Transmission security is the component of communications security (COMSEC) that results from the application of measures designed to protect transmissions from interception and exploitation by means other than cryptanalysis

Cryptanalysis

Cryptanalysis is the study of methods for obtaining the meaning of encrypted information, without access to the secret information that is normally required to do so. Typically, this involves knowing how the system works and finding a secret key...

. Goals of transmission security include:

• Low probability of interception (LPI
  LPI
  LPI may refer to:Places* Link%C3%B6ping SAAB Airport, Sweden * Linus Pauling Institute at Oregon State University* La Pyramide Inversée, a skylight constructed in an underground shopping mall in front of the Louvre Museum in France...
  
  )
• Low probability of detection (LPD)
• Antijam — resistance to jamming
  Radio jamming
  Radio jamming is the transmission of radio signals that disrupt communications by decreasing the signal to noise ratio. Unintentional jamming occurs when an operator transmits on a busy frequency without first checking whether it is in use, or without being able to hear stations using the frequency...
  
   (EPM or ECCM
  Electronic counter-countermeasures
  Electronic counter-countermeasures is a part of electronic warfare which includes a variety of practices which attempt to reduce or eliminate the effect of electronic countermeasures on electronic sensors aboard vehicles, ships and aircraft and weapons such as missiles. ECCM is also known as...
  
  )

Methods used to achieve transmission security include frequency hopping and spread spectrum

Spread spectrum

Spread-spectrum techniques are methods by which a signal generated in a particular bandwidth is deliberately spread in the frequency domain, resulting in a signal with a wider bandwidth...

 where the required pseudorandom sequence generation is controlled by a cryptographic algorithm and key

Key (cryptography)

In cryptography, a key is a piece of information that determines the functional output of a cryptographic algorithm or cipher. Without a key, the algorithm would produce no useful result. In encryption, a key specifies the particular transformation of plaintext into ciphertext, or vice versa...

. Such keys are known as transmission security keys (TSK). Modern U.S. and NATO TRANSEC-equipped radios include SINCGARS

SINCGARS

SINCGARS is a Combat Net Radio currently used by U.S. and allied military forces. The radios, which handle voice and data communications, are designed to be reliable, secure and easily maintained...

 and HAVE QUICK

HAVE QUICK

HAVE QUICK is a frequency-hopping system used to protect military UHF radio traffic.Since the end of World War II, U.S. and Allied military aircraft have used AM radios in the 225–400 MHz UHF band for short range air-to-air and ground-to-air communications...

.