while the invention is susceptible to various modifications and alternative constructions , certain illustrated embodiments thereof have been shown in the drawings and will be described below in detail . it should be understood , however , that there is no intention to limit the invention to the specific form disclosed , but , on the contrary , the invention is to cover all modifications , alternative constructions , and equivalents falling within the spirit and scope of the invention as defined in the claims . several preferred embodiments and features of the security holster are shown in fig1 - 10 . preferably , the security holster is made of a tough and durable plastic such as kydex ( manufactured by kleerdex company ). a thickness of ⅛ inch of this material has been found to be suitable for making the security holster . other materials or thicknesses of materials that have similar characteristics of rigidity , strength and weight would also be suitable . this might include metal such as aluminum or steel , other types of plastics or leather . the security holster shown in fig1 a , 1b and fig2 shows the general configuration of the security holster when used with a handgun 40 , and is designated as 10 in the figures . the security holster 10 is not limited strictly to handguns . in additional embodiments , the security holster 10 may be adapted to holster additional devices such as stun guns , tools , or other instruments that could benefit from the features of the present invention . the security holster 10 includes a holster body 12 and a holster outer cover 14 . the holster outer cover 14 wraps either fully or partially around the holster body 12 , and covers the locking mechanism . fig1 a shows the holster outer cover 14 cut away to reveal the locking mechanism of this embodiment . the locking mechanism of this embodiment is referred to as a spring plate assembly 16 . the spring plate assembly 16 includes three parts . the first part is a fixed plate 18 , which is rigidly and permanently attached to a bulge in the holster body 12 . the bulge in the holster body 12 forms a finger tube 46 , which is an opening or finger - receiving receptacle between the handgun and the holster body 12 , which allows insertion of a user &# 39 ; s index finger . the fixed plate 18 is preferably metal , and stainless steel has proven to be a suitable metal for its construction . opposite the fixed plate 18 is a first arm 20 , which is rigidly attached to a second arm 24 , both of which are attached by a hinge 30 to the fixed plate 18 . the second arm 24 includes a release tab 26 , which is semicircular in cross section and fitted to receive the index finger of a user , and lies over the trigger and trigger guard of a handgun 40 when it is fully inserted into the security holster 10 . the second arm 24 and its components are rigidly attached to the first arm 20 . the first arm 20 also includes locking tab mounting slots 32 , locking tab mounting screws 34 , and a locking tab 22 ( which is shown in fig3 b ). the locking tab 22 is mounted to the first arm 20 so that the screws 34 , which secure it , allow the locking tab 22 to move back and forth in the locking tab mounting slots 32 . the first arm 20 and second arm 24 are attached to the fixed plate 18 by a hinge 30 . the hinge 30 includes a spring 28 , which presses the first arm 20 against the holster body 12 . the spring 28 also pushes the second arm 24 away from the handgun and towards the finger tube 46 . the locking tab 22 passes through a locking tab passage 42 in the holster body 12 , in order to engage the ejection port of the handgun 40 and a portion of the holster body 12 . fig1 a also shows an insertion distance adjuster 72 . by adjusting the insertion distance adjuster , the user can select for himself the distance that the handgun must be pressed in before the locking tab snaps into place . in this way the user can select very little distance , even zero , or can select the maximum insertion distance available . in the example of the insertion distance adjuster shown in fig1 a , the deeper that the adjustment screw 74 is inserted , the less insertion distance is required to clear the undercut on the locking tab 22 . fig2 is a side view of the security holster of the invention showing the outer cover removed so that the locking mechanism is more visible . shown in fig2 is a trigger guard 76 of the handgun 40 . mounted to a spring bracket 44 is an adjustable travel stop 78 . by moving the adjustable travel stop in or out in relation to the trigger guard 76 , the distance that the handgun may be inserted into the holster is adjusted . the view shown in fig2 can be modified so that the adjustable travel stop includes a spring or other biasing member , by adjusting the adjustable travel stop and its associated spring or biasing member , the amount of force required to engage the locking tab 22 with the handgun 40 can be adjusted by the user . fig3 a and 3b show a detailed view of the spring plate assembly 16 . these views are of the right and left side of the spring plate assembly 16 , with the holster body 12 removed . as seen in fig3 a , the release tab 26 is semicircular in cross section to easily admit and receive