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Duplicating-Objects.md

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Duplicating Objects

Sample procedure to transfer an RSA key from one TPM to another. The RSA key never leaves the protection of the two TPMs at anytime and cannot be decoded or used on any other system even if it intercepted in transit.

This procedure will transfer an RSA key from TPM-A to TPM-B. The key can be generated via openssl and imported into TPM-A or generated directly on TPM-A.

TPM-B will provide TPM-A the public portion of a keypair it owns that allows a sealed transfer. This tutorial uses the following two APIs

On TPM-B

Create a parent object that will be used to wrap/transfer the PEM file

tpm2_createprimary -C o -g sha256 -G rsa -c primary.ctx

tpm2_create  -C primary.ctx -g sha256 -G rsa \
-r new_parent.prv  -u new_parent.pub \
-a "restricted|sensitivedataorigin|decrypt|userwithauth"

Copy new_parent.pub to TPM-A. The copy steps assumes attestation was done previously and that TPM-A trusts the new_parent.pub issued by TPM-B

On TPM-A

Create root object and auth policy allows duplication only

tpm2_createprimary -C o -g sha256 -G rsa -c primary.ctx

tpm2_startauthsession -S session.dat

tpm2_policycommandcode -S session.dat -L dpolicy.dat TPM2_CC_Duplicate

tpm2_flushcontext session.dat

rm session.dat

At this point, either generate an external RSA Keypair to transfer

openssl genrsa -out private.pem 2048

openssl rsa -in private.pem -outform PEM -pubout -out public.pem

tpm2_loadexternal -C n -Grsa -u public.pem -r private.pem \
-c key.ctx -L dpolicy.dat -a "sensitivedataorigin|userwithauth|decrypt|sign"

tpm2_readpublic -c key.ctx -o dup.pub
  • Or generate an RSA keypair on TPM to transfer (note the passphrase is 'foo')
tpm2_create -C primary.ctx -g sha256 -G rsa -p foo -r key.prv \
-u key.pub  -L dpolicy.dat -a "sensitivedataorigin|decrypt|userwithauth"

tpm2_load -C primary.ctx -r key.prv -u key.pub -c key.ctx

tpm2_readpublic -c key.ctx -o dup.pub

Test sign and encryption locally (so we can compare later in this)

echo "meet me at.." >file.txt

tpm2_rsaencrypt -c key.ctx  -o data.encrypted file.txt

tpm2_sign -c key.ctx -g sha256 -f plain -o sign.raw file.txt

Use openssl to sign

openssl dgst -sha256 -sign private.pem -out file.sign.sha256 file.txt

Compare the signatures are the same (meaning we imported the private key correctly).For example:

sha256sum sign.raw

a1b4e3fbaa29e6e46d95cff498150b6b8e7d9fd21182622e8f5a3ddde257879e

sha256sum file.sign.sha256

a1b4e3fbaa29e6e46d95cff498150b6b8e7d9fd21182622e8f5a3ddde257879e

Start an auth session and policy command to allow duplication

tpm2_startauthsession --policy-session -S session.dat

tpm2_policycommandcode -S session.dat -L dpolicy.dat TPM2_CC_Duplicate

Load the new_parent.pub file transferred from TPM-B

tpm2_loadexternal -C o -u new_parent.pub -c new_parent.ctx

Start the duplication

tpm2_duplicate -C new_parent.ctx -c key.ctx -G null  \
-p "session:session.dat" -r dup.dup -s dup.seed

Copy the following files to TPM-B:

  • dup.pub
  • dup.prv
  • dup.seed
  • (optionally data.encrypted just to test decryption)

On TPM-B

Start an auth,policy session

tpm2_startauthsession --policy-session -S session.dat

tpm2_policycommandcode -S session.dat -L dpolicy.dat TPM2_CC_Duplicate

Load the context we used to transfer

tpm2_flushcontext --transient-object

tpm2_load -C primary.ctx -u new_parent.pub -r new_parent.prv -c new_parent.ctx

Import the duplicated context against the parent we used

tpm2_import -C new_parent.ctx -u dup.pub -i dup.dup \
-r dup.prv -s dup.seed -L dpolicy.dat

Load the duplicated key context

tpm2_flushcontext --transient-object

tpm2_load -C new_parent.ctx -u dup.pub -r dup.prv -c dup.ctx

Test the imported key matches

  • Sign
echo "meet me at.." >file.txt

tpm2_sign -c dup.ctx -g sha256 -o sig.rss file.txt

dd if=sig.rss of=sign.raw bs=1 skip=6 count=256

Compare the signature file hash:

$ sha256sum sign.raw

a1b4e3fbaa29e6e46d95cff498150b6b8e7d9fd21182622e8f5a3ddde257879e
  • Decryption

If you used an external PEM file for the import, you can decrypt the data. Encrypted file transferred directly with out passphrase:

tpm2_flushcontext --transient-object

tpm2_rsadecrypt  -c dup.ctx -o data.ptext data.encrypted

# cat data.ptext 
meet me at..

If you used the TPM-generated RSA key on A that had the passphrase ('foo'), provide that during decryption

tpm2_rsadecrypt -p foo -c dup.ctx -o data.ptext data.encrypted

Author

@salrashid123