# Weak Cryptography

Start with version 10.4, qpdf is taking steps to reduce the likelihood of a user accidentally creating PDF files with insecure cryptography but will continue to allow creation of such files indefinitely with explicit acknowledgment.

The PDF file format makes use of RC4, which is known to be a weak cryptography algorithm, and MD5, which is a weak hashing algorithm. In version 10.4, qpdf generates warnings for some (but not all) cases of writing files with weak cryptography when invoked from the command-line. These warnings can be suppressed using the `--allow-weak-crypto` option.

It is planned for qpdf version 11 to be stricter, making it an error to write files with insecure cryptography from the command-line tool in most cases without specifying the `--allow-weak-crypto` flag and also to require explicit steps when using the C++ library to enable use of insecure cryptography.

Note that qpdf must always retain support for weak cryptographic algorithms since this is required for reading older PDF files that use it. Additionally, qpdf will always retain the ability to create files using weak cryptographic algorithms since, as a development tool, qpdf explicitly supports creating older or deprecated types of PDF files since these are sometimes needed to test or work with older versions of software. Even if other cryptography libraries drop support for RC4 or MD5, qpdf can always fall back to its internal implementations of those algorithms, so they are not going to disappear from qpdf.