November 2019
Every state that has developed intercontinental ballistic missiles has considered how to deliver a nuclear payload to its target without interference. Generally, states counter ballistic missile defence (BMD) by increasing the number of missiles they produce, developing countermeasures and penetration aides such as decoys or chaff - which confuse BMD sensors, or increasing the number of warheads in each missile's payload.
November 2019
While there is insufficient evidence to prove that North Korea is pursuing countermeasures or penetration aids (penaids), the lack of evidence certainly does not prove that they are not. The technology behind producing penaids - or still more dangerous - multiple independent reentry vehicles (MIRVs) is difficult, but not more difficult than what North Korea has already accomplished.