Glossary / RC4 Encryption
What Is RC4 Encryption in PDFs?
RC4 is a stream cipher that was historically used to encrypt PDF documents. It was introduced in PDF 1.1 with 40-bit keys and later supported 128-bit keys in PDF 1.4. RC4 has known cryptographic vulnerabilities and is considered deprecated in favor of AES encryption per ISO 32000-2.
How RC4 works in PDFs
RC4 generates a pseudorandom keystream from a password-derived key and XORs it with the document data. Each PDF object is encrypted individually using a key derived from the document encryption key and the object number. The simplicity of RC4 made it fast to implement, which is why early PDF versions adopted it.
Security concerns
RC4 has several known weaknesses: statistical biases in the keystream, vulnerability to related-key attacks, and insufficient key length in the 40-bit variant (which can be brute-forced in minutes on modern hardware). Security researchers and standards bodies, including NIST, recommend against using RC4 for any new applications.
Related tools on fixtdoc
- Protect PDF , encrypt PDFs using modern AES encryption
- Unlock PDF , remove encryption from legacy RC4-protected files