a user &# 39 ; s index finger . on one end of the release tab 26 is a fingertip flare 36 , which is also curved and serves to help the user position his / her finger on the release tab 26 . the release tab 26 may be engaged when a user flexes a finger in the direction of the release tab 26 . in the preferred embodiment , release is accomplished by flexing the finger in a direction normal to the plane of the security holster and corresponding handgun . flexing is defined as the bending of a portion of a finger about a joint that allows the finger to apply pressure against a surface or mechanism . a release tab 26 is positioned so that it can be released by a finger flexed in a single direction , which provides an additional safety feature . the tendons of a finger only allow a finger to be flexed in a direction toward the palm of the hand . as a result , an unauthorized person that is able to insert a finger in the finger tube of the security holster would probably not be able to easily release the gun by finger flexure . as shown in fig3 a , the locking tab mounting slots 32 allow the locking tab mounting screws 34 and the locking tab 22 to move back and forth in the locking tab mounting slots 32 . as shown in fig3 b , the locking tab 22 has several angled faces which facilitates the locking tab 22 in engaging and disengaging from a handgun feature . an engagement edge 60 faces towards an entry end 62 of the security holster 10 . fig4 a and 4b are views of the security holster 10 looking from the entry end 62 toward the barrel end 64 . they show the spring plate assembly 16 in two positions . fig4 a shows the spring plate assembly with the first arm 20 lowered into a locking position . if a handgun 40 were in the security holster 10 in this position , the locking tab 22 would be engaged with the ejection port of the handgun to secure the handgun 40 in the security holster 10 . additionally , the angled faces slide under the security holster body 12 preventing the locking tab 22 from being improperly removed . fig4 b shows the first arm 20 raised away from the holster body 12 into a release position . when the fixed arm 20 moves in relation to the holster body 12 , the second arm 24 also moves , since it is rigidly attached to the first arm 20 . in this position , the locking tab 22 is disengaged from the ejection port of the handgun 40 , and the handgun 40 could be withdrawn from the security holster 10 . the first arm essentially functions as a first class lever . a first class lever is defined as a lever where the fulcrum is between the load and the effort force . in the present invention , the hinge 30 ( the fulcrum ) is between the locking tab 22 ( the load ) and the release tab 26 ( effort force ). the lever is oriented so that if the gun in the holster is oriented with the barrel horizontal , the lever arm of the security holster is oriented vertically , with the spring on the hinge oriented horizontally . fig5 is a front view of the security holster 10 , looking from the barrel end 64 towards the entry end 62 . this view shows a first arm 20 in a locking position . it also shows a trigger guard spring mount 38 and a slide spring mount 52 . these are two alternative positions of a spring , which extends backward from the spring bracket 44 or the slide spring bracket 50 , with a short coil spring , which presses against the handgun 40 . in the preferred embodiment , this coil spring is approximately ¼ inch in diameter , and extends approximately ¾ inches toward the entry end 62 of the security holster 10 . when a handgun 40 is inserted into the security holster 10 , the spring is depressed until its length is about ⅜ inches . two positions are shown for a spring bracket and a spring . the reason for this is that some handguns 40 can be equipped with devices such as flashlights or laser sights , which mount to the front of a trigger guard of a handgun . with such a device , the trigger guard spring bracket 44 would be dispensed with only the slide spring bracket 50 , and the slide spring mount 52 would be utilized . these springs serve as a rebounding device or pressure switch . other rebounding devices could also be utilized such as a compressible material such as foam , or an elastomer would serve as such a rebounding device . in additional embodiments , the rebounding device could be a piston or air bag . use of the rebounding device may require a user to compress the rebounding device before the spring plate assembly 16 would engage to lock in the handgun . additionally , when a user desires to remove the handgun from the holster , it is necessary to first compress the rebounding device before the release tab could be used to disengage the locking tab and corresponding angled faces . in a preferred embodiment , the rebounding device is adjustable . the rebounding device may allow a user to define a minimum distance and / or force that must be exerted against the rebounding device before the locking mechanism and corresponding locking tab can be engaged and / or disengaged . a user that has a need of quick accessibility to his handgun may dial down the minimum compression force that must be exerted against the handgun in order to activate and disengage the