crypto.MessageSigner wrapper

go 1.25.1 introduced the crypto.MessageSigner which allows an implementation to perform the digest operation on the raw data to sign. see issues/63405

Previously when you used crypto.Signer, an implementation expected the data to already be hashed.

this is a really trivial wrapper allows you to pass a crypto.Signer implementation and make use of crypto.MessageSigner interface.

to be honest, i don't know of any real-life implementation of this and you'll only likley find the very few i authored here for some time now

• tpmsigner
• pkcssigner

(i mostly wrote this because i filed a git issue)

Anyway, to use this, just acquire a crypto.Singer and pass that as a crypto.MessageSigner. The default hash is using the options provided hash function. One copmplication of using the opts hashFunc is that for certain backends like a tpmsigner if the underlying key is restricted, you need to allow the TPM to hash the data and return a validation ticket.

import (
	"github.com/salrashid123/messagesigner"
)
    // get any private key;  rsa.privateKey implements crypto.Signer
	mysigner, err := rsa.GenerateKey(rand.Reader, 2048)

    // get the wrapper
	r, err := messagesigner.NewMessageSigner(&MessageSignerConfig{
		Signer: mysigner,
	})

    // raw data to sign
	rawData := []byte("foo")

    // hash it
	h := sha256.New()
	h.Write(rawData)
	digest := h.Sum(nil)

    // use as crypto.Signer on the digest
	s, err := r.Sign(rand.Reader, digest, crypto.SHA256)
	fmt.Printf("signature using Sign(): %s\n", base64.StdEncoding.EncodeToString(s))

    // now just pass the raw data in
    //    the SignMessage will autohash the data
	sr, err := r.SignMessage(rand.Reader, rawData, crypto.SHA256)

    // the signatures are the same (in this case, for rsassa)
	fmt.Printf("signature using MessageSigner: %s\n", base64.StdEncoding.EncodeToString(sr))

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