Using OpenSSL with ECDH (Elliptic Curve Diffie-Hellman) involves creating an ECDH key pair, performing key exchange, and optionally encrypting/decrypting data using the shared secret. Here's a step-by-step guide on how to do this:

1. Generate ECDH Key Pairs: You'll need to generate ECDH key pairs for both parties (e.g., Alice and Bob). You can use OpenSSL for this purpose.
   
   # Generate Alice's ECDH private key and public key
openssl ecparam -name secp256k1 -genkey -out alice_private.pem openssl ec -in alice_private.pem -pubout -out alice_public.pem

# Generate Bob's ECDH private key and public key

openssl ecparam -name secp256k1 -genkey -out bob_private.pem openssl ec -in bob_private.pem -pubout -out bob_public.pem

2. Replace secp256k1 with the curve of your choice. You can list available curves using openssl ecparam -list_curves

3. Exchange Public Keys: Alice and Bob exchange their public keys (alice_public.pem and bob_public.pem, respectively)

4. Compute the Shared Secret: Either Alice or Bob can compute the shared secret by combining their private key with the other party's public key.

   For example, if Alice wants to compute the shared secret:

   openssl pkeyutl -derive -inkey alice_private.pem -peerkey bob_public.pem -out shared_secret.bin

   This will store the shared secret in shared_secret.bin Bob can perform the same calculation using his private key and Alice's public key.

5. (Optional) Encrypt/Decrypt Data: If you want to encrypt and decrypt data using the shared secret, you can use symmetric encryption algorithms like AES.

   • Encrypt data with the shared secret:

     openssl enc -aes-256-cbc -in plaintext.txt -out ciphertext.bin -K $(xxd -p shared_secret.bin) -iv 0

     Replace plaintext.txt with your input file.

   • Decrypt data with the shared secret:

     openssl enc -d -aes-256-cbc -in ciphertext.bin -out decrypted.txt -K $(xxd -p shared_secret.bin) -iv 0

     Replace ciphertext.bin with your encrypted file.

   Note that using symmetric encryption like AES with ECDH allows both parties to securely exchange data without revealing the shared secret during transmission.

Make sure to protect your private keys and shared secrets, as they are sensitive information. Additionally, the choice of curve and encryption parameters may vary depending on your specific security requirements.

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