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Skype Encryption (Partly) Revealed

Posted by timothy on from the skyping-ahead dept.

TSHTF writes "Just weeks after Skype unveiled a public API for the service, a group of cryptographers led by Sean O'Neill have successfully reverse engineered the encryption used by the Skype protocol. Source code is available under a non-commercial license which details Skype's implementation of the RC4 cipher." The linked article cautions, however, that "initial analysis suggests that O'Neill's publication does not mean that Skype's encryption can be considered 'cracked'. Further study will be needed to determine whether key expansion and initialisation vector generation are secure."

2 of 151 comments (clear)

  1. Re:implications? by 0123456 · · Score: 5, Informative

    None of this harms Skype's existing security in any way. Encryption, if properly implemented, is secure even when all of the mechanisms are known

    ROT13 isn't secure when it's known.

    Like ROT13, RC4 is an antiquated cipher with many known issues; and a modified version of RC4 could be even less secure than the vanilla implementation. No-one should be using it these days when there are much better alternatives available.

  2. The key scheduling is what's important by bk2204 · · Score: 5, Informative

    The actual RC4 cipher has bad key scheduling issues. Because the initialization step doesn't mix the key bytes well enough into the S-box, the first bytes of the keystream (which is XOR'd with the plaintext to produce the ciphertext) leak lots of data about the key. This is a major problem with WEP (there are, of course, others). Cryptographers recommend discarding the beginning of the keystream because of this weakness. Nevertheless, RC4 is popular because it is byte-oriented and fast. Even 8-bit machines can implement it trivially.

    Ultimately, it comes down to the key scheduling. If Skype has a better key-scheduling algorithm, it may actually improve security over standard RC4.