Abstract:
Encryption of virtual disc image is accomplished by increasing the size of a virtual disc to support the inclusion of a master boot record and a decryption program. Encrypting portions of a virtual disc image on the virtual disc, but leaving the boot record and decryption program unencrypted and accessible, where the decryption program will decrypt the encrypted portions if the appropriate cryptographic key is supplied. Subsequent decryption is accomplished by initiating a boot sequence through the master boot record, receiving the appropriate cryptographic key, appropriately ordering the decrypted disc image.

Description:
FIELD OF THE INVENTION 
     The invention relates to virtualization software for computer systems. In particular the invention relates to protecting the contents of a virtual disc by encrypting and decrypting the virtual disc. 
     BACKGROUND 
     Setting up or installing large numbers of servers and computer systems with specific software and applications using conventional physical resources has become more and more costly. In recent years this process has become simplified by the use of virtualization technologies providing virtual discs or operating systems with preconfigured software packages and system configurations. 
     If a large number of like configured servers is needed, the same virtual disc can be used over and over again. Virtual disc images may be distributed via the internet. However the packets containing the virtual disc image may contain critical data and licenses. In addition, the unauthorized use of virtual disc images needs to be prevented. Currently the entire disc image is encrypted before being distributed. 
     SUMMARY OF THE INVENTION 
     The invention provides a computer-readable storage medium containing instructions for encrypting a virtual disc, a computer-readable storage medium containing instructions for decrypting an encrypted virtual disc, and an encrypted-virtual-disc computer-readable storage medium containing the virtual disc in the independent claims. Embodiments are given in the dependent claims. 
     Encrypting all of the virtual disc images before distribution has several disadvantages. First a virtual disc image in one format may not be converted into another without being decrypted. Secondly, an administrator is needed to decrypt and install the virtual disc image. This could be a problem, because a cryptographic key or credentials for decrypting the virtual disc image need to be provided to the administrator. The end user or operator of the virtual disc system may or may not wish to share the cryptographic key or credentials with the administrator. 
     Embodiments of the invention may solve these and other problems by placing an encryption-master-boot-record on the encrypted virtual disc image along with a decryption program for decrypting an at least partially encrypted virtual disc image. The encryption-master-boot-record is a master boot record which is used for booting the virtual machine. The virtual machine then decrypts the encrypted virtual disc image using the decryption program and cryptographic credentials. This eliminates the need to provide the operator with the cryptographic credentials. 
     Embodiments of the invention may have the advantage that the virtual disc may be encrypted or decrypted from the virtual disc. Embodiments of the invention may have the advantage that the method can be set up to encrypt only used blocks. This results in faster encryption and a reduction in the amount of data to encrypt. Embodiments of the invention may have the advantage that a virtual disc can be encrypted and decrypted on the fly. 
     Additionally, only portions of the virtual disc may be encrypted. For instance, the blocks of the virtual disc may be selectively encrypted. The portions of the virtual disc which contain data or records specific to a particular format of virtual disc can be left unencrypted. This allows the conversion of the virtual disc without decrypting the virtual disc. 
     A computer-readable storage medium as used herein encompasses any tangible storage medium which may store instructions which are executable by a processor of a computing device. The computer-readable storage medium may be referred to as a computer-readable non-transitory storage medium. The computer-readable storage medium may also be referred to as a tangible computer-readable medium. In some embodiments, a computer-readable storage medium may also be able to store data which is able to be accessed by the processor of the computing device. 
     Examples of computer-readable storage media include, but are not limited to: a floppy disc, a magnetic hard disc drive, a solid state hard disc, flash memory, a USB thumb drive, Random Access Memory (RAM) memory, Read Only Memory (ROM) memory, an optical disc, a magneto-optical disc, and the register file of the processor. Examples of optical discs include Compact Discs (CD) and Digital Versatile Discs (DVD), for example CD-ROM, CD-RW, CD-R, DVD-ROM, DVD-RW, or DVD-R discs. The term computer-readable storage medium also refers to various types of recording media capable of being accessed by the computer device via a network or communication link. For example a data may be retrieved over a modem, over the internet, or over a local area network. 
