Simplify and rename hash functions

pkg/attestation/sign.go

@@ -19,7 +19,7 @@ func SignDSSE(ctx context.Context, payload []byte, payloadType string, signer ds
 	encPayload := dsse.PAE(payloadType, payload)
 
 	// statement message digest
-	hash := util.S256(encPayload)
+	hash := util.SHA256(encPayload)
 
 	// sign message digest
 	sig, err := signer.Sign(ctx, hash)

pkg/attestation/verify.go

@@ -121,7 +121,7 @@ func verifySignature(ctx context.Context, sig Signature, payload []byte, keys Ke
 		return fmt.Errorf("error failed to decode signature: %w", err)
 	}
 	// verify payload ecdsa signature
-	ok = ecdsa.VerifyASN1(publicKey, util.S256(payload), signature)
+	ok = ecdsa.VerifyASN1(publicKey, util.SHA256(payload), signature)
 	if !ok {
 		return fmt.Errorf("payload signature is not valid")
 	}