Abstract. In this paper, we rst prove an explicit formula which bounds the degree of regularity of the family of HFEv (HFE with vinegar) and HFEv-(HFE with vinegar and minus) multivariate public key cryptosystems over a nite eld of size q. The degree of regularity of the polynomial system derived from an HFEv-system is less than or equal to (q − 1)(r + v + a − 1) 2 + 2 if q is even and r + a is odd,where the parameters v, D, q, and a are parameters of the cryptosystem denoting respectively the number of vinegar variables, the degree of the HFE polynomial, the base eld size, and the number of removed equations, and r is the rank paramter which in the general case is determined by D and q as r = log q (D − 1) + 1. In particular, setting a = 0 gives us the case of HFEv where the degree of regularity is bound by (q − 1)(r + v − 1) 2 + 2 if q is even and r is odd,This formula provides the rst solid theoretical estimate of the complexity of algebraic cryptanalysis of the HFEv-signature scheme, and as a corollary bounds on the complexity of a direct attack against the QUARTZ digital signature scheme. Based on some experimental evidence, we evaluate the complexity of solving QUARTZ directly using F4/F5 or similar Gröbner methods to be around 2 92 .