mTLS with TPM bound private key

Simple http client/server in golang where the private key used in the connection is generated and embedded within a Trusted Platform Module.

This repo mostly uses the crypto.Signer implementation from my own library implementing that interface for TPM ("github.com/salrashid123/signer/tpm") and not the one from from go-tpm-tools.

The steps here will create a client and server using a local software tpm swtpm. On that TPM, create two RSA keys, generate a CSR using those keys, then an external CA will issue an x509 cert using that csr.

Finally, the client will establish an mTLS https connection to the server

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• update 7/17/23: This sample use RSA keys involves several steps and a custom crypto.signer. If you want to see one-way TLS where the server's private key is embedded in a TPM and the private key is cryptographically verified (tpm remote attestation), please instead see https://github.com/salrashid123/tls_ak

for python, see Python mTLS client/server with TPM based key

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NOTE: this repo is not supported by Google

To use this sample, you'll need:

• golang
• openssl v3 (with https://github.com/tpm2-software/tpm2-openssl)
• software tpm (https://github.com/stefanberger/swtpm)

The TPM based private keys conforms to ASN.1 Specification for TPM 2.0 Key Files which in its basic mode is compatible with openssl

QuickStart

if you want to use the keys provided in this repo, just startup software TPMs:

The following will startup two software TPMs where the client and server keys reside

• Server:

cd certs/
swtpm socket --tpmstate dir=myvtpm --tpm2 --server type=tcp,port=2321 --ctrl type=tcp,port=2322 --flags not-need-init,startup-clear --log level=2

export TPM2TOOLS_TCTI="swtpm:port=2321"
export TPM2OPENSSL_TCTI="swtpm:port=2321"
export TPM2TSSENGINE_TCTI="swtpm:port=2321"
export OPENSSL_MODULES=/usr/lib/x86_64-linux-gnu/ossl-modules/   # or wherever tpm2.so sits, eg /usr/lib/x86_64-linux-gnu/ossl-modules/tpm2.so


$ openssl list  -provider tpm2  -provider default  --providers
Providers:
  default
    name: OpenSSL Default Provider
    version: 3.2.0
    status: active
  tpm2
    name: TPM 2.0 Provider
    version: 1.2.0-25-g87082a3
    status: active

$ openssl rsa -provider tpm2  -provider default -in server_key.pem --text

### test with openssl server
cd certs/
openssl s_server  -provider tpm2  -provider default  \
        -cert server.crt \
      -key server_key.pem \
      -port 8081 \
      -CAfile ca/root-ca.crt \
      -tlsextdebug \
      -tls1_3  \
      -trace \
      -WWW

## or golang server
# go run src/server/server.go -cacert certs/ca/root-ca.crt \
#    -servercert certs/server.crt \
#     --severkey=certs/server_key.pem -port :8081 \
#       --tpm-path="127.0.0.1:2321"

You can test the config locally using the pre-generated client certificates provided in this repo

curl -v -H "Host: server.domain.com"  --resolve  server.domain.com:8081:127.0.0.1 \
   --cert certs/user10.crt --key certs/user10.key \
    --cacert ca/root-ca.crt https://server.domain.com:8081/index.html

• Client:

cd certs/
swtpm socket --tpmstate dir=myvtpm2 --tpm2 --server type=tcp,port=2341 --ctrl type=tcp,port=2342 --flags not-need-init,startup-clear --log level=2

export TPM2TOOLS_TCTI="swtpm:port=2341"
export TPM2OPENSSL_TCTI="swtpm:port=2341"
export TPM2TSSENGINE_TCTI="swtpm:port=2341"
export OPENSSL_MODULES=/usr/lib/x86_64-linux-gnu/ossl-modules/ 

go run src/client/client.go -cacert certs/ca/root-ca.crt \
  --clientkey=certs/client_key.pem --pubCert=certs/client.crt  \
   --address localhost --tpm-path="127.0.0.1:2341"

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Appendix

The following sewts up your own certs and software TPMs

Server

The following will setup a server cert where the private key is on a TPM

## if you'd rather use a software tpm than a real one, set the following and use --tpm-path="127.0.0.1:2321"

mkdir myvtpm
sudo swtpm_setup --tpmstate myvtpm --tpm2 --create-ek-cert
sudo swtpm socket --tpmstate dir=myvtpm --tpm2 --server type=tcp,port=2321 --ctrl type=tcp,port=2322 --flags not-need-init,startup-clear --log level=2

export TPM2TOOLS_TCTI="swtpm:port=2321"
export TPM2OPENSSL_TCTI="swtpm:port=2321"
export TPM2TSSENGINE_TCTI="swtpm:port=2321"
export OPENSSL_MODULES=/usr/lib/x86_64-linux-gnu/ossl-modules/   # or wherever tpm2.so sits, eg /usr/lib/x86_64-linux-gnu/ossl-modules/tpm2.so
# export TSS2_LOG=esys+debug

printf '\x00\x00' > unique.dat
tpm2_createprimary -C o -G ecc -g sha256  -c rprimary.ctx -a "fixedtpm|fixedparent|sensitivedataorigin|userwithauth|noda|restricted|decrypt" -u unique.dat

tpm2_create -G rsa2048:rsapss:null -g sha256 -u server.pub -r server.priv -C rprimary.ctx
tpm2_flushcontext -s && tpm2_flushcontext -t && tpm2_flushcontext -l
tpm2_load -C rprimary.ctx -u server.pub -r server.priv -c server.ctx
tpm2_flushcontext -s && tpm2_flushcontext -t && tpm2_flushcontext -l