     Computer memory is an example of a computer-readable storage medium. Computer memory is any memory which is directly accessible to a processor. Examples of computer memory include, but are not limited to: RAM memory, registers, and register files. 
     Computer storage is an example of a computer-readable storage medium. Computer storage is any non-volatile computer-readable storage medium. Examples of computer storage include, but are not limited to: a hard disc drive, a USB thumb drive, a floppy drive, a smart card, a DVD, a CD-ROM, and a solid state hard drive. In some embodiments computer storage may also be computer memory or vice versa. 
     A computing device or computer system as used herein refers to any device comprising a processor. A processor as used herein encompasses an electronic component which is able to execute a program or machine executable instruction. References to the computing device comprising “a processor” should be interpreted as possibly containing more than one processor. The term computing device should also be interpreted to possibly refer to a collection or network of computing devices each comprising a processor. Many programs have their instructions performed by multiple processors that may be within the same computing device or which may even be distributed across multiple computing devices. 
     A user interface as used herein is an interface which allows a user or operator to interact with a computer or computer system. A user interface may provide information or data to the operator and/or receive information or data from the operator. The display of data or information on a display or a graphical user interface is an example of providing information to an operator. The receiving of data through a keyboard, mouse, trackball, touchpad, pointing stick, graphics tablet, joystick, gamepad, webcam, headset, gear sticks, steering wheel, pedals, wired glove, dance pad, remote control, and accelerometer are all examples of receiving information or data from an operator. 
     Virtualization software, a virtualization program, and a virtualization module as used herein all refer to software or computer executable instructions which allow a computer system to run a virtual computer system. A virtual machine or virtual computer system as used herein encompasses a computer system which is implemented virtually or simulated by software running on a computer system. 
     A cryptographic key or cryptographic credential as used herein encompasses a key, credential, or password which may be used by a decryption algorithm to decrypt a data file. 
     A virtual disc as used herein encompasses data which may be used by a virtualization program as a virtual disc image. A virtual disc may contain a file system which may be accessed by the virtualization system or virtual system. A virtual disc may also contain a bootable operating system. 
     In one aspect the invention provides for a computer-readable storage medium containing machine executable instructions that when executed by a processor cause a processor to encrypt a virtual disc. The virtual disc comprises a virtual disc image. The virtual disc image is an image of an existing disc file system or a disc file system which is constructed for the purpose of creating the virtual disc image. Execution of the machine executable instructions causes the processor to receive the virtual disc. Execution of the machine executable instructions further causes the processor to increase the size of the virtual disc. The virtual disc may essentially be a file stored on the computer-readable storage medium or a different computer-readable storage medium. The size of the virtual disc may be increased by adding blocks at the beginning or the end of the virtual disc. Execution of the machine executable instructions further cause the processor to write a decryption-master-boot-record and a decryption program to the virtual disc. The decryption-master-boot-record is a master boot record that a virtual computer or computer system boots into when using the virtual disc image. The decryption-master-boot-record allows the virtual computer system to run the decryption program for decrypting the virtual disc. Execution of the machine executable instructions further causes the processor to encrypt at least a portion of the virtual disc image. The decryption program comprises decryption-machine-executable-instructions for decrypting the at least partially encrypted virtual disc image in accordance with a cryptographic key. The virtual disc image is encrypted such that the virtual disc image may be decrypted using the decryption program in accordance with the cryptographic key. 
     In some embodiments the entire virtual disc image is encrypted. In other embodiments only certain portions of the virtual disc image are encrypted. For instance, if portions of the virtual disc are unused, these portions of the disc image need not be encrypted. It may also be able to be determined if certain portions of the virtual disc contain sensitive information that will be desirable to protect by encryption, for instance application programs or sensitive data. The decryption of the virtual disc image could be speeded up by selectively using those portions of the virtual disc which need to be protected and not encrypting those portions which do not need to be protected. 
     Embodiments of the invention have several advantages. For instance, adding the decryption-master-boot-record to the virtual disc and the decryption program enables an end user to perform the decryption of the virtual disc image. This eliminates the need for an administrator to perform this task. 
