diff --git a/core/embed/extmod/modtrezorcrypto/modtrezorcrypto-ed25519.h b/core/embed/extmod/modtrezorcrypto/modtrezorcrypto-ed25519.h
index a09c18e919..fb3fa4c7c8 100644
--- a/core/embed/extmod/modtrezorcrypto/modtrezorcrypto-ed25519.h
+++ b/core/embed/extmod/modtrezorcrypto/modtrezorcrypto-ed25519.h
@@ -100,16 +100,16 @@ STATIC MP_DEFINE_CONST_FUN_OBJ_VAR_BETWEEN(mod_trezorcrypto_ed25519_sign_obj, 2,
 #if !BITCOIN_ONLY
 
 /// def sign_ext(
-///     secret_key: bytes, secret_extension: bytes, message: bytes
+///     secret_scalar: bytes, secret_extension: bytes, message: bytes
 /// ) -> bytes:
 ///     """
-///     Uses secret key to produce the cardano signature of message.
+///     Uses extended secret key to produce the cardano signature of message.
 ///     """
-STATIC mp_obj_t mod_trezorcrypto_ed25519_sign_ext(mp_obj_t secret_key,
+STATIC mp_obj_t mod_trezorcrypto_ed25519_sign_ext(mp_obj_t secret_scalar,
                                                   mp_obj_t secret_extension,
                                                   mp_obj_t message) {
   mp_buffer_info_t sk = {0}, skext = {0}, msg = {0};
-  mp_get_buffer_raise(secret_key, &sk, MP_BUFFER_READ);
+  mp_get_buffer_raise(secret_scalar, &sk, MP_BUFFER_READ);
   mp_get_buffer_raise(secret_extension, &skext, MP_BUFFER_READ);
   mp_get_buffer_raise(message, &msg, MP_BUFFER_READ);
   if (sk.len != 32) {
diff --git a/core/mocks/generated/trezorcrypto/ed25519.pyi b/core/mocks/generated/trezorcrypto/ed25519.pyi
index 451b346821..735c4dce3b 100644
--- a/core/mocks/generated/trezorcrypto/ed25519.pyi
+++ b/core/mocks/generated/trezorcrypto/ed25519.pyi
@@ -24,10 +24,10 @@ def sign(secret_key: bytes, message: bytes, hasher: str = "") -> bytes:
 
 # extmod/modtrezorcrypto/modtrezorcrypto-ed25519.h
 def sign_ext(
-    secret_key: bytes, secret_extension: bytes, message: bytes
+    secret_scalar: bytes, secret_extension: bytes, message: bytes
 ) -> bytes:
     """
-    Uses secret key to produce the cardano signature of message.
+    Uses extended secret key to produce the cardano signature of message.
     """
 
 
diff --git a/crypto/ed25519-donna/ed25519.c b/crypto/ed25519-donna/ed25519.c
index 6b7413421d..9f7abfb1ae 100644
--- a/crypto/ed25519-donna/ed25519.c
+++ b/crypto/ed25519-donna/ed25519.c
@@ -101,23 +101,17 @@ ED25519_FN(ed25519_cosi_sign) (const unsigned char *m, size_t mlen, const ed2551
 }
 
 void
-ED25519_FN(ed25519_sign_ext) (const unsigned char *m, size_t mlen, const ed25519_secret_key sk, const ed25519_secret_key skext, ed25519_signature RS) {
+ED25519_FN(ed25519_sign_ext) (const unsigned char *m, size_t mlen, const ed25519_secret_key secret_scalar, const ed25519_secret_key skext, ed25519_signature RS) {
 	ed25519_hash_context ctx;
 	bignum256modm r = {0}, S = {0}, a = {0};
 	ge25519 ALIGN(16) R = {0};
 	ge25519 ALIGN(16) A = {0};
 	ed25519_public_key pk = {0};
-	hash_512bits extsk = {0}, hashr = {0}, hram = {0};
-
-	/* we don't stretch the key through hashing first since its already 64 bytes */
-
-	memcpy(extsk, sk, 32);
-	memcpy(extsk+32, skext, 32);
-
+	hash_512bits hashr = {0}, hram = {0};
 
 	/* r = H(aExt[32..64], m) */
 	ed25519_hash_init(&ctx);
-	ed25519_hash_update(&ctx, extsk + 32, 32);
+	ed25519_hash_update(&ctx, skext, 32);
 	ed25519_hash_update(&ctx, m, mlen);
 	ed25519_hash_final(&ctx, hashr);
 	expand256_modm(r, hashr, 64);
@@ -128,8 +122,7 @@ ED25519_FN(ed25519_sign_ext) (const unsigned char *m, size_t mlen, const ed25519
 	ge25519_pack(RS, &R);
 
 	/* a = aExt[0..31] */
-	expand256_modm(a, extsk, 32);
-	memzero(&extsk, sizeof(extsk));
+	expand256_modm(a, secret_scalar, 32);
 
 	/* A = aB */
 	ge25519_scalarmult_base_niels(&A, ge25519_niels_base_multiples, a);
diff --git a/crypto/ed25519-donna/ed25519.h b/crypto/ed25519-donna/ed25519.h
index 46544dda08..defaf43639 100644
--- a/crypto/ed25519-donna/ed25519.h
+++ b/crypto/ed25519-donna/ed25519.h
@@ -22,7 +22,7 @@ void ed25519_publickey_ext(const ed25519_secret_key extsk, ed25519_public_key pk
 
 int ed25519_sign_open(const unsigned char *m, size_t mlen, const ed25519_public_key pk, const ed25519_signature RS);
 void ed25519_sign(const unsigned char *m, size_t mlen, const ed25519_secret_key sk, ed25519_signature RS);
-void ed25519_sign_ext(const unsigned char *m, size_t mlen, const ed25519_secret_key sk, const ed25519_secret_key skext, ed25519_signature RS);
+void ed25519_sign_ext(const unsigned char *m, size_t mlen, const ed25519_secret_key secret_scalar, const ed25519_secret_key skext, ed25519_signature RS);
 
 int ed25519_scalarmult(ed25519_public_key res, const ed25519_secret_key sk, const ed25519_public_key pk);