Asymmetric ciphering¶

This section covers two asymmetric-key operations:

RSA decryption
ECDH shared-secret derivation

Both use asymmetric keys, but they serve different purposes: RSA decrypts data, while ECDH derives a shared secret.

Prepare sample data¶

echo "This is a test string. Be safe, be secure." > data

RSA-OAEP is the sane default¶

First export the public key:

pkcs11-tool --read-object --pin 648219 --id 1 --type pubkey > 1.der
openssl rsa -inform DER -outform PEM -in 1.der -pubin > 1.pub

Encrypt on the host:

openssl pkeyutl -encrypt \
  -inkey 1.pub \
  -pubin \
  -pkeyopt rsa_padding_mode:oaep \
  -pkeyopt rsa_oaep_md:sha256 \
  -pkeyopt rsa_mgf1_md:sha256 \
  -in data \
  -out data.crypt

Decrypt in Pico HSM:

pkcs11-tool \
  --id 1 \
  --pin 648219 \
  --decrypt \
  --mechanism RSA-PKCS-OAEP \
  -i data.crypt

RSA-PKCS and RSA-X.509 exist, but carefully¶

Upstream examples also show RSA-PKCS and RSA-X-509.

Use the following guidance when choosing between these mechanisms:

RSA-PKCS exists largely for compatibility
RSA-X-509 is low-level and easy to misuse
neither should be your first recommendation when OAEP is available

ECDH is not encryption¶

With elliptic-curve keys, the usual operation is shared-secret derivation:

openssl ecparam -genkey -name prime192v1 > bob.pem
openssl ec -in bob.pem -pubout -outform DER > bob.der

pkcs11-tool \
  --pin 648219 \
  --id 11 \
  --derive \
  -i bob.der \
  -o mine-bob.der

And compare against the host-side derived secret:

openssl pkeyutl -derive -out bob-mine.der -inkey bob.pem -peerkey 11.pub
cmp bob-mine.der mine-bob.der

When cmp prints nothing, the two derived secrets match.

Practical caution¶

Asymmetric ciphering is where users often discover that:

the firmware is capable
the host tool syntax is obscure
the intended application still needs adaptation

This is common with low-level cryptographic tools, so test the full application path as well as the primitive itself.