Abstract. WG-7 is a stream cipher based on WG Stream Cipher and has been designed by Y. Luo, Q. Chai, G. Gong, and X. Lai in 2010. This cipher is designed for low cost and lightweight applications (RFID tags and mobile phones, for instance). This paper addresses cryptographic weaknesses of WG-7 Stream Cipher. We show that the key stream generated by WG-7 can be distinguished from a random sequence after knowing 2 13.5 keystream bits and with a negligible error probability. Also, we investigate the security of WG-7 against algebraic attacks. An algebraic key recovery attack on this cipher is proposed. The attack allows to recover both the internal state and the secret key with the time complexity about 2 27 .

Abstract. RC4(n, m) is a stream cipher based on RC4 and is designed by G. Gong et al.. It can be seen as a generalization of the famous RC4 stream cipher designed by Ron Rivest. The authors of RC4(n, m) claim that the cipher resists all the attacks that are successful against the original RC4. The paper reveals cryptographic weaknesses of the RC4(n, m) stream cipher. We develop two attacks. The first one is based on non-randomness of internal state and allows to distinguish it from a truly random cipher by an algorithm that has access to 2 4·n bits of the keystream. The second attack exploits low diffusion of bits in the KSA and PRGA algorithms and recovers all bytes of the secret key. This attack works only if the initial value of the cipher can be manipulated. Apart from the secret key, the cipher uses two other inputs, namely, initial value and initial vector. Although these inputs are fixed in the cipher specification, some applications may allow the inputs to be under the attacker control. Assuming that the attacker can control the initial value, we show a distinguisher for the cipher and a secret key recovery attack that for the L-bit secret key, is able to recover it with about (L/n) · 2 n steps. The attack has been implemented on a standard PC and can reconstruct the secret key of RC(8,32) in less than a second.

Abstract. Rakaposhi is a synchronous stream cipher, which uses three main components a non-linear feedback shift register (NLFSR), a dynamic linear feedback shift register (DLFSR) and a non-linear filtering function (N LF ). NLFSR consists of 128 bits and is initialised by the secret key K. DLFSR holds 192 bits and is initialised by an initial vector (IV ). N LF takes 8-bit inputs and returns a single output bit. The work identifies weaknesses and properties of the cipher. The main observation is that the initialisation procedure has the so-called sliding property. The property can be used to launch distinguishing and key recovery attacks. The distinguisher needs four observations of the related (K, IV ) pairs. The key recovery algorithm allows to discover the secret key K after observing 2 9 pairs of (K, IV ). In the proposed relatedkey attack, the number of related (K, IV ) pairs is 2 (128+192)/4 pairs. The key recovery algorithm allows to discover the secret key K after observing 2 9 related (K, IV ) pairs. Further the cipher is studied when the registers enter short cycles. When NLFSR is set to all ones, then the cipher degenerates to a linear feedback shift register with a non-linear filter. Consequently, the initial state (and Secret Key and IV ) can be recovered with complexity 2 63.87 . If DLFSR is set to all zeros, then N LF reduces to a low non-linearity filter function. As the result, the cipher is insecure allowing the adversary to distinguish it from a random cipher after 2 17 observations of keystream bits. There is also the key recovery algorithm that allows to find the secret key with complexity 2 54 .

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