diff --git a/ChangeLog.d/ecp_write_key.txt b/ChangeLog.d/ecp_write_key.txt
new file mode 100644
index 0000000..73354c8
--- /dev/null
+++ b/ChangeLog.d/ecp_write_key.txt
@@ -0,0 +1,8 @@
+Features
+   * The new function mbedtls_ecp_write_key_ext() is similar to
+     mbedtls_ecp_write_key(), but can be used without separately calculating
+     the output length.
+
+New deprecations
+   * mbedtls_ecp_write_key() is deprecated in favor of
+     mbedtls_ecp_write_key_ext().
diff --git a/docs/psa-transition.md b/docs/psa-transition.md
index e89128c..94b57eb 100644
--- a/docs/psa-transition.md
+++ b/docs/psa-transition.md
@@ -845,7 +845,6 @@
 
 ```
 unsigned char buf[PSA_BITS_TO_BYTES(PSA_VENDOR_ECC_MAX_CURVE_BITS)];
-size_t length = PSA_BITS_TO_BYTES(mbedtls_pk_bitlen(&pk));
 mbedtls_ecp_keypair *ec = mbedtls_pk_ec(&pk);
 psa_ecc_curve_t curve;
 {
@@ -862,7 +861,8 @@
     mbedtls_ecp_point_free(&Q);
     mbedtls_mpi_free(&d);
 }
-mbedtls_ecp_write_key(ec, buf, length);
+size_t length;
+mbedtls_ecp_write_key_ext(ec, &length, buf, sizeof(buf));
 psa_key_attributes_t attributes = PSA_KEY_ATTRIBUTES_INIT;
 psa_set_key_type(&attributes, PSA_KEY_TYPE_ECC_KEY_PAIR(curve));
 psa_set_key_usage_flags(&attributes, PSA_KEY_USAGE_... | ...);
@@ -900,8 +900,8 @@
 // Omitted: fill ec with key material
 // (the public key will not be used and does not need to be set)
 unsigned char buf[PSA_BITS_TO_BYTES(PSA_VENDOR_ECC_MAX_CURVE_BITS)];
-size_t length = PSA_BITS_TO_BYTES(mbedtls_pk_bitlen(&pk));
-mbedtls_ecp_write_key(&ec, buf, length);
+size_t length;
+mbedtls_ecp_write_key_ext(&ec, &length, buf, sizeof(buf));
 psa_ecc_curve_t curve = ...; // need to determine the curve family manually
 psa_key_attributes_t attributes = PSA_KEY_ATTRIBUTES_INIT;
 psa_set_key_attributes(&attributes, PSA_KEY_TYPE_ECC_KEY_PAIR(curve));
@@ -1300,7 +1300,7 @@
 The PSA API is a cryptography API, not an arithmetic API. As a consequence, there is no PSA equivalent for the ECC arithmetic functionality exposed by `ecp.h`:
 
 * Manipulation of point objects and input-output: the type `mbedtls_ecp_point` and functions operating on it (`mbedtls_ecp_point_xxx`, `mbedtls_ecp_copy`, `mbedtls_ecp_{set,is}_zero`, `mbedtls_ecp_tls_{read,write}_point`). Note that the PSA export format for public keys corresponds to the uncompressed point format (`MBEDTLS_ECP_PF_UNCOMPRESSED`), so [`psa_import_key`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__import__export/#group__import__export_1ga0336ea76bf30587ab204a8296462327b), [`psa_export_key`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__import__export/#group__import__export_1ga668e35be8d2852ad3feeef74ac6f75bf) and [`psa_export_public_key`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__import__export/#group__import__export_1gaf22ae73312217aaede2ea02cdebb6062) are equivalent to `mbedtls_ecp_point_read_binary` and `mbedtls_ecp_point_write_binary` for uncompressed points. The PSA API does not currently support compressed points, but it is likely that such support will be added in the future.
-* Manipulation of key pairs as such, with a bridge to bignum arithmetic (`mbedtls_ecp_keypair` type, `mbedtls_ecp_export`). However, the PSA export format for ECC private keys used by [`psa_import_key`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__import__export/#group__import__export_1ga0336ea76bf30587ab204a8296462327b), [`psa_export_key`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__import__export/#group__import__export_1ga668e35be8d2852ad3feeef74ac6f75bf) is the same as the format used by `mbedtls_ecp_read_key` and `mbedtls_ecp_write_key`.
