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https://en.wikipedia.org/wiki/Emergency_Action_Message

EAMs use cryptographic protocols (including such methods as digital signatures) to authenticate the messages,[1] thereby ensuring that they cannot be forged or altered.

In the United States, the EAM will be issued from the National Military Command Center (NMCC) at the Pentagon or, if it has been destroyed by an enemy first strike, by the Alternate National Military Command Center - Site R at Raven Rock, Pennsylvania or by the Boeing E-4B National Airborne Operations Center (NAOC).

The messages are sent in digital format to nuclear-capable major commands. The messages are then relayed to aircraft that are on alert by the U.S. Strategic Command at Offutt Air Force Base in Omaha, Nebraska, via single-sideband modulation radio transmitters of the High Frequency Global Communications System (formerly known as the Global High Frequency Service). The EAM is relayed to missile-firing nuclear submarines via special transmitters designed for communication with submarines. The transmitters include those designed to operate at Very Low Frequency (VLF). The submarines pick up the message via special antennas.

Nuclear-capable forces will then be expected to carry out an EAM without fail. Manned bombers may be recalled, but missiles fired from land-based silos or from submarines cannot be recalled.

Skyking messages are also read on the same network as EAMs, also known as “Foxtrot Broadcasts”. These messages will interrupt an EAM if needed to be read. They contain a higher priority and time-sensitive code for orders that need immediate attention.[1]