X25519
X25519 is the Diffie-Hellman function over Curve25519, defined in RFC 7748, used to establish a shared secret from a 32-byte public key and a 32-byte private key. Fast and resistant to many implementation pitfalls, it is the default elliptic-curve key exchange in TLS 1.3, SSH, WireGuard, and the Signal protocol, replacing older Diffie-Hellman groups.
Role in post-quantum hybrids
On its own X25519 is a classical scheme broken by Shor's algorithm, so it is being combined rather than replaced. Current TLS deployments run X25519 alongside ML-KEM-768 in a hybrid key-exchange group named X25519MLKEM768, which concatenates both shared secrets so the handshake stays secure if either component holds. TLS libraries and browsers have shipped this hybrid as a default group since 2024, making it one of the most widely deployed pieces of hybrid cryptography. The X25519 half preserves today's classical security while ML-KEM adds resistance to a future quantum computer.
Sources
Cite this entry
"X25519." postquantum.wiki. Updated July 11, 2026. https://postquantum.wiki/x25519@misc{pqwiki-x25519,
title = {X25519},
howpublished = {\url{https://postquantum.wiki/x25519}},
year = {2026},
note = {postquantum.wiki, updated 2026-07-11}
}