“We cannot allow anyone to evade sanctions, because the consequences of North Korea obtaining funding, technology, and information to further its desire to build nuclear weapons put the world at risk,” said William F. Sweeney Jr. , an assistant director-in-charge at the Federal Bureau of Investigation. “It’s even more egregious that a U.S. citizen allegedly chose to aid our adversary.”

A lawyer for Mr. Griffith did not respond to requests for comment on Friday.

In a statement, the Ethereum Foundation said Mr. Griffith acted on his own. “We can confirm that the foundation was not represented in any capacity at the events outlined in the Justice Department’s filing, and that the foundation neither approved nor supported any such travel, which was a personal matter,” the group said.

Hacker magazine, 2600, where Mr. Griffith was a contributing writer, issued a statement on Twitter on Friday saying that his arrest was “an attack on all of us.”

The magazine’s editor, who uses the pen name Emmanuel Goldstein, said on Twitter that what Mr. Griffith had done — explaining the concept of cryptocurrency — was not a crime. In a phone interview on Friday, Mr. Goldstein said he had socialized with Mr. Griffith the night before he met with the F.B.I., and Mr. Griffith had insisted on telling federal authorities “the truth” without a lawyer.

“I kept warning him it was a trap,” Mr. Goldstein wrote on Twitter.

Mr. Goldstein said Mr. Griffith was incapable of doing what federal investigators have accused him of. “He would not help a murderous dictator,” he said. “He’s a typical hacker who loves technology and adventure.”

The United States and the United Nations Security Council have imposed tight sanctions on North Korea to try to force the country to curb its nuclear and ballistic missile programs.

Last year, Washington amended its sanctions to prohibit “a U.S. person, wherever located” from giving technology to North Korea. Prosecutors are charging Mr. Griffith’s presentation at the conference amounted to a transfer of technology.