function ShortCurve(conf) {
  Base.call(this, 'short', conf);

  this.a = new BN(conf.a, 16).toRed(this.red);
  this.b = new BN(conf.b, 16).toRed(this.red);
  this.tinv = this.two.redInvm();

  this.zeroA = this.a.fromRed().cmpn(0) === 0;
  this.threeA = this.a.fromRed().sub(this.p).cmpn(-3) === 0;

  // If the curve is endomorphic, precalculate beta and lambda
  this.endo = this._getEndomorphism(conf);
  this._endoWnafT1 = new Array(4);
  this._endoWnafT2 = new Array(4);
}
Is the string a hex string.

@method check if string is hex string of specific length
@param {String} value
@param {Number} length
@returns {Boolean} output the string is a hex string