A.2 The function MUL(V, P, c)
3GPP55.251GEA5 and GIA5 algorithm specificationSpecification of the GEA5 and GIA5 encryption algorithms for General Packet Radio Service (GPRS)TS
This clause only available under licence.
See http://www.etsi.org/about/what-we-do/security-algorithms-and-codes/cellular-algorithm-licences.
Annex B (informative):
Implementation options for some operations of the GIA5 algorithm