-----Original Message----- From: Dhawale, Dinkar Sent: Tuesday, January 8, 2019 1:56 AM To: Roberts, William C <william.c.roberts(a)intel.com&gt;; Rawat, Arvind <arvind.rawat(a)intel.com&gt;; tpm2(a)lists.01.org Cc: Ghosh, Arijit <arijit.ghosh(a)intel.com&gt; Subject: RE: Need help in understanding tpm2_makecredential command + Adding alias _____________________________________________ From: Dhawale, Dinkar Sent: Tuesday, January 8, 2019 12:13 AM To: Roberts, William C <william.c.roberts(a)intel.com <mailto:william.c.roberts@intel.com> >; Rawat, Arvind <arvind.rawat(a)intel.com <mailto:arvind.rawat@intel.com> > Cc: Ghosh, Arijit <arijit.ghosh(a)intel.com <mailto:arijit.ghosh@intel.com> > Subject: RE: Need help in understanding tpm2_makecredential command Hi William, We fixed following after parsing last 256 bytes for Endorsement Key Modulus from tpm2_readpublic. However, we have landed in strange issue. We have java implementation of tpm2_makecredentials in our code as it is done on Attestation Server (which can be installed on VM). This implementation needed few output formatting changes as per new format of tpm2_activatecredentials. We did those changes and these were working fine on Purley server. However, same changes are not working on Simics server. Simics server crashes while activating credentials and we get following exception on console, ASSERTION ERROR: Key is too large. D:\mp\builds\4653165\simics-base\src\extensions\crypto- engine\backends/openssl-wrap.c:599: Simics core build 5189 Module: Fri Nov 09 13:14:08 2018 Please report this. #0 0x000000006a87fad0 (C:\PROGRA~1\Simics\SIMICS~1\SIMICS~1.165\win64\bin\libsimics-common.dll + 0x3fad0) The simulation state has been corrupted. Simulation cannot continue. Please restart Simics. Surprisingly, we did not face this issue when we used tpm2_makecredentials commands itself. Could you please review our attached implementation (from line no 100)? and let us know if we need to make any changes with upgraded tpm2 tools. << File: Tpm2.java >> Thanks, -Dinkar -----Original Message----- From: Roberts, William C Sent: Monday, December 31, 2018 9:01 PM To: Rawat, Arvind <arvind.rawat(a)intel.com <mailto:arvind.rawat@intel.com> > Cc: Dhawale, Dinkar <dinkar.dhawale(a)intel.com <mailto:dinkar.dhawale@intel.com> >; Ghosh, Arijit <arijit.ghosh(a)intel.com <mailto:arijit.ghosh@intel.com> > Subject: RE: Need help in understanding tpm2_makecredential command From what I understand, this TPM2_MakeCredential is a convenience function that can be done externally from the TPM as described in section 24 of: https://trustedcomputinggroup.org/wp-content/uploads/TPM-Rev-2.0-Part-1- Architecture-01.38.pdf The only other thing I could think of making something end up at size 256 is: 1. The key length of an RSA2048 key (as you pointed out, modulus size) 2. The name-algorithm as part of the KDFa routine to generate the symmetric key. We use sha256 as the default name algorithm in all places (if we don't that's a bug). Bill > -----Original Message----- > From: Rawat, Arvind > Sent: Friday, December 28, 2018 8:21 AM > To: Roberts, William C <william.c.roberts(a)intel.com <mailto:william.c.roberts@intel.com> > > Cc: Dhawale, Dinkar <dinkar.dhawale(a)intel.com <mailto:dinkar.dhawale@intel.com> >; Ghosh, Arijit > <arijit.ghosh(a)intel.com <mailto:arijit.ghosh@intel.com> > > Subject: Need help in understanding tpm2_makecredential command > > Hi William, > > > > I was trying to code a software implementation of tpm2_makecredential > command. From what I understood by looking at the tpm2_makecredential > code under tpm2-tools, I think that the command is performing RSA > encryption on random data and generating an encrypted secret along > with a credential blob with a 8 byte header appended at the beginning. > I tried to do the same in my software implementation as well. I am > using the same set of input in my implementation as used by tpm2_makecredentail command from tpm2-tools. > > > > However, I found that the output generated from my implementation and > tpm2- tools implementation are different. I tried comparing the hex > bytes of the two output blobs and I found that the encrypted secret > part is different in the two blobs. With tpm2-tools implementation, I > am getting encrypted secret of size 256 bytes, whereas with my > implementation, the encrypted secret size is coming as > 316 bytes. I did some research on the RSA encryption and found that > the encrypted secret size depends on the modulus size. If a key > modulus is 256 bytes in size, then it will generate 256 bytes > encrypted secret no matter what the input data size is. > > > > Based on the above research, I tried to found the modulus size of EK > public key which is passed as an input to tpm2_makecredential command, > and found that the modulus size is 316 bytes. Hence, I am getting > encrypted secret of size 316 bytes with my implementation. But my > question is, since I am passing same EK public key to tpm2-tools > implementation as well, then how come there the encrypted secret is > getting generated of size 256 bytes. Ideally it should also be of > 316 bytes. Can you tell me what additional thing the tpm2-tools > implementation of tpm2_makecredential command is doing other than RSA > encryption that is causing the encrypted secret size to be of 256 bytes instead of 316 bytes. > > > > I am using tpm2-tools version 3.0.4 with tpm2-abrmd service. > > > > Thanks, > > Arvind