When thy talk about backdoors I don't think they mean the encryption scheme, they mean the proprietary software that uses it. For example Skype has backdoors that allow the NSA to eavesdrop on conversations. They are not breaking encryption, they are circumventing it with the help of Microsoft.
Many, many articles. Here are a few that are first on a search engine.
This is probably common knowledge in government cryptographic circles in various countries and may even be what motivated a Chinese national to create Litecoin shortly after bitcoin.