City-based Zoho Corp’s email and chat services are one of the handful of services, which the U.S. National Security Agency (NSA) has found it difficult to crack under its mass surveillance programme.

According to a report by German newsmagazine Der Spiegel, NSA has classified the encryption and security-breaking problems it encountered on a scale of 1 to 5, from ‘trivial’ to ‘catastrophic.’ Facebook chat, for example, was considered ‘trivial.’ The report was based on the documents obtained from former NSA contractor and whistleblower Edward Snowden.

The NSA had major problem at the fourth level with Zoho, an encrypted email service, the report said.

Encryption — the use of mathematics to protect communications from spying — is used for electronic transactions of all types, by governments, firms and private users alike. This comes as a strong testimonial for Zoho, which competes with Microsoft and Google in the mail and office suite space. The firm has over 10 million users, and mainly focuses on small and medium enterprises in the U.S. and other global markets. Zoho declined to comment on this story.

In an August 2013 blog post, Zoho’s founder Sridhar Vembu noted that his company remained the only major email service provider that never displayed any ads. “In fact, there are no ads inside any of our products. We don’t have an incentive to look inside your data ourselves. While Google has gone on record to say you can’t expect privacy from Google itself, we can assure you that we guarantee your privacy, at least from Zoho itself, if not from the government,” he added in the post.

Der Spiegel report also mentions Tor, the network and software that help users browse the Internet anonymously, which NSA found it difficult to crack.

Although the documents are around two years’ old, experts consider it unlikely the agency's digital spies had made much progress in cracking these technologies, the report adds.