Many people seem to be of the opinion that the First Amendment (of the
United States Constitution) grants people the right to anonymity. This
is very much not the case.
The First Amendment does not say that one has a right to speak
anonymously. In fact, a person is often put into a situation where
their identity is compelled, especially if they are related to a case
where a felony has been committed.
The right to privacy is used somewhat interchangeably with anonymity,
but that is not proven in the reading of our Constitution. The right
to privacy comes from the Fourth Amendment, which guarantees a security
within their persons, houses, papers and effects.
There is also the right to remain silent, written into the Fifth
Amendment, which protects against a situation where someone is compelled
to supply information about themselves or their conduct. Metaphorically,
this can be read as an extension of the Fourth Amendment into someone's
thoughts: "a brain cannot be seized and searched, one is secure within
their own mind."
Lastly, there is a right to face one's accuser; the Sixth Amendment
speficially grants the accused all manners of due process. In such a
situation, there is no right to anonymity: a witness must divulge their
identity to make a credible accusation. The United States has the Federal Witness Protection Program, a program that tries
to secure high-profile testimony without endangering the witness, by
helping the witness "disappear" with a new identity after that explicit
testimony is rendered.
A person is always to be held accountable for their own actions
in a court of law; there is no right to being free from accountability.