I was just reading RFC 2440 and it points out that when creating a digital signature for text (such as in email), the eoln sequences are normalized (treated as CR LF no matter now they are actually stored) and trailing whitespace on a line is ignored when computing the signature.

Now, think about how Bleach converts a program to a "blank" file. It uses spaces and tabs to encode bits.

This kind of space/tab trailing could be used to attach a covert message to email or other digitally signed text, without invalidating the signature.

—John

In reply to Acme::Bleach and covert messages by John M. Dlugosz

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