Solidity wrapper for Ethereum Byzantium's BigInt `modexp` built-in contract 0x5

ModExp.sol

pragma solidity ^0.4.17;

contract ModExp {

    // Wrapper for built-in bigint_modexp (contract 0x5) as described here https://github.com/ethereum/EIPs/pull/198
    function modexp(bytes memory _base, bytes memory _exp, bytes memory _mod) public view returns(bytes memory ret) {
    
        uint256 bl = _base.length;
        uint256 el = _exp.length;
        uint256 ml = _mod.length;
        assembly {
            // Free memory pointer is always stored at 0x40
            let freemem := mload(0x40)
            
            // arg[0] = base.length @ +0
            mstore(freemem, bl)
            
            // arg[1] = exp.length @ +32
            mstore(add(freemem,32), el)
            
            // arg[2] = mod.length @ +64
            mstore(add(freemem,64), ml)
            
            // arg[3] = base.bits @ + 96
            // Use identity built-in (contract 0x4) as a cheap memcpy
            let success := call(450, 0x4, 0, add(_base,32), bl, add(freemem,96), bl)
            
            // arg[4] = exp.bits @ +96+base.length
            let size := add(96, bl)
            success := call(450, 0x4, 0, add(_exp,32), el, add(freemem,size), el)
            
            // arg[5] = mod.bits @ +96+base.length+exp.length
            size := add(size,el)
            success := call(450, 0x4, 0, add(_mod,32), ml, add(freemem,size), ml)
            
            // Total size of input = 96+base.length+exp.length+mod.length
            size := add(size,ml)
            // Invoke contract 0x5, put return value right after mod.length, @ +96
            success := call(sub(gas, 1350), 0x5, 0, freemem, size, add(96,freemem), ml)
            
            // Return the location of the return value (length, bits)
            ret := add(64,freemem)
        }
    }
  
}

@adolfommoyano Sorry, I haven't looked into Solidity for years, and am not really interested in doing so ever again. I suggest you join the Gitter channel https://gitter.im/ethereum/solidity/