     In another embodiment the virtual disc image is divided into first and second parts. The virtual disc is divided into first, second, third, fourth, and fifth portions. The virtual disc image originally spans the first, second, and third portions of the virtual disc. The second part of the virtual disc image is stored in a third portion of the virtual disc. Execution of the instructions further cause the processor to copy the first part of the virtual disc image to a fourth portion of the virtual disc. The first part of the virtual disc image is copied or moved from the first and second portions of the virtual disc to the fourth portion of the virtual disc. The decryption-master-boot-record is written to the first portion of the virtual disc. When the virtual disc is loaded into a virtual system and the virtual system boots into the virtual disc the decryption-master-boot-record will cause the virtual system to run the decryption program. 
     The decryption program is written to the second portion of the virtual disc. As mentioned above, the first part of the virtual disc image is copied from the first and second portions of the virtual disc. The first part of the virtual disc image is copied from the first and second portions of the virtual disc before the decryption-master-boot-record and the decryption program are written to the first and second portions of the virtual disc respectively. Execution of the instructions further causes the processor to at least partially encrypt the first and second parts of the virtual disc. The size of the virtual disc is increased to create the fourth portion of the virtual disc and a fifth portion of the virtual disc. The size of the fifth portion is larger than or equal to the second portion. The combined size of the first and second portions is less than or equal to the size of the fourth portion. This embodiment of the invention may be advantageous because the decryption-master-boot-record is in the first portion and will cause the virtual system to boot into the decryption program. 
     In another embodiment the first part of the virtual disc image is encrypted together. In this embodiment the entire first part of the virtual disc image is encrypted as a single encrypted data file. 
     In another embodiment the second part of the virtual disc image is encrypted together. In this embodiment the second part of the virtual disc image is encrypted as a single data file. 
     In another embodiment the virtual disc image is divided into blocks. As used herein a block is a portion or sub-division of data of a disc or a virtual disc image. The data in a block is addressable by the disc or the virtual disc. The blocks are selectively encrypted in accordance with a predetermined block encryption list. The block encryption list is a list of blocks which are to be encrypted during the encryption of the virtual disc image. For instance an operator could determine which blocks of the virtual disc image contain data which is desired to be protected by encryption. For instance these blocks may contain sensitive data or information. Likewise these blocks may contain applications for which a license is to be purchased. If the program is transmitted across the internet it would be desirable to protect the executable version of the code or data. 
     In another embodiment the virtual disc image is divided into blocks. Execution of the instructions causes the processor to examine each of the blocks and create a list of unused blocks. Particular blocks are encrypted only if they are not found in the list unused blocks. This embodiment is particularly advantageous because the computer-readable storage medium avoids encrypting blocks which are not used. Since the blocks are not used there is no need to protect them. In some virtual file systems unused data may be part of the file system but not used. By not encrypting these portions of the file system the virtual disc may be smaller. 
     The aforementioned embodiments of the computer-readable storage medium also provide for other aspects of the invention. For instance a computer system is provided for by the invention which contains or comprises the machine readable instructions contained on a computer-readable storage medium according to an embodiment of the invention. Likewise execution of the machine executable instructions causes the processor to perform various steps or actions which also provide for a method and computer-implemented methods. The executable instructions on the computer-readable storage medium also provide for a computer program product and/or a computer system. 
     Another aspect of the invention provides for a computer-readable storage medium containing machine executable instructions that when executed by a processor cause the processor to decrypt an encrypted virtual disc. The virtual disc comprises a decryption-master-boot-record, a decryption program, and an at least partially encrypted virtual disc image. The decryption program comprises decryption-machine-executable-instructions for decrypting the at least partially encrypted virtual disc image in accordance with a cryptographic key. Execution of the machine executable instructions causes the processor to receive the encrypted virtual disc. 