+* Manipulation of key pairs as such, with a bridge to bignum arithmetic (`mbedtls_ecp_keypair` type, `mbedtls_ecp_export`). However, the PSA export format for ECC private keys used by [`psa_import_key`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__import__export/#group__import__export_1ga0336ea76bf30587ab204a8296462327b), [`psa_export_key`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/group/group__import__export/#group__import__export_1ga668e35be8d2852ad3feeef74ac6f75bf) is the same as the format used by `mbedtls_ecp_read_key` and `mbedtls_ecp_write_key_ext`.
 * Elliptic curve arithmetic (`mbedtls_ecp_mul`, `mbedtls_ecp_muladd` and their restartable variants).
 
 ### Additional information about RSA
diff --git a/include/mbedtls/ecp.h b/include/mbedtls/ecp.h
index 0201963..d8f73ae 100644
--- a/include/mbedtls/ecp.h
+++ b/include/mbedtls/ecp.h
@@ -24,6 +24,7 @@
 #include "mbedtls/private_access.h"
 
 #include "mbedtls/build_info.h"
+#include "mbedtls/platform_util.h"
 
 #include "mbedtls/bignum.h"
 
@@ -1327,10 +1328,11 @@
 int mbedtls_ecp_read_key(mbedtls_ecp_group_id grp_id, mbedtls_ecp_keypair *key,
                          const unsigned char *buf, size_t buflen);
 
+#if !defined(MBEDTLS_DEPRECATED_REMOVED)
 /**
  * \brief           This function exports an elliptic curve private key.
  *
- * \note            Note that although this function accepts an output
+ * \deprecated      Note that although this function accepts an output
  *                  buffer that is smaller or larger than the key, most key
  *                  import interfaces require the output to have exactly
  *                  key's nominal length. It is generally simplest to
@@ -1338,6 +1340,10 @@
  *                  checking that the output buffer is large enough.
  *                  See the description of the \p buflen parameter for
  *                  how to calculate the nominal length.
+ *                  To avoid this difficulty, use mbedtls_ecp_write_key_ext()
+ *                  instead.
+ *                  mbedtls_ecp_write_key() is deprecated and will be
+ *                  removed in a future version of the library.
  *
  * \note            If the private key was not set in \p key,
  *                  the output is unspecified. Future versions
@@ -1367,8 +1373,31 @@
  *                  representation is larger than the available space in \p buf.
  * \return          Another negative error code on different kinds of failure.
  */
-int mbedtls_ecp_write_key(mbedtls_ecp_keypair *key,
-                          unsigned char *buf, size_t buflen);
+int MBEDTLS_DEPRECATED mbedtls_ecp_write_key(mbedtls_ecp_keypair *key,
+                                             unsigned char *buf, size_t buflen);
+#endif /* MBEDTLS_DEPRECATED_REMOVED */
+
+/**
+ * \brief           This function exports an elliptic curve private key.
+ *
+ * \param key       The private key.
+ * \param olen      On success, the length of the private key.
+ *                  This is always (`grp->nbits` + 7) / 8 bytes
+ *                  where `grp->nbits` is the private key size in bits.
+ * \param buf       The output buffer for containing the binary representation
+ *                  of the key.
+ * \param buflen    The total length of the buffer in bytes.
+ *                  #MBEDTLS_ECP_MAX_BYTES is always sufficient.
+ *
+ * \return          \c 0 on success.
+ * \return          #MBEDTLS_ERR_ECP_BUFFER_TOO_SMALL if the \p key
+ *                  representation is larger than the available space in \p buf.
+ * \return          #MBEDTLS_ERR_ECP_BAD_INPUT_DATA if no private key is
+ *                  set in \p key.
+ * \return          Another negative error code on different kinds of failure.
+ */
+int mbedtls_ecp_write_key_ext(const mbedtls_ecp_keypair *key,
+                              size_t *olen, unsigned char *buf, size_t buflen);
 
 /**
  * \brief           This function exports an elliptic curve public key.
diff --git a/library/ecp.c b/library/ecp.c
index 66b3dc1..427059b 100644
--- a/library/ecp.c
+++ b/library/ecp.c
@@ -3302,6 +3302,7 @@
 /*
  * Write a private key.
