How to create a shared secret with DH

[mramonlopez]

I'm trying to generate a shared secret between two elliptic curve key pairs and I can't get it to work. The secrets generated are always different. It doesn't work for me either using pkcs11-tool or in my Go code.

This is what I'm doing on the command line with pkcs11-tool:

rm *.der *.raw

# Generate test keys
sudo pkcs11-tool --modul /usr/lib/softhsm/libsofthsm2.so --login -p "123456" --login-type user \
    --keypairgen --id 1 --label "bob" --key-type EC:prime256v1
sudo pkcs11-tool --modul /usr/lib/softhsm/libsofthsm2.so --login -p "123456" --login-type user \
    --keypairgen --id 2 --label "alice" --key-type EC:prime256v1

# Extract public keys
sudo pkcs11-tool --modul /usr/lib/softhsm/libsofthsm2.so --label "bob" \
    --read-object --type pubkey -o bob-public.der
sudo pkcs11-tool --modul /usr/lib/softhsm/libsofthsm2.so --label "alice" \
    --read-object --type pubkey -o alice-public.der

# DH: Bob priv key with Alice pub key
sudo pkcs11-tool --modul /usr/lib/softhsm/libsofthsm2.so --login -p "123456" --login-type user \
    --label "bob" --derive -i alice-public.der -m ECDH1-DERIVE -o bob_shared_secret.raw

# DH: Alice priv key with Bob pub key
sudo pkcs11-tool --modul /usr/lib/softhsm/libsofthsm2.so --login -p "123456" --login-type user \
    --label "alice" --derive -i bob-public.der -m ECDH1-DERIVE -o alice_shared_secret.raw

echo "################################################################"
xxd -p -c 256 bob_shared_secret.raw
xxd -p -c 256 alice_shared_secret.raw
echo "################################################################"

What am I doing wrong?

[popovec]

You cannot use the --label switch, you must use the --id switch:

pkcs11-tool --modul /usr/lib/softhsm/libsofthsm2.so --login -p "123456" --login-type user \
    --id 1 --derive -i alice-public.der -m ECDH1-DERIVE -o bob_shared_secret.raw

pkcs11-tool --modul /usr/lib/softhsm/libsofthsm2.so --login -p "123456" --login-type user \
    --id 2 --derive -i bob-public.der -m ECDH1-DERIVE -o alice_shared_secret.raw

[mramonlopez]

Hi popovec,

this not change the result. By running this code:

# DH: Bob priv key with Alice pub key
sudo pkcs11-tool --modul /usr/lib/softhsm/libsofthsm2.so --login -p "123456" --login-type user \
    --label "bob" --derive -i alice-public.der -m ECDH1-DERIVE -o bob_shared_secret.raw

# DH: Alice priv key with Bob pub key
sudo pkcs11-tool --modul /usr/lib/softhsm/libsofthsm2.so --login -p "123456" --login-type user \
    --label "alice" --derive -i bob-public.der -m ECDH1-DERIVE -o alice_shared_secret.raw

sudo pkcs11-tool --modul /usr/lib/softhsm/libsofthsm2.so --login -p "123456" --login-type user \
    --id 1 --derive -i alice-public.der -m ECDH1-DERIVE -o bob_shared_secret2.raw

# DH: Alice priv key with Bob pub key
sudo pkcs11-tool --modul /usr/lib/softhsm/libsofthsm2.so --login -p "123456" --login-type user \
    --id 2 --derive -i bob-public.der -m ECDH1-DERIVE -o alice_shared_secret2.raw

echo "################################################################"
xxd -p -c 256 bob_shared_secret.raw
xxd -p -c 256 alice_shared_secret.raw
xxd -p -c 256 bob_shared_secret2.raw
xxd -p -c 256 alice_shared_secret2.raw
echo "################################################################"

