se050 ed448 question

[Changhawn]

Dear 

I am finding conflicting information across documents regarding the SE050E's support for the Ed448 curve, and my own attempts to implement it have been unsuccessful.

Could you please confirm once again whether the SE050E supports Ed448?

When I execute the SE050E API to verify a certificate signature, the `WriteECKey` operation fails and returns error code `0x6985` (curve `0x42`), which indicates "Conditions of use not satisfied."
Upon further investigation, it appears that the "448" support on the SE050E refers to X448 DH rather than Ed448 signatures; could you please verify this?

[Kan_Li · Accepted solution]

Hi @Changhawn ,

Thank you for reaching out and for your thorough investigation. Your conclusion is correct, and I'd like to confirm the exact situation definitively.

The SE050E does NOT support Ed448 (Edwards Curve 448) for EdDSA digital signatures.

The "448" support advertised for the SE050E refers exclusively to X448 (Montgomery Curve 448) for Diffie-Hellman key exchange (ECDH). The two are distinct:

• X448 (Montgomery Curve448, curve ID 0x43) — fully supported on SE050E for ECDH/DH key exchange ✓
• Ed448 (Edwards Curve448, curve ID 0x42) — NOT available for EdDSA sign/verify operations ✗

This explains the 0x6985 ("Conditions of use not satisfied") error you are seeing on WriteECKey with curve 0x42. The curve ID 0x42 (ID_ECC_ED_448) exists in the applet's curve table but is explicitly marked as "Not available for use".

Regarding the conflicting documentation: Application Note AN13014 mentions "support for Edwards Curve448," which is misleading. The more accurate sources — the Plug & Trust middleware header se05x_enums.h and the APDU specification (AN12436, AN12973) — only define Montgomery Curve448 (0x43) for key exchange, with no EdDSA signing counterpart. We apologize for any confusion this documentation inconsistency may have caused.

Recommended workaround: Please implement Ed448 EdDSA in software. A well-tested option is the mbedTLS library, which provides a complete Ed448 implementation. The SE050E can continue to handle X448 key exchange in hardware, while the signing operations are handled in software on the host side.

Please let me know if you have any further questions.

Have a great day,
Kan

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