On long walks or simply sitting in the sun, Peter Schweitzer let his mind roam through the world of math. As a cryptographer, he worked with companies on encryption and breaking codes, matters beyond the grasp of most of those he encountered.
“Although I couldn’t understand the intricacies of encryption, I could hold his hand, so to speak, when he was working on something difficult,’’ said Eleanor Perrone, his longtime partner. “Quite often he would come back from a walk and say, ‘I’ve broken it.’ That was a good thing, I soon learned. Next morning, he would say, ‘I was mistaken.’ I learned to wait for a few cycles of this before accepting the final pronouncement of, ‘I’ve really broken it.’ ’’
Mr. Schweitzer, who also was a well-regarded freelance photographer, died of a stroke Oct. 13 in Mount Auburn Hospital in Cambridge. He was 80 and lived in Watertown.
Described by family, friends, and colleagues as something of a modern renaissance man, he was knowledgeable about an array of subjects, from music to science, animal rights to politics.
“It’s a huge loss,’’ said longtime friend Liz Thomas of Peterborough, N.H. “Never mind the friendship or the love you feel for people; to lose that much information is terrible.’’
A specialist at breaking complex codes, Mr. Schweitzer was often called upon to test their strength, said H.F. Mattson Jr., a friend and a professor emeritus of electrical engineering and computer science at Syracuse University in New York.
Two years ago, Mr. Schweitzer began working as chief cryptanalyst for Revere Security. He had been an independent security consultant for much of his life. Early in his career, he worked at the Air Force Cambridge Research Center under the late Horst Feistel, a renowned cryptographer. Mr. Schweitzer was part of the team that developed the foundations of modern symmetric encryption for security systems.
He also was a member of Information Security Systems Inc., an invitation-only consulting group founded by Jim Frazer, who formerly worked for the National Security Agency and helped create the field of modern cryptographic evaluation.
“I always found Peter’s views to be refreshingly different than those of others, which is a good thing in our business,’’ Bob McNelis, a colleague with Information Security Systems, wrote in an e-mail. “They were curiously insightful and reflected a deep understanding and breadth of knowledge that he had in the work in which we were engaged. Peter was a gem, indeed a gentleman and scholar.’’
Born a US citizen in Berlin, Mr. Schweitzer lived in Paris during his early childhood. He used to tell family that he made it out of Europe in 1939, practically on the last boat, as World War II engulfed the continent.
As a child, Mr. Schweitzer became interested in music the day he heard Beethoven’s Fifth Symphony on an old record player.
“In some odd way, that initial exposure and reaction to music established his . . . profound connection to it,’’ Perrone said.
Mr. Schweitzer graduated from The Cambridge School of Weston in 1947 and was 16 when he entered Harvard College. Graduating in 1951 with a bachelor’s degree in mathematics, he stayed at Harvard and received a master’s in math in 1953.
At Harvard, he often would sneak into Paine Hall in the middle of the night to practice piano and fell just shy of also graduating with a bachelor’s in music. The only course he missed was orchestration.
Mr. Schweitzer studied math at MIT and the University of Zurich, taking courses in doctoral programs at each school.
In 1952, he married Susanna Reichenbach, who died in 1994. They raised three daughters in Cambridge.
Their daughter Jana of Portland, Ore., said he was an interested and involved father who attended her piano recitals and even played duets with her in a high school concert.
In his spare time, Mr. Schweitzer was a freelance photographer. Specializing in the arts, he was known for capturing performances live, without posing or changes in lighting. His photographs appeared in publications such as the Globe, The New York Times, Dance Magazine, and Opera News.
He met Perrone when a friend referred her to him for publicity photos. After making the recommendation, Perrone recalled, the friend paused, and cautioned, “He’s kind of artistic.’’
“Isn’t that a good thing in a photographer?’’ she responded.
“Yeah, but he wears all black and wears a beret,’’ her friend said.
Perrone said, “It took me years to get him to try occasionally wearing a color.’’
Perrone and Mr. Schweitzer were longtime partners, and though they never married, they referred to each other as wife and husband.
“We spent much time together, since we both worked from home, and would offer each other support and advice with work-related issues, as well as all the personal issues,’’ she said.
They hiked together and looked after their seven cats. Four had been scheduled to be euthanized, but were rescued at the last minute by Mr. Schweitzer, a longtime animal lover.
Mr. Schweitzer’s daughter said he crusaded for restrictions on sales of wild animals as pets.
Though described as unpretentious by family and friends, Mr. Schweitzer valued quality in everything. He was so fond of good sharp knives that he brought them with him to restaurants and to friends’ houses for dinner. To cook a perfect piece of seared meat, he timed the recipe to the second, using the best possible wristwatch. He told friends he was dismayed by the rise of digital photography, which he found inferior to film.
In addition to Perrone and his daughter Jana, Mr. Schweitzer leaves two other daughters, Alexandra of Lexington and Taryn Gillis of Billerica; and four grandchildren.
His family will announce a memorial concert in the spring.
When solving complex problems for work, Mr. Schweitzer usually mulled the equations in his head first, often while basking in the sun or going for walks up nearby Belmont Hill.
“He sat outside with his eyes shut,’’ said Thomas, a friend since their days at the Cambridge School. “It looked like someone taking a nap, but he was just doing higher math in his head.’’
Said Jana: “He would literally lie around sunbathing, and meanwhile his mind would be going a zillion miles an hour.’’
Correction: Because of a reporting error, an earlier version of this obituary on Peter Schweitzer, a code breaker and photographer, provided an incorrect residence for Liz Thomas. She lives in Peterborough, N.H. Also, the caption of a photo by Mr. Schweitzer misidentified the dance company. Two performers from Concert Dance Company were featured in the photo.