I get

Using slot 0 with a present token (0x393c5500)
Using slot 0 with a present token (0x393c5500)
Using slot 0 with a present token (0x393c5500)
Using derive algorithm 0x00001050 ECDH1-DERIVE
Using slot 0 with a present token (0x393c5500)
Using derive algorithm 0x00001050 ECDH1-DERIVE
Using slot 0 with a present token (0x393c5500)
Using derive algorithm 0x00001050 ECDH1-DERIVE
Using slot 0 with a present token (0x393c5500)
Using derive algorithm 0x00001050 ECDH1-DERIVE
################################################################
d67e515f48ce6232fe4729484fb50e47beb3411def6de44935347c56e0235f3b
ee38f303ba94b33845b711755ee757152aa08e000387d88a044b9c731f3d2aa7
d67e515f48ce6232fe4729484fb50e47beb3411def6de44935347c56e0235f3b
e765501cd10ba4453b02519124f25813000b2fa3ca5db6232dcec797144db104
################################################################

There is no difference by using -I or --label, and 'shared' secrets are not the same for Alice and Bob

[popovec]

this is my test script:
(note that the script also contains softhsm initialization, I will not run it under root/via sudo)

#!/bin/bash
set -e
SOPIN="12345678"
PIN="123456"

P11LIB=/usr/lib/softhsm/libsofthsm2.so

echo "directories.tokendir = .tokens/" > .softhsm2.conf
export SOFTHSM2_CONF=".softhsm2.conf"

rm -rf .tokens
mkdir .tokens
softhsm2-util --init-token --slot 0 --label "SC test" --so-pin="$SOPIN" --pin="$PIN"

pkcs11-tool --modul /usr/lib/softhsm/libsofthsm2.so --login -p "123456" --login-type user \
    --keypairgen --id 1 --label "bob" --key-type EC:prime256v1

pkcs11-tool --modul /usr/lib/softhsm/libsofthsm2.so --login -p "123456" --login-type user \
    --keypairgen --id 2 --label "alice" --key-type EC:prime256v1


pkcs11-tool --modul /usr/lib/softhsm/libsofthsm2.so --label "bob" \
    --read-object --type pubkey -o bob-public.der
    
pkcs11-tool --modul /usr/lib/softhsm/libsofthsm2.so --label "alice" \
    --read-object --type pubkey -o alice-public.der
    
pkcs11-tool --modul /usr/lib/softhsm/libsofthsm2.so --login -p "123456" --list-objects --type privkey

pkcs11-tool --modul /usr/lib/softhsm/libsofthsm2.so --login -p "123456" --login-type user \
    --id 1 --derive -i alice-public.der -m ECDH1-DERIVE -o bob_shared_secret.raw

pkcs11-tool --modul /usr/lib/softhsm/libsofthsm2.so --login -p "123456" --login-type user \
    --id 2 --derive -i bob-public.der -m ECDH1-DERIVE -o alice_shared_secret.raw
    

echo "################################################################"
xxd -p -c 256 bob_shared_secret.raw
xxd -p -c 256 alice_shared_secret.raw
diff -a bob_shared_secret.raw alice_shared_secret.raw && echo "OK"
echo "################################################################"

derived keys are the same.

$ softhsm2-util --version
2.6.1

[mramonlopez]

It worked!

In the initialization script of my Dev Container I was creating two other key pairs with ids 1 and 2. My bad

It seems that if you don't use -i it defaults to '1', maybe?

Is the mandatory use of -i documented anywhere? Is it a pkcs11, softhsm2 or pkcs11-tool requirement?

A few days ago I tried it with IBM HPCS and the same thing happened

Thank you very much! Now I'm going to try to make it work in my Go code

[popovec]

In my experience, using "--label" on various tokens did not always work correctly for me, here is an example, but from another tool (pkcs15-tool):

I can generate two different beams with the same label:

pkcs15-init --generate-key ec:prime256v1 --label key1 --id 22 --auth-id=1
pkcs15-init --generate-key ec:prime256v1 --label key1 --id 23 --auth-id=1

But the keys cannot have the same ID:

pkcs15-init --generate-key ec:prime256v1 --label key3 --id 22 --auth-id=1
Failed to generate key: Non unique object ID

I haven't found out yet if the "label" needs to be unique.