## convert rkey.pub rkey.priv to PEM format
## using https://github.com/salrashid123/tpm2genkey/releases
./tpm2genkey --mode=tpm2pem --public=server.pub --private=server.priv --out=server_key.pem

# create a csr using the tpm key...i have it in this repo:
openssl rsa -provider tpm2  -provider default -in server_key.pem --text

export SAN="DNS:server.domain.com"
openssl req -new  -provider tpm2  -provider default    -config server.conf \
  -out server.csr  \
  -key server_key.pem  -reqexts server_reqext   \
  -subj "/C=US/O=Google/OU=Enterprise/CN=server.domain.com" 

openssl req -in server.csr -noout -text

openssl ca \
    -config single-root-ca.conf \
    -in server.csr \
    -out server.crt  \
    -extensions server_ext

cd certs/
openssl s_server  -provider tpm2  -provider default  \
        -cert server.crt \
      -key server_key.pem \
      -port 8081 \
      -CAfile ca/root-ca.crt \
      -tlsextdebug \
      -tls1_3  \
      -trace \
      -WWW

# run the server as go
go run src/server/server.go -cacert certs/ca/root-ca.crt \
   -servercert certs/server.crt \
    --severkey=certs/server_key.pem -port :8081 \
      --tpm-path="127.0.0.1:2321"

Client

For the client,

mkdir myvtpm2
sudo swtpm_setup --tpmstate myvtpm2 --tpm2 --create-ek-cert
sudo swtpm socket --tpmstate dir=myvtpm2 --tpm2 --server type=tcp,port=2341 --ctrl type=tcp,port=2342 --flags not-need-init,startup-clear --log level=2

export TPM2TOOLS_TCTI="swtpm:port=2341"
export TPM2OPENSSL_TCTI="swtpm:port=2341"
export TPM2TSSENGINE_TCTI="swtpm:port=2341"
export OPENSSL_MODULES=/usr/lib/x86_64-linux-gnu/ossl-modules/   # or wherever tpm2.so sits, eg /usr/lib/x86_64-linux-gnu/ossl-modules/tpm2.so

printf '\x00\x00' > unique.dat
tpm2_createprimary -C o -G ecc -g sha256  -c rcprimary.ctx -a "fixedtpm|fixedparent|sensitivedataorigin|userwithauth|noda|restricted|decrypt" -u unique.dat

tpm2_create -G rsa2048:rsapss:null -g sha256 -u client.pub -r client.priv -C rcprimary.ctx
tpm2_flushcontext -s && tpm2_flushcontext -t && tpm2_flushcontext -l
tpm2_load -C rcprimary.ctx -u client.pub -r client.priv -c client.ctx
tpm2_flushcontext -s && tpm2_flushcontext -t && tpm2_flushcontext -l

## convert rkey.pub rkey.priv to PEM format
## using https://github.com/salrashid123/tpm2genkey/releases
./tpm2genkey --mode=tpm2pem --public=client.pub --private=client.priv --out=client_key.pem

# create a csr using the tpm key...i have it in this repo:
openssl rsa -provider tpm2  -provider default -in client_key.pem --text

export SAN="DNS:client.domain.com"
openssl req -new  -provider tpm2  -provider default    -config client.conf \
  -out client.csr  \
  -key client_key.pem  -reqexts client_reqext   \
  -subj "/C=US/O=Google/OU=Enterprise/CN=client.domain.com" 

openssl req -in client.csr -noout -text

openssl ca \
    -config single-root-ca.conf \
    -in client.csr \
    -out client.crt  \
    -extensions client_ext

go run src/client/client.go -cacert certs/ca/root-ca.crt \
  --clientkey=certs/client_key.pem --pubCert=certs/client.crt  \
   --address localhost --tpm-path="127.0.0.1:2341"

At this point, you should see a simple 'ok' from the sever

References

Other references:

• Trusted Platform Module (TPM) and Google Cloud KMS based mTLS auth to HashiCorp Vault
• Docker daemon mTLS with Trusted Platform Module
• TPM TLS with nginx, openssl: https://github.com/salrashid123/go_tpm_https#nginx]

RSA-PSS padding:

• Synthesized PSS support
• PSS advertising during TLS handshake for TPM signing
• TLS salt length auto detection, switch from DIGEST to AUTO