_toBuffer (plainValue) {
    if (plainValue instanceof Buffer) {
      return plainValue
    }

    if (typeof plainValue === 'object') {
      return Buffer.from(JSON.stringify(plainValue), 'utf-8')
    }

    return Buffer.from(`${plainValue}`, 'utf8')
  }
Convert secret value to buffer
@param {(string|Buffer|object)} plainValue - plain secret value
@returns {Buffer} Buffer containing value