     Execution of the machine executable instructions further causes the processor to boot a virtual machine using the decryption-master-boot-record. Execution of the machine executable instructions further causes the processor to receive the cryptographic key. The order of receiving the cryptographic key is not critical. For instance the processor could receive the cryptographic key at any point before the virtual disc image is decrypted. Execution of the machine executable instructions further cause the processor to decrypt the at least partially encrypted virtual disc image in accordance with the cryptographic key and the decryption program. The machine executable instructions cause the processor to boot the virtual machine and the virtual machine boots into the operating system on the encrypted virtual disc via the decryption-master-boot-record. This then causes the virtual machine to run the decryption program. The decryption program then decrypts the at least partially encrypted virtual disc image. Both the cryptographic key and the decryption program are needed for decrypting the at least partially encrypted virtual disc image. 
     In another embodiment the decryption of the at least partially encrypted virtual disc image is performed during deployment of the virtual machine. In the current state of the art an administrator will receive an encrypted virtual disc and the administrator is responsible for decrypting it. This is however undesirable in many circumstances because the end user or operator of the virtual machine relies on an administrator to perform the decryption. Embodiments of the invention may have the advantage that the end user or operator can perform the decryption his or herself. 
     In another embodiment the virtual disc comprises a first portion containing the decryption-master-boot-record. The virtual disc further comprises a second portion containing the decryption program. The virtual disc further comprises a third portion containing a second part of the virtual disc image. The virtual disc further comprises a fourth portion containing a first part of the virtual disc. The first part of the virtual disc may contain in some embodiments a master boot record for booting into an operating system contained in the virtual disc image. This master boot record in the fourth portion may be used to boot the virtual machine once the decryption of the at least partially encrypted virtual disc is completed. 
     The virtual disc comprises a fifth portion containing storage space. The size of the fifth portion is larger than the second portion. The combined size of the first and second portions is less than or equal to the size of the fourth portion. The combined size of the first and second portions is less than or equal to the size of the fourth portion. The at least partially encrypted disc image is decrypted by decrypting the second part of the virtual disc image. The at least partially encrypted virtual disc image is further decrypted by copying the decryption program to the fifth portion of the virtual disc. The at least partially encrypted virtual disc image is decrypted by decrypting a portion of the first part of the virtual disc image. 
     The at least partially encrypted virtual disc image is further decrypted by copying the decrypted portion of the first part of the virtual disc image to the second portion of the virtual disc. The virtual disc image is further decrypted by decrypting the remainder of the first part of the virtual disc image. The virtual disc image is further decrypted by copying the decrypted remainder of the first part of the virtual disc image to the first portion of the virtual disc. Performing the decryption in this manner may have the advantage that the decryption can be interrupted at any point in time. For instance the fifth portion may contain a data file which maintains a status of the decryption process. 
     In another embodiment execution of the instructions further causes the processor to erase data in the fourth and fifth portions of the virtual disc after copying the decrypted remainder of the first part of the virtual disc image to the first portion of the virtual disc. 
     In another embodiment execution of the instructions further causes the virtual machine to reboot after decrypting the at least partially encrypted virtual disc image. 
     The aforementioned embodiments of the computer-readable storage medium also provide for other aspects of the invention. For instance a computer system is provided for by the invention which contains or comprises the machine readable instructions contained on a computer-readable storage medium according to an embodiment of the invention. Likewise execution of the machine executable instructions causes the processor to perform various steps or actions which also provide for a method and computer-implemented methods. The executable instructions on the computer-readable storage medium also provide for a computer program product and/or a computer system. 
     In another aspect the invention provides for an encrypted-virtual-disc computer-readable storage medium containing a virtual disc. The virtual disc comprises a decryption-master-boot-record, a decryption program, and an at least partially encrypted virtual disc image. The decryption program comprises machine executable instructions for decrypting the at least partially encrypted virtual disc in accordance with a cryptographic key. In other words the combination of the decryption program and the cryptographic key are used for decrypting the at least partially encrypted virtual disc image. 
     The decryption program comprises machine executable instructions that when executed by a processor cause the processor to receive a cryptographic key. The cryptographic key may in some embodiments be prompted to be entered by the decryption program or the cryptographic key may be passed to the decryption program by another program. For instance virtualization software for running a virtual computer system may pass a cryptographic key on to the decryption program. Further execution of the machine executable instructions of the decryption program cause the processor to decrypt the at least partially encrypted virtual disc image in accordance with the cryptographic key and the decryption program. 
