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i am trying to generate the nonce for AES-GCM. I know people usually generate it by Counters and LFSRs. I am confused about the meaning of Counters. Is that means Counter (CTR)? Why it can generate unique nonce?

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  • $\begingroup$ Thank you so much! $\endgroup$ – FRANKfisher Apr 12 '20 at 11:03
  • $\begingroup$ How do you do AES-GCM on linux? Which program? $\endgroup$ – eli Apr 13 '20 at 15:13
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Counters are storing values and have usually clear, set, and increment functions. A counter can be implemented in software very easily by using an integer with enough size or using a library like Java BigInteger or Gnu/GMP for C/C++.

  • clear() makes the all bits zero: unsigned int counter=0
  • set(y) set the counter value to y : counter = y
  • increment() increase the current value by 1; counter++

For GCM one needs 32 bits counter and a total 128-bit for the CTR mode's counter with the 96-bit nonce.

Take a 128-bit unsigned integer and init to zero. Then GCM will use it like $ J_0 = IV || 0^{31} ||1 $ so during the implementation, you need to take a copy of the counter, shift 32 to left, and OR with 0x01.

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