WASHINGTON, April 26 -- Signal, an end-to-end encrypted messaging app long considered one of the most secure in the world, has recently faced attacks from hackers accused of links to Russia.

Top German officials on Saturday blamed Moscow-backed groups for phishing attacks targeting senior politicians on the messaging app, raising questions about how secure Signal really is.

Similar phishing cases have been reported by Dutch and American users, with Google in February sounding the alarm over cyberattacks from Russia-aligned groups.

But what makes Signal different from other messaging apps, and how could one of the world's most secure messaging apps be so widely targeted?

How does it work? 

Signal's end-to-end encryption means tha...