The Guardian and The New York Times, which obtained the documents, reported that the NSA successfully got NIST to adopt its version of a security standard in 2006.
That standard included vulnerabilities that NSA hackers could later exploit to spy on private communications, the papers reported.
NIST is not a regulatory agency — it only helps private groups agree on voluntary standards and guidelines. If outside groups stop trusting the NIST, it could undermine the agency's usefulness.
"NIST would not deliberately weaken a cryptographic standard," the agency said in the statement. "We will continue in our mission to work with the cryptographic community to create the strongest possible encryption standards for the U.S. government and industry at large."
But NIST acknowledged that it is legally required to consult with the NSA and that the NSA helps develop security codes "because of its recognized expertise."