Libgcrypt CTR Mode Counter Value

[sudoer]

Hello All,

I'm using libgcrypt-1.4.4 and wanted to know if the counter source for the
CTR mode is internal or external. As far as I see in the code, the counter
has to be set using the gcry_cipher_setctr() function. So, it sounds like
"external" counter source, correct?

But the CAVP or FIPS algorithm certificate for Libgcrypt's AES mentions that
its CTR mode uses internal counter. Can someone please explain how?

Also, if the counter is indeed sourced internally, how does Libgcrypt ensure
that the counter values are always unique as specified in NIST SP800-38a
standard?

If someone can provide some code pointers for this, it would be great.

I appreciate your time and help.

Thanks.
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