It\'s my first venture into cryptography, and I\'m struggling with how to use a

Question

It's my first venture into cryptography, and I'm struggling with how to use a nonce in an encryption standard between two digital radios.

The two radios support AES encryption, only requiring a 128-bit key and a nonce for the encryption/decryption of each packet. To my understanding, the key should be the same for either radio, all the time. The nonce however needs to change with every packet sent/received. My question is: how will the two radios agree on a continuously changing nonce for any given transmission/reception? Example: when Radio A picks its nonce to encrypt its message, how should Radio B know which nonce to use to decrypt the message?

The radios may not necessarily be synced in time so that can't be used. Is there any way to do this without using time as the nonce? Does the nonce need to be agreed upon ahead of time in an un-encrypted exchange?

Any guidance on this is appreciated.

Explanation / Answer

The standard approach is to have the sender pick his nonce (either randomly, or as a counter), and send it with the packet. The decryptor then knows what nonce to use to decrypt, because it's right there. Because nonces aren't assumed to be secret, this works.

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