     In another embodiment the virtual disc comprises a first portion containing the decryption-master-boot-record. The virtual disc further comprises a second portion containing the decryption program. The virtual disc further comprises a third portion containing a second part of the virtual disc image. The virtual disc further comprises a fourth portion containing a first part of the virtual disc. The virtual disc further comprises a fifth portion containing storage space. The size of the fifth portion is larger than or equal to the second portion. The combined size of the first and second portions is less than or equal to the size of the fourth portion. 
     In another embodiment the at least partially encrypted virtual disc image is decrypted by decrypting the second part of the virtual disc image. The virtual disc is further decrypted by copying the decryption program to the fifth portion of the virtual disc. The virtual disc image is further decrypted by decrypting a portion of the first part of the virtual disc image. The virtual disc is further decrypted by copying the decrypted portion of the first part of the virtual disc image to the second portion of the virtual disc. The virtual disc is further decrypted by copying the decrypted portion of a first part of the virtual disc image to the second portion of the virtual disc. The virtual disc image is further decrypted by decrypting the remaining of the first part of the virtual disc image. The virtual disc image is further decrypted by copying the decrypted remainder of the first part of the virtual disc image to the first portion of the virtual disc. 
     In another embodiment the virtual disc contains a decryption-status-data-file for storing the progress of the decryption of the at least partially encrypted virtual disc image. Execution of the machine executable instructions of the decryption program further cause the processor to update the decryption-status-data-file during decryption of the at least partially encrypted virtual disc image. Execution of the machine executable instructions of the decryption program further cause the processor to check the decryption-status-data-file when starting the decryption of the at least partially encrypted virtual disc image. By checking the status of the decryption-status-data-file the decryption can be started at an intermediate point if the decryption was originally interrupted. 
     In another embodiment execution of the instructions further cause the processor to erase data in the fourth and fifth portion of the virtual disc after copying the decrypted remainder of the first part of the virtual disc image to the first portion of the virtual disc. 
     In another embodiment execution of the instructions further cause a virtual machine executing the decryption program to reboot after decrypting the at least partially encrypted virtual disc image. 
     The aforementioned embodiments of the computer-readable storage medium also provide for other aspects of the invention. For instance a computer system is provided for by the invention which contains or comprises the machine readable instructions contained on a computer-readable storage medium according to an embodiment of the invention. Likewise execution of the machine executable instructions causes the processor to perform various steps or actions which also provide for a method and computer-implemented methods. The executable instructions on the computer-readable storage medium also provide for a computer program product and/or a computer system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the following, preferred embodiments of the invention will be described in greater detail by way of example only making reference to the drawings in which: 
         FIG. 1  illustrates the decryption of a virtual disc  100  according to an embodiment of the invention, 
         FIG. 2  illustrates an example of decryption during importation of the virtual system, 
         FIG. 3  illustrates the decryption of the virtual disc image during deployment, 
         FIGS. 4 a -4 e    illustrate a method of block-based encryption of a virtual disc image according to an embodiment of the invention, 
         FIGS. 5 a -5 d    illustrate the decryption of the virtual disc image encrypted in  FIGS. 4 a   - 4   e,    
         FIG. 6  shows a flow diagram which illustrates a method of encrypting a virtual disc image according to a further embodiment of the invention, 
         FIG. 7  shows a flow diagram which illustrates a method of decrypting a virtual disc image according to a further embodiment of the invention, 
         FIG. 8  shows a flow diagram which illustrates a method of decrypting a virtual disc image according to a further embodiment of the invention, and 
         FIG. 9  illustrates a first computer system for encrypting a virtual disc and a second computer system for decrypting a virtual disc. 
     
    
    
     DETAILED DESCRIPTION 
     In the following, like numbered elements in these figures are either similar elements or perform an equivalent function. Elements which have been discussed previously will not necessarily be discussed in later figures if the function is equivalent. 
