encrypting it with the recovered keystream, without knowing the encryption key. Like most ciphers in use for bulk data transfer today, CipherSaber is a symmetric-key cipher. Thus, each pair of communicating users must somehow securely agree on an encryption key, and each user must securely store the encryption keys of those they are to communicate with. Agreeing on encryption keys when the only communications channels available are insecure is the classic chicken-and-egg problem solved by public key cryptography as provided by PGP-like programs. Avoiding the need for secure symmetric key agreements between every pair of users is of considerable convenience and generally improves security. A protocol typically used to achieve good efficiency and convenience is to use a public key cipher such as RSA for key exchange, then a symmetric-key cipher such as CipherSaber for bulk data transfer using the negotiated key. The short key-setup RC4 used in CipherSaber-1 is broken: RC4's original key scheduling is now known to be too weak to protect a large number of ciphertexts encrypted using the same key. CipherSaber-2 modifies CipherSaber-1's key setup procedure by repeating it multiple times in the hope of improving its security (the result is equivalent to using conventional RC4 starting with a key that's been preprocessed by a complex algorithm). While this procedure is believed to close the RC4 key scheduling vulnerability, its effectiveness has not been proven. Like any chosen-key cipher, both versions of CipherSaber are vulnerable to dictionary attack if the chosen key (which would normally be a password or passphrase) does not have sufficient entropy. Symmetric-key cryptography implementations usually include a facility for generating random keys when high security is required. The CipherSaber site recommends generating high entropy random passphrases using Diceware. Like most other cryptosystems (including PGP), CipherSaber makes no provisions at all to prevent attackers from detecting (as opposed to decrypting) the encrypted messages. This is a potentially serious problem in some situations for which CipherSaber was designed: if the government has banned the distribution of cryptographic software, it probably also will want to pounce on anyone who it finds sending encrypted messages. See traffic analysis and steganography for more about these issues and their countermeasures. References Further reading Cryptographic software Encryption debate