Seal info w EK pub or cert on systems without TPM

[akutz]

This patch introduces support for sealing information to an endorsement key without needing to be on a system with a TPM. This feature enables support for clients that want to encrypt small amounts of data in order to bootstrap a virtual machine that may only have an endorsement key prior to first boot.

For example, the following command, when executed from the root of this project, produces output that can only be unsealed on the system with a TPM that has the specified endorsement key:

1. For example, on a system without a TPM, the following command seals the specified plain-text data to the provided endorsement key RSA certificate:

   echo "Hello, world." | go run ./examples/tpm2-ekseal/main.go -ek-pem ./tpm2/testdata/ek-rsa-crt.pem -f 0

   The use of the -f 0 tells the program to emit the encrypted, duplicated object as base64-encoded data, its parts delimited by @@NULL@@:

   AE4ACAALAAAEAAAgAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAEAAgjD2tBVBgi06poK/qVwaY8tEKr/NRmEFkNlW3P2Bgy3M=@@NULL@@AFoAIOR+5y1YYp28HDEIaBM/NqsiZ5oY55Qw/5LqQg/L/9ZHd81bf0xE1rkhcOo4PwknfpNevbU9vwY+w2l/xwSq5qjhDVzL5rfCUaYD/kDr7T/FONLm+dSz1jo=@@NULL@@AQASLSEbE2TSHJozv1IhIhvjp0bhy7WUAQatA4/zUxgrqXO9QuFkO5YbtAVd/yukMAbVgIiaHr/miQHlEfU01AU/FDKKM0dT117m0+fEhIBlGhuTfwwg6/rU+FLTDBUyV6HluYdtHMgMlHPi9X67gRIhlZfPJYnlfwDE7kU8AXJydqSIanomFG5KU4IGclBTZGsOIdzJ3F6KB1hG44wxeoCi6yFha08G+I5wb0I7xOkiVb4GkW2KGEYMAjBJTbMJcUcm1pQQQsRL8jHJRMze5Etj5MPjKhwdAGU7wYis77wynBB5fa2QX0rECHN8IhPXhBvun4CW7F6NcdV/MM3IecpM

2. On the system with the TPM and matching endorsement key, the file ./examples/tpm2-ekseal/tpm2-unseal.sh may be used with its -0 flag to unseal the data:

   echo 'AE4ACAALAAAEAAAgAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAEAAgjD2tBVBgi06poK/qVwaY8tEKr/NRmEFkNlW3P2Bgy3M=@@NULL@@AFoAIOR+5y1YYp28HDEIaBM/NqsiZ5oY55Qw/5LqQg/L/9ZHd81bf0xE1rkhcOo4PwknfpNevbU9vwY+w2l/xwSq5qjhDVzL5rfCUaYD/kDr7T/FONLm+dSz1jo=@@NULL@@AQASLSEbE2TSHJozv1IhIhvjp0bhy7WUAQatA4/zUxgrqXO9QuFkO5YbtAVd/yukMAbVgIiaHr/miQHlEfU01AU/FDKKM0dT117m0+fEhIBlGhuTfwwg6/rU+FLTDBUyV6HluYdtHMgMlHPi9X67gRIhlZfPJYnlfwDE7kU8AXJydqSIanomFG5KU4IGclBTZGsOIdzJ3F6KB1hG44wxeoCi6yFha08G+I5wb0I7xOkiVb4GkW2KGEYMAjBJTbMJcUcm1pQQQsRL8jHJRMze5Etj5MPjKhwdAGU7wYis77wynBB5fa2QX0rECHN8IhPXhBvun4CW7F6NcdV/MM3IecpM' | \
   tpm2-ekunseal.sh -0 2>/dev/null

   The above command will emit the original, sealed text:

   Hello, world.

The above, end-to-end example used RSA, but ECC is possible as well:

1. On a system without the TPM, seal the data while specifying the ECC public certificate:

   echo "Hello, world." | go run ./examples/tpm2-ekseal/main.go -ek-pem ./tpm2/testdata/ek-ecc-crt.pem -f 0 

2. Echo the emitted data on the system with the TPM, this time using the -G ecc flag to specify the algorithm:

   echo 'AE4ACAALAAAEAAAgAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAEAAg7gVvQhmvIP9CvGZgrK+X3Ivcm1+pyBu5IVwV+WU5zZo=@@NULL@@AFoAIIYRbmB7f32ATtXs0ITU7cKyW+ihpDw3YrO+KMNtno15l5L4B0HmOVkBbBkmq9FI+P0BNt1dUlmskEiLY2l3HTCrXCSrnc2cmpIY/1Qj5xab4GBknH4zwO8=@@NULL@@AEQAID4Mo1N3w133AnMkBdqxEedDDhDZIUC07RDMTjTwLFOMACA0Bx74PwXK5ve4IUhGDb1TXptQGlNUTKf0y3u/ULKMfA==' | \
   tpm2-ekunseal.sh -0 -G ecc 2>/dev/null

   The above command will emit the original, sealed text:

   Hello, world.

Special kudos/shout-out to my colleague @jessepool for helping with this. I would have been lost without him!