Apple and Google are appealing to U.S. President Barack Obama to reject proposals to allow encryption "back doors" in mobile devices.A letter signed by Apple and Google to be sent Tuesday is aimed at protecting privacy and limiting law enforcement access to encrypted data, according to a report in The Washington Post."Strong encryption is the cornerstone of the modern information economy's security," the paper quotes the letter as saying.Over 140 technology companies, technologists and civil society groups also signed it, calling on the president to not "in any way subvert, undermine, weaken or make vulnerable" security software.Apple and Google did not immediately reply to a request for more information about the report.The signatories include three people on a five-member panel appointed by Obama to review government surveillance programs following the leaks by former contractor Edward Snowden, the paper said.Kevin Bankston, policy director at the New America Foundation's Open Technology Institute, organized the letter, according to the article, which he posted on Twitter.The development follows a request from the U.S. Federal Bureau of Investigation to the U.S. Congress to mandate encryption workarounds in smartphones as a tool to prevent serious crimes. The appeal has drawn criticism from lawmakers such as U.S. Representative Ted Lieu, a Democrat from California, who called the idea "technologically stupid."Michael Daniel, the White House's cybersecurity coordinator, said in March that there is "no scenario" in which the U.S. government wants weaker encryption.But the president and administration officials including FBI director James Comey have raised concerns about encrypted data that is beyond the reach of law enforcement.Last year, Apple and Google added more data protection to mobile devices, with passcode-based encryption for iPhones and iPads running iOS 8 and default data encryption in a new version of Android."If you have a back door, somebody will find it, and that somebody may be a bad guy," Vinton Cerf, Google's chief Internet evangelist and the co-creator of TCP\/IP, said in a speech earlier this month. "Creating this kind of technology is super, super risky."Tim Hornyak covers Japan and emerging technologies for The IDG News Service. Follow Tim on Twitter at @robotopia.