I'm a little bit confused. I'm using AES in CTR mode. Do i need an mechanism to send the nonce with the cypertext? Or expressed differently: does every participant has to know the current nonce to encrypt/decrypt the message?
Normally the nonce would be 0 as in the NIST documentation:
In the recommended usage scenario, the party encrypting maintains an integer counter, nonce , initially 0, and produces the string ctr as the 128-bit string which encodes the number nonce 2 64 . (In other words, nonce is regarded as a 64-bit binary number, and ctr is constructed by appending to this number 64 zero-bits.) The number nonce is incremented following each encryption. Typically, one transmits C along with a string which encodes nonce.
If you choose to use an other nonce you would have to send this to the decrypter as well. More can be found in this NIST document.