The known designs of digital signature schemes with a hidden group, which use finite non-commutative algebras as algebraic support, are based on the computational complexity of the so-called hidden discrete logarithm problem. A similar design, used to develop a signature algorithm based on the difficulty of solving a system of many quadratic equations in many variables, is introduced. The significant advantage of the proposed method compared with multivariate-cryptography signature algorithms is that the said system of equations, which occurs as the result of performing the exponentiation operations in the hidden group, has a random look and is specified in a finite field of a higher order. This provides the ability to develop post-quantum signature schemes with significantly smaller public-key sizes at a given level of security.