locking mechanism . in most circumstances the direction of the required rebounding force is parallel to the long axis of the security holster toward a bottommost portion of the security holster . in other circumstances , a user may want to adjust the rebounding device so that no force or distance requirement is needed to draw the handgun . an adjustment mechanism for the rebounding device may function by tightening or loosening the tension on a spring or other rebounding device as previously defined . fig6 a , 6b and 6 c illustrate the interaction of the security holster 10 , the locking tab 22 , and the handgun ejection port 54 of the handgun 40 . when seen from the top view , the security holster 10 has a locking tab passage 42 , which is adjacent the locking tab 22 and allows the locking tab 22 to pass through the wall of the security body 12 . the handgun 40 includes a handgun slide 56 and a handgun ejection port 54 , which permits a spent cartridge ( not shown ) to pass through the handgun ejection port 54 and through the handgun slide 56 . fig8 shows these handgun parts to advantage . as a handgun 40 is inserted into the security holster 10 , the handgun slide 56 lifts the locking tab 22 , and slides past it as it enters the security holster 10 . fig6 a shows a portion of the handgun slide 56 lifting the locking tab 22 . the handgun ejection port 54 is visible in the locking tab passage 42 of the security holster 10 . as shown in fig6 b , when the handgun 40 has been fully inserted into the security holster 10 , the locking tab 22 is past the handgun slide 56 , and can drop into place in the handgun ejection port 54 . when this happens , the first arm 20 falls into place against the side of the holster body 12 as shown in fig6 b . the locking tab 22 includes an engagement notch 58 , which is adjacent the interface of the locking tab 22 and the first arm 20 . at this point , the handgun 40 cannot be removed without first depressing the release tab 26 . if the release tab 26 were depressed , the locking tab 22 would move into the position shown in fig6 a , and the handgun ejection port 54 would not be engaged , and the handgun 40 could be removed . in one preferred embodiment of the security holster 10 of the invention , the security holster 10 is provided with one or more springs . a spring can be mounted on the trigger guard spring bracket 44 , and / or could be mounted on the slide spring bracket 50 , which was shown in fig5 . in the embodiment which utilizes one or more springs , once the handgun had been fully inserted as shown in fig6 b , the user could release the handgun 40 and the spring would press the handgun back in the security holster 10 until the engagement notch 58 of the locking tab 22 engaged with the holster body 12 which was exposed in the locking tab passage 42 . this position is shown in fig6 c . when springs are used as described above , the user would need to compress the springs a small amount by pressing down on the handgun 40 , in order to engage or release the locking tab 22 and thus the handgun by pressing the release tab 26 . this creates a secondary locking system by physically linking the locking tab 22 to the holster body 12 . when this happens , if force is applied to the handgun to withdraw it without first releasing the locking tab 22 , the force is transmitted to the holster body , and the first arm 20 does not bear this force without support . when the handgun 40 is inserted into the security holster , a similar minimum biasing force must be exerted against the handgun 40 in order for the locking tab 22 to engage the ejection port . a second preferred embodiment of the invention is shown in fig7 and 8 , in which the spring plate assembly is a t - shaped torsion spring 66 . depressing a second arm 26 of the torsion spring 66 lifts the first arm 20 and its attached locking tab 22 out of the ejection port 54 of the handgun to release the handgun 40 . fig9 shows the security holster 10 of the invention with a first security lock 68 and a second security lock 70 . each of these security locks allow a user to lock the first and second arm from movement , and thus lock a handgun 40 in the security holster 10 , and prevent the release of the locking tab 22 . in either security lock , a tab can be moved forward or backward which physically prevents the locking tab 22 from being lifted , and thus prevents the handgun 40 from being released and withdrawn . fig1 a , 10b and 10 c show three embodiments of the interface between the locking tab 22 and the holster body 12 . fig1 a shows the locking tab 22 having a sloping surface ; fig1 b shows a lap joint type interface ; and fig1 c shows a tongue and groove type connection . also shown in fig1 c is the insertion distance adjuster 72 and the adjustment screw 74 . from the foregoing description , it will be apparent that various changes may be made without departing from the spirit and scope of the invention as defined by the following claims . while there is shown and described the present preferred embodiment of the invention , it is to be distinctly understood that this invention is not limited thereto but may be variously embodied to practice within the scope of the following claims .