Being too strong can be a waste of time

The short story is to use Elliptic curves with size 256 or 384, and avoid wasting time investigating why using other sizes doesn’t always work.

The long story – part 1 the server

I had a RACF server certificate defined as

SIZE(521) NISTECC …

and using Chrome browser it failed. I did a lot of digging around and could see the gsktrace had

EXIT gsk_get_ec_parameters_info(): <— Exit status 0x00000000 (0) EC curve type 34, key size 521
ERROR send_v3_alert(): Sent SSL V3 alert 40 to 10.1.0.2[38736]

INFO edit_ciphers(): Server certificate ec curve 0034 not in supported ecurve tls extension. EC cipher suites disabled

Where curve type 34, also known as 0x0019 = secp521r.

When I changed the size to 256 it worked.

Digging in to this, the Client Hello part of the TLS handshake sent from Chrome had

Supported Groups (4 groups)
Supported Group: Reserved (GREASE) (0x2a2a)
Supported Group: x25519 (0x001d)
Supported Group: secp256r1 (0x0017)
Supported Group: secp384r1 (0x0018)

This is missing the 0x0019.

For Firefox, which worked, it had

Supported Groups (6 groups)
Supported Group: x25519 (0x001d)
Supported Group: secp256r1 (0x0017)
Supported Group: secp384r1 (0x0018)
Supported Group: secp521r1 (0x0019)
Supported Group: ffdhe2048 (0x0100)
Supported Group: ffdhe3072 (0x0101)

So after a couple of hours working on this ( and an overnight sleep-on-it) the solution was to use secp256r1 instead of secp521.

The long story – part 2 – the client

I also tried using a client certificate which was defined as a secp521r1. This has similar problems. When I defined it as secp256r1 or secp384r1 it worked.

The gsktrace included

INFO read_v3_certificate(): Certificate key algorithm 13, Signature algorithm 84
ENTRY gsk_get_ec_parameters_info(): —> keyInfo size 12
EXIT gsk_get_ec_parameters_info(): <— Exit status 0x00000000 (0) EC curve type 34, key size 521
ERROR read_v3_certificate(): Client certificate elliptic curve not in clients supported elliptic curve list

I also know more about the TLS hand shake – which has more twists and turns every time I look at it!