       FIG. 1  illustrates the decryption of a virtual disc  100  according to an embodiment of the invention. The virtual disc  100  comprises a virtual disc image  102  which is encrypted and a decryption program  104  for decrypting the virtual disc image  102 .  FIG. 1  also shows a computer system  106  which contains virtualization software for running virtual computer systems or machines. Shown in  FIG. 1  is an operator  108  and a user  110 .  FIG. 1  illustrates the actions taken by the operator  108  and the user  110  when using a virtual disc  100  according to an embodiment of the invention. The steps shown in  FIG. 1  are for the first booting of the virtual disc and its decryption. Step  1  is labeled  112 . In this step the operator  108  stores the virtual disc  100  on the computer system  106 . In step  2  ( 114 ), the user  110  boots the virtual machine using the virtualization software on a computer system  106 . In step  3  ( 116 ), the decryption program  104  starts and requests credentials or a password from the user  110 . In step  4  ( 118 ), the user  110  provides the credentials or password to the decryption program  104 . In step  5  ( 120 ), the decryption program  104  decrypts the virtual disc image  102  using the credentials or password. In step  6  ( 122 ), the virtual machine reboots and the virtual machine boots from the decrypted virtual disc image  102 . 
     During use of a virtual disc  100 , according to an embodiment of the invention, two different use scenarios are possible. There may be decryption during the import of the virtual disc image  102  or there may be decryption during deployment of the virtual disc image  102 . If the virtual disc image  102  is decrypted during the importation of the virtual system the operator knows the credentials and passes this to the program or programs for managing the virtual systems. The programs for managing the virtual system then import the image and decrypt it on the fly. For the second possibility for decryption during deployment, the operator does not know the credentials and asks to import the images without decrypting. The virtual disc image is stored in a database of virtual systems. At deployment time the user is prompted for the credentials. 
       FIG. 2  shows an example of decryption during importation of the virtual system. Shown in  FIG. 2  is a computer system  200  that functions as a virtual system image server  202 . The virtual system image server  202  serves images of virtual systems when requested by an operator  208 . Also stored or able to be accessed by the computer system  200  is a virtual disc repository  204 , which is a repository of virtual discs accessible via the virtual system image server  202 . There is also a decryption module  206  that is equivalent to the decryption program  104  shown in  FIG. 1 . In step  1  the operator  208  downloads a virtual disc. In step  2  ( 212 ), the operator requests the importation of the virtual disc into the system managed by the virtual system image server  202 . In step  3  ( 214 ), the virtual system image server  202  requests credentials or a password from the operator  208 . In step  4  ( 216 ), the operator  208  provides the credentials or passwords to the virtual system image server  202 . In step  5  ( 218 ), the virtual system image server  202  imports the virtual disc image from the virtual disc and decrypts the virtual disc image on the fly using the credentials or passwords provided by the operator  208 . 
       FIG. 3  illustrates the decryption of the virtual disc image during deployment. Shown in this figure is a computer system  200  with a virtual system image server  202  that manages virtual disc images stored in a virtual disc repository  204 . In the example shown in  FIG. 3  there is a second computer system  300  that is used for decryption during the deployment of the virtual disc image. The second computer system  300  is for running a target virtual system  302 . Within the target system  302  is an operating system deployment tool  301 . The operating system deployment tool  301  is provided to deploy a virtual system in a virtual disc image. 
     Also within the target virtual system is a decryption module  303 . The decryption module is a software module or decryption program for decrypting a virtual disc image using a password or a cryptographic credential. An operator  304  and a user  306  are shown. In a first step  1  designated by reference  308 , the operator  304  requests or triggers the deployment of a virtual system on the second computer system  300 . In a second step  2  (designated by reference  310 ) the target or virtual system boots on the operating system deployment tool  301 . In a third step  3  (designated by reference  312 ), the operating system deployment tool  301  requests a cryptographic password or credentials from the user  306 . In a fourth step  4  (designated by reference  314 ), the user  306  provides the cryptographic password or credentials to the operating system deployment tool  301 . In a fifth step  5  (also referred to as  316 ), the operating tool downloads and decrypts the virtual disc image using the cryptographic password or credentials and the decryption module  303 . In step  6  (also referred to as  318 ), after the virtual disc image has been decrypted, the deployment of the virtual system continues on the decrypted virtual disc image. 
