Sets the {@link Document}'s body to an encrypted+signed OpenPGP message containing {@code body}.
@param keySet the {@link UnlockedKeySet} of the {@link User} that owns this{@link Document}
@param random a {@link SecureRandom} instance
@param body the unencrypted document body
@throws CryptographicException if the owner's {@link KeySet} cannot be unlocked with {@code ownerPassphrase}
@see MessageWriter
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