TASK

Use ECDH for key exchange

ECDH (Elliptic Curve Diffie-Hellman) establishes shared secrets using elliptic curves. Smaller keys, same security as DH.

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ECDH: Alice has private key a, public key A = a*G. Bob has private key b, public key B = b*G. They exchange public keys. Alice computes shared = a*B = a*b*G. Bob computes shared = b*A = b*a*G. Both get the same point — the x-coordinate is the shared secret. Use Curve25519 for ECDH — it is fast, constant-time, and secure.
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Random Bytes Generator →
Generate the private key — a random integer in [1, n-1] where n is the curve order. For Curve25519, the private key is 32 random bytes with specific bit clamping. Use a CSPRNG — the private key must be unpredictable.
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Hash Calculator →
Hash the shared secret to derive a symmetric key. The raw x-coordinate of the shared point is not uniformly distributed — hash it with SHA-256 to get a uniformly random key. Use HKDF for key derivation: K = HKDF(shared_secret, info="encryption key").

Frequently asked questions

ECDH vs DH?: ECDH uses elliptic curve groups — smaller keys for equivalent security (256-bit ECDH ≈ 3072-bit DH). ECDH is faster and uses less bandwidth. Use ECDH with Curve25519 for new systems. Classic DH with large primes is still secure but slower.
What is X25519?: X25519 is the Diffie-Hellman key exchange function using Curve25519. It is specified in RFC 7748 and used in TLS 1.3, SSH, and Signal. It is fast, constant-time (resistant to timing attacks), and handles all edge cases (no invalid curve attacks). Use X25519 for ECDH in new systems.

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Learn more

Want to understand why?

How to Generate Cryptographic Nonces

An intermediate-level guide covering nonce, random, cryptography. Includes step-by-step instructions, common errors, troubleshooting tips, and security considerations for nonce.

How to Generate CUIDs

A beginner-level guide covering cuid, id, unique. Includes step-by-step instructions, common errors, troubleshooting tips, and security considerations for cuid.

Base36 represents numbers with 36 characters (0-9 and a-z), mixing digits and letters, and is commonly used for short links and compact IDs.

Base62 represents numbers with 62 characters (0-9, a-z, A-Z); it is case-sensitive, denser than Base36, and widely used for short links and ID generation.

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