You get warnings in your log file:
Missing character: There is no <B9> in font lmsy9!
Missing character: There is no <B9> in font lmsy9!
Missing character: There is no <BA> in font lmsy9!
Missing character: There is no <B9> in font lmsy9!
Missing character: There is no <BA> in font lmsy9!
Missing character: There is no <B9> in font lmsy9!
Missing character: There is no <BA> in font lmsy9!
(the actual output may differ in how it displays <B9>
and <BA>
).
The problem is that the acmart
document class loads newtxmath
with the option libertine
, which assigns different mathcodes to (
and )
, precisely "42B9
for )
and 52BA
for )
and this explains the “missing character” warnings.
The standard mathcodes are, respectively, "4028
and "5029
. You might get away with such problems by redefining the mathcodes:
\DeclareMathDelimiter{(}{\mathopen} {operators}{"28}{largesymbols}{"00}
\DeclareMathDelimiter{)}{\mathclose}{operators}{"29}{largesymbols}{"01}
Here's the complete code:
\documentclass[sigconf, anonymous=true]{acmart}
\usepackage{lmodern}
\DeclareMathDelimiter{(}{\mathopen} {operators}{"28}{largesymbols}{"00}
\DeclareMathDelimiter{)}{\mathclose}{operators}{"29}{largesymbols}{"01}
\begin{document}
\title[XYZ]{XYZ}
\maketitle
\begin{equation}
Output = AES_{K}(0),AES_{K}(1),AES_{K}(2),...
\end{equation}
\bibliographystyle{ACM-Reference-Format}
\bibliography{PRNGbib}
\end{document}
Note, however, that it's possible to have other mismatches between the defined mathcodes and those expected by lmodern
. Also some parts of the document may use Libertine nonetheless.

Final advice: the acmart
class has been carefully written to comply with the standards of ACM and affiliate conferences. Changing the appearance of the output by loading different fonts than what's defined in the class should not be done and it could be a good reason for rejection of a submission.
acmart
. – egreg Mar 5 '18 at 17:48acmart
document class loads thelibertine
text font package and thenewtxmath
math font package (with optionlibertine
). You should under no circumstance be loading thelmodern
font package while using theacmart
document class. If, for some reason, you simply must use thelmodern
font package, don't use theacmart
document class. Incidentally, I get no error or warning message when running your code without the instruction\usepackage{lmodern}
. Which TeX distribution do you employ? – Mico Mar 5 '18 at 17:59MiKTeX
. I have enhanced the MWE to include the document title part now and commenting the packagelmodern
gives me this error "pdftex error (font expansion): auto expansion is only possible with scalabale fonts
". – Azuru Mar 6 '18 at 19:37\usepackage{lmodern}
directive) on both a MikteX2.9 and a MacTeX2017 system and am unable to produce any error messages. Sorry. – Mico Mar 6 '18 at 21:48lmodern
package, no output pdf output is produced i.e. 0 pages. Though the compiler doesn't gives an error, the log file do mentions this error "!pdfTeX error: pdflatex.exe (file LinLibertineT-tlf-ot1--base): Font LinLibertineT-tlf-ot1--base at 360 not found ==> Fatal error occurred, no output PDF file produced!
". So as you already suggested this seems to be a case of some components missing from the underlyinglibertine
font package. – Azuru Mar 7 '18 at 0:01