) == 0) ||
    (
        // Minimum and maximum size constraints.
        sigSize(sig) >= 9 && sigSize(sig) <= 73 &&

        // A signature is of type 0x30 (compound).
        getElem(sig,0) == 0x30 &&

        // Make sure the length covers the entire signature.
        getElem(sig,1) == sigSize(sig) - 3 &&

        // Make sure the length of the S element is still inside the signature.
        lenR(sig) + 5 < sigSize(sig) &&

        // Verify that the length of the signature matches the sum of the length
        // of the elements.
        static_cast<size_t>(lenS(sig) + lenR(sig) + 7) == sigSize(sig) &&

        // Check whether the R element is an integer.
        getElem(sig,2) == 0x02 &&

        // Zero-length integers are not allowed for R.
        lenR(sig) != 0 &&

        // Negative numbers are not allowed for R.
        (getElem(sig,4) & 0x80) == 0 &&

        // Zero bytes at the start of R are not allowed, unless it would
        // otherwise be interpreted as a negative number.
        !(lenR(sig) > 1 && (getElem(sig,4) == 0x00) && !(getElem(sig,5) & 0x80)) &&

        // Check whether the S element is an integer.
        getElem(sig,lenR(sig) + 4) == 0x02 &&

        // Zero-length integers are not allowed for S.
        lenS(sig) != 0 &&

        // Negative numbers are not allowed for S.
        (getElem(sig,lenR(sig) + 6) & 0x80) == 0 &&

        // Null bytes at the start of R are not allowed, unless it would otherwise be
        // interpreted as a negative number.
        !(lenS(sig) > 1 && (getElem(sig,lenR(sig) + 6) == 0x00) && !(getElem(sig,lenR(sig) + 7) & 0x80))

      );
}

template <char... Args>
std::vector<unsigned char> operator "" _der()
{
  constexpr std::array<char, sizeof...(Args)> temp{{Args...}};
  static_assert(IsDERSignature(std::array<char,temp.size()>(temp)), "invalid der");
  std::vector<unsigned char> ret(sigSize(temp));
  for (auto i : ret)
    ret[i] = getElem(temp, i);
  return ret;
}

int main()
{
  typedef std::vector<unsigned char> der_type;
  der_type foo = 0x304402202cb265bf10707bf49346c3515dd3d16fc454618c58ec0a0ff448a676c54ff71302206c6624d762a1fcef4618284ead8f08678ac05b13c84235f1654e6ad168233e8201_der;
}