A method is provided for obtaining a secret value for use as a key in a cryptographic operation, the secret value combining a private key, x, of one computing device with a public key, Y, of another computing device to obtain a secret value xY. The method includes obtaining a pair of scalars x0, x1 such that x=x0+x1 t where t is a scaling factor; obtaining a supplementary public key tY; combining the scalars and the public keys to obtain a representation of the secret value xY as a linear combination of the scalars and the public keys; and utilising the secret value as a key in a cryptographic operation performed by the one computing device.