Expanding Weak PRF with Small Key Size
We propose modes for weakly-secure block ciphers that take one block input to provide output of arbitrary length. Damgård and Nielsen proposed such a mode called the Pseudorandom Tree (PRT) mode, and demonstrated that PRT could be used to establish a communication channel that is secure against Chosen-Plaintext Attacks, if the underlying block cipher is secure against any Known-Plaintext Attacks. We present a mode that reduces the key size of PRT to about 60% without any additional computation. We call this the Extended PRT (ERT) mode and prove its security. One drawback of PRT and ERT is that their key sizes are not much small under small expansion, since functions with small expansion are important from practical point of view. We also present a mode that greatly reduces the key size under small expansion.
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