       FIGS. 4 a  to 4 e    illustrate a method of block-based encryption of a virtual disc image according to an embodiment of the invention. In  FIG. 4 a   , a virtual disc  400  and a virtual disc image  402  are shown. The blocks which make up the virtual disc image are labeled 1-n. To encrypt the virtual disc image the user starts an encryption tool or program. In a first step the decryption tool increases the size of the virtual disc. This is illustrated in  FIG. 4 b   . At the end of the virtual disc  400  a region of empty operating system blocks  404  is created. In a next step the virtual disc image is divided into a first part  406  and a second part  408 . The first part of the virtual disc image  406  is copied to the empty operating system blocks  404  at the end of the virtual disc  400 . 
     In  FIG. 4 d   , it is shown that the encryption tool copies a decryption master boot record  410  and a decryption program  412  to a first and second part of the virtual disc  400 .  FIG. 4 e    illustrates the final step. The encryption tool or software encrypts the first part of the virtual disc image  406 ′ and encrypts the second part of the virtual disc image  408 ′. All of the blocks of the encrypted virtual disc image  406 ′,  408 ′ may be encrypted or the blocks may be selectively encrypted. In  FIG. 4 e    the virtual disc  400  is also shown as being divided into five portions. The first portion of the virtual disc  414  contains the decryption-master-boot-record  410 . The second portion of the virtual disc  416  contains the decryption program  412 . The third portion of the virtual disc  418  contains the encrypted  408 ′ second part of the virtual disc image. The fourth portion of the virtual disc  420  contains the encrypted  406 ′ first part of the virtual disc image. The fifth portion of the virtual disc  422  is at the end of the virtual disc  400 . In various embodiments the fifth portion  422  may contain data recording the decryption state, journaling data, temporary data used in the decryption, and combinations thereof. 
       FIGS. 5 a -5 d    illustrate the decryption of the virtual disc image  406 ′,  408 ′ of a virtual disc  400  when booted from a virtual machine.  FIG. 5 a    is identical with  FIG. 4 e   . In a first step the virtual machine boots on the virtual disc  400  and boots into the virtual disc master boot record  410 . Next the master boot record  410  loads the decryption program  412 . The decryption program  412  then requests cryptographic credentials or a password for use for decrypting the encrypted virtual disc image  406 ′,  408 ′. In  FIG. 5 b    the decryption process is illustrated. Two different views of the virtual disc  400  are shown. Blocks labeled  500  are decrypted blocks of the second part of the virtual disc image. Blocks labeled  502  are encrypted blocks of the second part of the virtual disc image. In the top view shown in  FIG. 5 b    only the block labeled 4 is a decrypted block  500 . The remainder of the second part of the virtual disc image is encrypted. The bottom part of  FIG. 5 b    shows that all blocks of the second part of the virtual disc image  408  are decrypted blocks  500 . 
       FIG. 5 c    shows further progress in decrypting the virtual disc  400 . After all blocks of the second part of the virtual disc image  408  have been decrypted the decryption program  412  is copied to the fifth portion  422  of the virtual disc. Next a portion of the first part of the virtual disc image  406 ′ is decrypted and copied to the second portion  416  of the virtual disc. The remainder  506  of the first part of the virtual disc image  406 ′ is decrypted and copied to the first portion  414  of the virtual disc. The remainder of the first part of the virtual disc image  506  in this embodiment has overwritten the master boot record  410 . The encrypted first part of the virtual disc image  406 ′ and the decryption program  412  may be overwritten leaving empty operating system blocks  404 . The Fig. shown in  5   d  is equivalent with that shown in  FIG. 4 b   . This shows how the method illustrated in  FIG. 5  has been used to decrypt the at least partially encrypted virtual disc image  402  of the virtual disc  400 . 
       FIG. 6  shows a flow diagram which illustrates an embodiment of encrypting a virtual disc image according to the invention. In step  600  a virtual disc is received. The virtual disc comprises a virtual disc image. In step  602  the size of the virtual disc is increased. In step  604  a decryption-master-boot-record and a decryption program are written to the virtual disc. In step  606  at least a portion of the virtual disc image is encrypted. 
