Add sntrup761x25519-sha512 post-quantum key exchange #335
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Common algorithm specific code is moved from common-kex.c to kex-*.c send_msg_kexdh_init() is simplified - parameters are no longer reused between first-follows and a subsequent kexdh_init. The default gen_kexcurve25519_param() is relatively fast, so recomputing is OK.
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This implements draft-kampanakis-curdle-ssh-pq-ke-04, a mechanism combining a post-quantum KEM with x25519. Adding a post-quantum KEM can avoid harvest-now-decrypt-later style attacks (captured traffic decrypted in future by a quantum computer, if they are created). Combining it with existing x25519 ensures that security is not weakened compared to the present status, if a weakness in the PQ KEM is discovered. A future commit will add concrete PQ methods, currently this code is unused. x25519 could be generalised later if needed.
This follows draft-ietf-sshm-ntruprime-ssh-01, using the sntrup761 implementation from supercop. It is available under both sntrup761x25519-sha512 and [email protected] names. Interoperability has been tested against OpenSSH 9.8 (client/server) and PuTTY 0.82 client. sntrup761.sh is taken from OpenSSH, to extract the code from the supercop distribution. KEX hash buffer size calculation has been updated to current algorithm limits, since sntrup761 was larger than the previous limit. Code size increases by approx 9kB (209 to 218kB) for a 32-bit armv7 build.
Change to q_s and q_c since pqhybrid isn't only EC.
Use separate fuzzers for client vs server since KEMs have an asymmetric API.
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