  */
+#if !defined MBEDTLS_DEPRECATED_REMOVED
 int mbedtls_ecp_write_key(mbedtls_ecp_keypair *key,
                           unsigned char *buf, size_t buflen)
 {
@@ -3332,6 +3333,39 @@
 
     return ret;
 }
+#endif /* MBEDTLS_DEPRECATED_REMOVED */
+
+int mbedtls_ecp_write_key_ext(const mbedtls_ecp_keypair *key,
+                              size_t *olen, unsigned char *buf, size_t buflen)
+{
+    size_t len = (key->grp.nbits + 7) / 8;
+    if (len > buflen) {
+        /* For robustness, ensure *olen <= buflen even on error. */
+        *olen = 0;
+        return MBEDTLS_ERR_ECP_BUFFER_TOO_SMALL;
+    }
+    *olen = len;
+
+    /* Private key not set */
+    if (key->d.n == 0) {
+        return MBEDTLS_ERR_ECP_BAD_INPUT_DATA;
+    }
+
+#if defined(MBEDTLS_ECP_MONTGOMERY_ENABLED)
+    if (mbedtls_ecp_get_type(&key->grp) == MBEDTLS_ECP_TYPE_MONTGOMERY) {
+        return mbedtls_mpi_write_binary_le(&key->d, buf, len);
+    }
+#endif
+
+#if defined(MBEDTLS_ECP_SHORT_WEIERSTRASS_ENABLED)
+    if (mbedtls_ecp_get_type(&key->grp) == MBEDTLS_ECP_TYPE_SHORT_WEIERSTRASS) {
+        return mbedtls_mpi_write_binary(&key->d, buf, len);
+    }
+#endif
+
+    /* Private key set but no recognized curve type? This shouldn't happen. */
+    return MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
+}
 
 /*
  * Write a public key.
diff --git a/library/pk.c b/library/pk.c
index 8a5124f..003ef4a 100644
--- a/library/pk.c
+++ b/library/pk.c
@@ -675,10 +675,7 @@
 #if defined(MBEDTLS_PK_USE_PSA_EC_DATA)
             psa_ecc_family_t from_family = pk->ec_family;
 #else /* MBEDTLS_PK_USE_PSA_EC_DATA */
-            /* We're only reading the key, but mbedtls_ecp_write_key()
-             * is missing a const annotation on its key parameter, so
-             * we need the non-const accessor here. */
-            mbedtls_ecp_keypair *ec = mbedtls_pk_ec_rw(*pk);
+            const mbedtls_ecp_keypair *ec = mbedtls_pk_ec_ro(*pk);
             size_t from_bits = 0;
             psa_ecc_family_t from_family = mbedtls_ecc_group_to_psa(ec->grp.id,
                                                                     &from_bits);
@@ -704,12 +701,9 @@
                 return MBEDTLS_ERR_PK_TYPE_MISMATCH;
             }
             unsigned char key_buffer[PSA_BITS_TO_BYTES(PSA_VENDOR_ECC_MAX_CURVE_BITS)];
-            /* Make sure to pass the exact key length to
-             * mbedtls_ecp_write_key(), because it writes Montgomery keys
-             * at the start of the buffer but Weierstrass keys at the
-             * end of the buffer. */
-            size_t key_length = PSA_BITS_TO_BYTES(ec->grp.nbits);
-            int ret = mbedtls_ecp_write_key(ec, key_buffer, key_length);
+            size_t key_length = 0;
+            int ret = mbedtls_ecp_write_key_ext(ec, &key_length,
+                                                key_buffer, sizeof(key_buffer));
             if (ret < 0) {
                 return ret;
             }
diff --git a/library/pkwrite.c b/library/pkwrite.c
index b9ddcf1..5e009c5 100644
--- a/library/pkwrite.c
+++ b/library/pkwrite.c
@@ -202,7 +202,7 @@
         mbedtls_ecp_keypair *ec = mbedtls_pk_ec_rw(*pk);
