This kind of PASERK serves as a pointer to another PASERK, which is in
turn intended for public
PASETOs.
This is for serializing secret keys in a keypair.
k[version].sid.[data]
See ID for the algorithm.
k1.sid.aBIISj44Kjs5PmIPh1q6DtTOTy2sz2ov14JJTch_5DHv
k2.sid.9wfgiRJhydmagHQ9kKOOxQm3OXRTCPxkelCzxw1sJRkV
k3.sid.mNalRnF8T60OMPdi1TWSMcub-51v3Au2VB1MOqPrw8zG
k4.sid.gHYyx8y5YzqKEZeYoMDqUOKejdSnY_AWhYZiSCMjR1V5