       FIG. 7  shows a flow diagram which illustrates a method of decrypting a virtual disc according to an embodiment of the invention. In step  700  an encrypted virtual disc is received. In step  702  a virtual machine is booted using a decryption-master-boot-record contained on the virtual disc. In step  704  a cryptographic key is received. The virtual disc comprises an at least partially encrypted virtual disc. In step  706  the at least partially encrypted virtual disc is decrypted using a decryption program which is on the virtual disc. The decryption program uses the cryptographic key for decrypting with the decryption program for performing the decryption of the at least partially encrypted virtual disc. 
       FIG. 8  shows a flow diagram which illustrates a method of decrypting an encrypted virtual disc according to a further embodiment of the invention. In step  800  an encrypted virtual disc is received. In step  802  a virtual machine is booted using the decryption-master-boot-record. In step  804  a cryptographic key is received. In step  806  a second part of the virtual disc image is decrypted using the cryptographic key and a decryption program which is located on the virtual disc. In step  808  the decryption program is copied to a fifth portion of the virtual disc. In step  810  a portion of a first part of the virtual disc image is decrypted. In step  812  the decrypted portion of the first part of the virtual disc image is copied to the second portion of the virtual disc. In step  814  the remainder of the first part of the virtual disc image is decrypted. In step  816  the decrypted remainder of the first part of the virtual disc image is copied to the first portion of the virtual disc. In step  818  the virtual machine is rebooted. The method illustrated in  FIG. 8  is analogous to the method illustrated by  FIG. 5 . 
       FIG. 9  shows two computer systems, a first computer system  900  for encrypting a virtual disc and a second computer system  902  for decrypting a virtual disc. There is a network communication  904  between the first computer system  900  and the second computer system  902 . The first computer system has a network interface  906  for connecting to the computer network  904  and the second computer system  902  has a network interface  908  for connecting to the computer interface  904 . The network connection  904  can be any standard computer interface such as an Ethernet connection or an internet connection. The first computer system  900  has a processor  910  that is connected to a user interface  912  and the network interface  906 . The processor  910  is also connected to computer storage  914  and computer memory  916 . 
     Within the computer storage  914  is an unencrypted virtual disc  918 . The unencrypted virtual disc contains an unencrypted virtual disc image. Also within the computer storage  914  is a decryption-master-boot-record  920 . Also within the computer storage  914  is a decryption program  922 . Also within the computer storage  914  is an encrypted virtual disc  924 . The encrypted virtual disc  924  contains a decryption-master-boot-record  920 , a decryption program  922 , and an at least partially encrypted virtual disc image  923 . The encrypted virtual disc  924  may also contain an at least partially encrypted virtual disc image. The computer memory  916  contains an encryption tool  926 . An encryption tool  926  is a software module or program containing machine executable instructions that cause the processor  910  to create the encrypted virtual disc  924  using the unencrypted virtual disc  918 , the decryption-master-boot-record  920 , and the decryption program  922 . The encryption tool  926  may be used to implement the methods illustrated in  FIGS. 4 and 6 . In some embodiments the computer memory  916  also contains a cryptographic module and a cryptographic credential generation module  930 . The cryptographic module  928  is used for encrypting the unencrypted virtual disc  918 . The cryptographic credential generation module  930  is an optional module and may be used for generating cryptographic credentials. For instance the cryptographic credential generation module may be used to generate a cryptographic key pair for an asymmetric encryption algorithm. 
     The second computer system  902  also contains a processor  932 . The processor  932  is connected to the network interface  908  and the user interface  934 . The processor  932  is also connected to computer storage  936  and computer memory  938 . The computer storage  936  contains the encrypted virtual disc  924  from the first computer system  900 . In this embodiment the network connection  904  was used to transfer the encrypted virtual disc  924 . Also within the computer storage  936  is an encryption cryptographic key  944 . Computer memory  938  contains a virtualization module  942 . The virtualization module  942  allows the processor  932  to run and operate a virtual computer system. As can be seen, all that is needed to decrypt the encrypted virtual disc  924  is the virtualization module  942  and the cryptographic key  944 . This Fig. also illustrates how an end user may be able to decrypt the encrypted virtual disc  924  without the aid of an operator.