Collusion Attack on a Server-Aided Unbalanced RSA Key Generation Protocol

To generate the RSA keys efficiently on a low-power handheld device, Modadugu et al. proposed two RSA key generationprotocols. These protocols work with the help of the untrusted servers. Recently, Chen et al. proposed two improved protocols and claimed that their protocols are secure against the collusion attack. The one is a standard RSA key generation protocol and the other is an unbalanced version. This letter point out a weakness in Chen et al.'s unbalanced RSA key generation protocol. If the servers collude with each other they can derive the user's secret prime with high probability that enable the decryption of any ciphertext.