September 29, 1997


Cylink Corporation
910 Hermosa Court
Sunnyvale, CA 94086

Gentlemen:

         At your request,  we have examined the  Registration  Statement on Form
S-8 executed by you on September 29, 1997,  and to be filed with the  Securities
and Exchange  Commission (the "SEC") in connection with the  registration  under
the Securities Act of 1933, as amended,  of an aggregate of 2,410,000  shares of
your Common Stock, $.01 par value (the "Common Stock") issuable upon exercise of
options which have been and will be granted pursuant to the Amended and Restated
1994 Flexible Stock  Incentive Plan and the Cylink/ARL 1997  Nonqualified  Stock
Option Plan (the "Plans").

         As your counsel in connection with the Registration  Statement, we have
examined the  proceedings  taken by you in  connection  with the adoption of the
Plans and the  authorization  of the  issuance of the Common Stock or options or
warrants to purchase  shares of Common Stock under the Plans (the "Plan Shares")
and such documents as we have deemed necessary to render this opinion.

         Based upon the foregoing,  it is our opinion that the Plan Shares, when
issued  and  outstanding  pursuant  to the terms of the  Plans,  will be validly
issued, fully paid and nonassessable shares of Common Stock.

         We consent to the use of this opinion as an exhibit to the Registration
Statement.

                                             Very truly yours,

                                             /s/ Morrison & Foerster LLP



                                   Exhibit 5.1
                                Page 6 of 7 Pages