Question: Who was the first Mafia boss to turn informant ("pentito") and explain the inner workings of the organisation, who told  that Geraci took care of the fugitive Riina while he stayed in Partinico?

Context paragraphs:
The American Black Chamber: The American Black Chamber is a 1931 book by Herbert O. Yardley.  The book describes the inner workings of the interwar American governmental cryptography organization called the Black Chamber.  The cryptography historian David Kahn called the book "the most famous book on cryptology ever published."  By describing the inner workings of the organization, the book created large interest public awareness of the United States's cryptographic abilities.  In particular, the Japanese government became aware of the extent of experience the American government had with cryptography and increased the strength of their own knowledge in cryptography in response.  Reviewers suggested the book may have cost the United States significantly in the Pacific theater against Japan in World War II.

Tommaso Buscetta: Tommaso Buscetta (] ; 13 July 1928 – 2 April 2000) was an Italian gangster, a member of the Sicilian Mafia, who became the first Mafia boss to turn informant ("pentito") and explain the inner workings of the organisation.

Vito Ciancimino: Vito Alfio Ciancimino (April 2, 1924 – November 19, 2002) was an Italian politician close to the Mafia leadership who became notorious for enriching himself and his associates by corruptly granting planning permission.  An abrasive personality, he served briefly as mayor of Palermo, Sicily as a Christian Democrat.  Ciancimino was close to Mafia boss and perennial fugitive Bernardo Provenzano, but regarded Salvatore Riina as irrational.

Nenè Geraci: Antonio Geraci (Partinico, January 2, 1917 – Partinico, February 6, 2007), better known as Nenè or "il vecchio" (the old one), is the historical boss of the Mafia in Partinico, in the province of Palermo.  Geraci sat on the Sicilian Mafia Commission since the mid-1970s and belonged to the hard line faction allied with the Corleonesi of Totò Riina and Bernardo Provenzano.  According to the pentito Tommaso Buscetta, Geraci took care of the fugitive Riina while he stayed in Partinico.

Vito Roberto Palazzolo: Vito Roberto Palazzolo (born July 31, 1947) is an Italian businessman previously living in South Africa.  Born in Terrasini, he moved to South Africa in the mid-1980s.  He also goes by the name Robert von Palace Kolbatschenko.  He is alleged to be a member of the Sicilian Mafia, an allegation he denies.  In March 2009, the highest court in Italy confirmed a 2006 nine-year sentence for association with the Mafia.  He is alleged to be a Mafia treasurer linked to Bernardo Provenzano, the Mafia boss arrested in April 2006 in Corleone, Sicily, and his predecessor Salvatore Riina, both serving life sentences in Italian jails.  Palazzolo maintains his innocence and claims that he has been persecuted by the Italian government, the press, lawyers, politicians, and "opportunists" for the last 19 years.  On March 30, 2012 Palazzolo was arrested by Interpol at Suvarnabhumi Airport in Bangkok, Thailand.

Baldassare Di Maggio: Baldassare Di Maggio (San Giuseppe Jato, November 19, 1954), also known as Balduccio, is a member of the Mafia, who became a government witness (pentito).  He helped the police to capture the head of Cosa Nostra, Totò Riina, and claimed that Riina respectfully kissed seven-time Prime Minister Giulio Andreotti when they met in 1987.

Stephen Flemmi: Stephen Joseph "The Rifleman" Flemmi (born June 9, 1934) is an American gangster and close associate of Winter Hill Gang boss Whitey Bulger.  Beginning in 1975, Flemmi was a top echelon informant for the Federal Bureau of Investigation (FBI).  Despite delivering a great deal of intelligence about the inner workings of the Patriarca crime family, Flemmi's own criminal activities proved a public relations nightmare for the FBI.  For this reason, he was prosecuted under the Racketeer Influenced and Corrupt Organizations Act (RICO) and sentenced to a long term of incarceration.

Larry Grathwohl: Larry David Grathwohl (October 13, 1947 – July 18, 2013) was a United States Army veteran and an FBI informant during the 1970s.  He is best known as the informant who infiltrated the Weather Underground, a radical political organization in the United States.  His exploits were documented in the 1976 book, "Bringing Down America", where he exposed the inner workings of the Weather Underground and the personal activities of many of its members, including Bill Ayers and Bernardine Dohrn.  He died, aged 65, in his Cincinnati apartment on July 18, 2013 of natural causes.

Giovanni Inzerillo: Giovanni Inzerillo (born in Brooklyn, New York, April 30, 1972), is a construction entrepreneur and son of Salvatore Inzerillo, a notorious Sicilian Mafia boss who was killed in May 1981 on the orders of Mafia boss Salvatore "Totò" Riina during the Second Mafia War.

Matteo Messina Denaro: Matteo Messina Denaro (born 26 April 1962), also known as "Diabolik", is a Sicilian Mafia boss.  He got his nickname from the Italian comic book character of the same name.  He is considered to be one of the new leaders of Cosa Nostra after the arrest of Bernardo Provenzano on 11 April 2006, and the most powerful after the arrest of Salvatore Lo Piccolo in November 2007.  Matteo Messina Denaro became known nationally on 12 April 2001 when the magazine "L'Espresso" put him on the cover with the headline: "Ecco il nuovo capo della Mafia" ("Here is the new Mafia boss").  He has been a fugitive since 1993 and according to "Forbes magazine" he is among the ten most wanted criminals in the world.

Supporting paragraphs: Nenè Geraci Tommaso Buscetta
Answer: Tommaso Buscetta