With the help of the eavesdropping, Der Spiegel reports, Israeli leaders were better able to understand Kerry's plans for his efforts during the negotiations and prepare for them. At times, the material collected even served to help Israel attack the US diplomat.
The author of the profile is Holger Stark, one of the magazine's senior writers and Germany's "Journalist of the Year" in 2013. Stark exposed the American eavesdropping of German Chancellor Angela Merkel's cell phone – which led to a severe crisis in the relations between the two allies.
Stark is the current Washington, D.C. bureau chief for Der Spiegel.
According to the profile which will be published in Monday's paper, at the height of US-led nine-month period of negotiations Kerry was making daily phone calls to his colleagues in the Middle East.
When he was in the United States, most of the calls were placed from his office or – as would happen often due to the time difference – from his home in Georgetown. Both locations have secure phone lines.
But during Kerry's numerous flights his plane was not equipped with an encrypted phone line and the US diplomat made calls using a regular satellite phone which Israeli intelligence – according to Der Spiegel – managed to intercept.
According to the report, Israel was one of two countries which eavesdropped on the phone calls but the profile of Kerry does not name the second country.
Der Spiegel notes that the transcripts of the conversations were not kept within the Israeli intelligence community and were passed on to the political establishment, which employed them for its own purposes. Israel and the US refused to respond to Der Spiegel on the matter.