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In cryptography, a security parameter is a way of measuring of how "hard" it is for an adversary to break a cryptographic scheme. There are two main types of security parameter: computational and statistical, often denoted by κ {\displaystyle \kappa } and λ {\displaystyle \lambda } , respectively. Roughly speaking, the computational security parameter is a measure for the input size of the computational problem on which the cryptographic scheme is based, which determines its computational complexity, whereas the statistical security parameter is a measure of the probability with which an adversary can break the scheme.
Security parameters are usually expressed in unary representation - i.e. κ {\displaystyle \kappa } is expressed as a string of κ {\displaystyle \kappa } 1 {\displaystyle 1} s, κ = 1 ⋯ 1 {\displaystyle \kappa =1\cdots 1} , conventionally written as 1 κ {\displaystyle 1^{\kappa }} - so that the time complexity of the cryptographic algorithm is polynomial in the size of the input.