Cloud KMS SOFTWARE Ed25519: exact asymmetricSign message-size limit?

[Joseph_Essert]

I’m implementing message-size validation before Cloud KMS signing and need an authoritative limit for this configuration:

• Purpose: ASYMMETRIC_SIGN

• Algorithm: EC_SIGN_ED25519 (PureEdDSA)

• Protection: SOFTWARE

• Location: us-central1

• API: REST cryptoKeyVersions.asymmetricSign

The request sends the original UTF-8 message as base64 in data, with dataCrc32c. It does not send a digest.

Could a Google Cloud KMS specialist confirm:

1. The exact maximum raw-message size in bytes.

2. Whether the limit counts decoded message bytes, base64 characters, or the complete serialized request.

3. Whether the maximum is inclusive and whether a separate REST request-size limit applies.

4. Whether this is a fixed limit or adjustable quota.

5. An authoritative reference specifically applicable to SOFTWARE Ed25519 signing.

We reviewed these references but could not establish that exact contract:

• Key purposes and algorithms  |  Cloud Key Management Service  |  Google Cloud Documentation

• Method: cryptoKeyVersions.asymmetricSign  |  Cloud Key Management Service  |  Google Cloud Documentation

• Creating and validating digital signatures  |  Cloud Key Management Service  |  Google Cloud Documentation

• Quotas  |  Cloud Key Management Service  |  Google Cloud Documentation

This concerns message size, not operation-rate quotas. We need to reject oversized messages locally before requesting signatures.

Thank you.