Re: aes/gcm maximum plaintext size; handling of large files

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I will look into ChaCha20.  Thank-you everyone who offered suggestions.

.. Lana ([email protected])




On Wed, Aug 13, 2025 at 6:46=E2=80=AFAM Jeffrey Walton <[email protected]>=
 wrote:

>
>
> On Tue, Aug 12, 2025 at 5:07=E2=80=AFPM Lana Deere <[email protected]>=
 wrote:
>
>> On Tuesday, August 12, 2025 at 8:10:03=E2=80=AFAM UTC-4 Jeffrey Walton w=
rote:
>>
>> GCM plaintext maximum length is specified in bits, not bytes. See
>> SP800-39D, Section 5.2.1.1 Input Data, p. 8, <
>> https://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication800-38=
d.pdf>.
>> That leads to:
>>
>>     2^39 - 256 =3D 549755813632
>>     549755813632 / 8 =3D 68719476704
>>
>>
>> Is there a standard practice for handling AES encryption of large files?
>> E.g., create a new IV and resume encryption?  Use something other than G=
CM
>> which has a higher limit?
>>
>
> The limit you are encountering in GCM mode is due to an internal 32-bit
> counter. The limit is not directly due to AES.
>
> You could use a block cipher and mode of operation with a 64-bit counter,
> or a stream cipher with a 64-bit counter. Bernstein's original ChaCha20 h=
as
> an internal 64-bit counter. So Bernstein's ChaCha20 paired with Poly1305
> could be a good replacement for you. See <
> https://www.cryptopp.com/wiki/ChaCha20>.
>
> The IETF's version of ChaCha20 used in TLS regresses to a 32-bit counter,
> so don't use it.
>
> Jeff
>
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<div dir=3D"ltr"><div>I will look into ChaCha20.=C2=A0 Thank-you everyone w=
ho offered suggestions.</div><div><div dir=3D"ltr" class=3D"gmail_signature=
" data-smartmail=3D"gmail_signature"><br>.. Lana (<a href=3D"mailto:lana.de=
[email protected]" target=3D"_blank">[email protected]</a>)<br><br><br></div=
></div><br></div><br><div class=3D"gmail_quote gmail_quote_container"><div =
dir=3D"ltr" class=3D"gmail_attr">On Wed, Aug 13, 2025 at 6:46=E2=80=AFAM Je=
ffrey Walton &lt;<a href=3D"mailto:[email protected]">[email protected]</=
a>&gt; wrote:<br></div><blockquote class=3D"gmail_quote" style=3D"margin:0p=
x 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><d=
iv dir=3D"ltr"><div dir=3D"ltr"><br></div><br><div class=3D"gmail_quote"><d=
iv dir=3D"ltr" class=3D"gmail_attr">On Tue, Aug 12, 2025 at 5:07=E2=80=AFPM=
 Lana Deere &lt;<a href=3D"mailto:[email protected]" target=3D"_blank">l=
[email protected]</a>&gt; wrote:<br></div><blockquote class=3D"gmail_quot=
e" style=3D"margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204)=
;padding-left:1ex"><div><div dir=3D"auto">On Tuesday, August 12, 2025 at 8:=
10:03=E2=80=AFAM UTC-4 Jeffrey Walton wrote:<br></div><blockquote style=3D"=
margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-lef=
t:1ex"><div dir=3D"ltr"><div><div>GCM plaintext maximum length is specified=
 in bits, not bytes. See SP800-39D, Section 5.2.1.1 Input Data, p. 8, &lt;<=
a href=3D"https://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublicatio=
n800-38d.pdf" rel=3D"nofollow" target=3D"_blank">https://nvlpubs.nist.gov/n=
istpubs/Legacy/SP/nistspecialpublication800-38d.pdf</a>&gt;. That leads to:=
</div><div><br></div><div>=C2=A0 =C2=A0 2^39 - 256 =3D=C2=A0<span>549755813=
632</span></div><div><span>=C2=A0 =C2=A0 549755813632 / 8 =3D=C2=A0</span><=
span>68719476704</span></div></div></div></blockquote><div><br></div><div>I=
s there a standard practice for handling AES encryption of large files?=C2=
=A0 E.g., create a new IV and resume encryption?=C2=A0 Use something other =
than GCM which has a higher limit?</div></div></blockquote><div><br></div><=
div>The limit you are encountering in GCM mode is due to an internal 32-bit=
 counter. The limit is not directly due to AES.</div><div><br></div><div>Yo=
u could use a block cipher and mode of operation with a 64-bit counter, or =
a stream cipher with a 64-bit counter. Bernstein&#39;s original ChaCha20 ha=
s an internal 64-bit counter. So Bernstein&#39;s ChaCha20 paired with Poly1=
305 could be a good replacement for you. See &lt;<a href=3D"https://www.cry=
ptopp.com/wiki/ChaCha20" target=3D"_blank">https://www.cryptopp.com/wiki/Ch=
aCha20</a>&gt;.</div><div><br></div><div>The IETF&#39;s version of ChaCha20=
 used in TLS regresses to a 32-bit counter, so don&#39;t use it.</div><div>=
<br></div><div>Jeff</div></div></div>

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RnUtqPecxTW_k-%3DmBNaXLt8Mq1A%40mail.gmail.com</a>.<br>
</blockquote></div>

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