         byte_length = (ec->grp.pbits + 7) / 8;
 
-        ret = mbedtls_ecp_write_key(ec, tmp, byte_length);
+        ret = mbedtls_ecp_write_key_ext(ec, &byte_length, tmp, sizeof(tmp));
         if (ret != 0) {
             goto exit;
         }
diff --git a/library/psa_crypto_ecp.c b/library/psa_crypto_ecp.c
index 7edea81..e373ad9 100644
--- a/library/psa_crypto_ecp.c
+++ b/library/psa_crypto_ecp.c
@@ -281,20 +281,8 @@
 
         return status;
     } else {
-        if (data_size < PSA_BITS_TO_BYTES(ecp->grp.nbits)) {
-            return PSA_ERROR_BUFFER_TOO_SMALL;
-        }
-
         status = mbedtls_to_psa_error(
-            mbedtls_ecp_write_key(ecp,
-                                  data,
-                                  PSA_BITS_TO_BYTES(ecp->grp.nbits)));
-        if (status == PSA_SUCCESS) {
-            *data_length = PSA_BITS_TO_BYTES(ecp->grp.nbits);
-        } else {
-            memset(data, 0, data_size);
-        }
-
+            mbedtls_ecp_write_key_ext(ecp, data_length, data, data_size));
         return status;
     }
 }
@@ -359,14 +347,11 @@
     }
 
     status = mbedtls_to_psa_error(
-        mbedtls_ecp_write_key(&ecp, key_buffer, key_buffer_size));
+        mbedtls_ecp_write_key_ext(&ecp, key_buffer_length,
+                                  key_buffer, key_buffer_size));
 
     mbedtls_ecp_keypair_free(&ecp);
 
-    if (status == PSA_SUCCESS) {
-        *key_buffer_length = key_buffer_size;
-    }
-
     return status;
 }
 #endif /* MBEDTLS_PSA_BUILTIN_KEY_TYPE_ECC_KEY_PAIR_GENERATE */
diff --git a/library/ssl_tls12_server.c b/library/ssl_tls12_server.c
index 53a9ce2..5bee188 100644
--- a/library/ssl_tls12_server.c
+++ b/library/ssl_tls12_server.c
@@ -2703,8 +2703,7 @@
                              PSA_KEY_TYPE_ECC_KEY_PAIR(ssl->handshake->xxdh_psa_type));
             psa_set_key_bits(&key_attributes, ssl->handshake->xxdh_psa_bits);
 
-            key_len = PSA_BITS_TO_BYTES(key->grp.pbits);
-            ret = mbedtls_ecp_write_key(key, buf, key_len);
+            ret = mbedtls_ecp_write_key_ext(key, &key_len, buf, sizeof(buf));
             if (ret != 0) {
                 mbedtls_platform_zeroize(buf, sizeof(buf));
                 break;
diff --git a/tests/suites/test_suite_ecp.data b/tests/suites/test_suite_ecp.data
index 1dd963a..fd63657 100644
--- a/tests/suites/test_suite_ecp.data
+++ b/tests/suites/test_suite_ecp.data
@@ -888,6 +888,109 @@
 depends_on:MBEDTLS_ECP_DP_CURVE448_ENABLED
 ecp_write_key:MBEDTLS_ECP_DP_CURVE448:"0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000080":55:MBEDTLS_ERR_ECP_BUFFER_TOO_SMALL
 
+ECP write key ext: secp256r1, nominal
+depends_on:MBEDTLS_ECP_DP_SECP256R1_ENABLED
+ecp_write_key_ext:MBEDTLS_ECP_DP_SECP256R1:"f12a1320760270a83cbffd53f6031ef76a5d86c8a204f2c30ca9ebf51f0f0ea7":32:0
+
+ECP write key ext: secp256r1, output longer by 1
+depends_on:MBEDTLS_ECP_DP_SECP256R1_ENABLED
+ecp_write_key_ext:MBEDTLS_ECP_DP_SECP256R1:"f12a1320760270a83cbffd53f6031ef76a5d86c8a204f2c30ca9ebf51f0f0ea7":33:0
+
+ECP write key ext: secp256r1, output short by 1
+depends_on:MBEDTLS_ECP_DP_SECP256R1_ENABLED
+ecp_write_key_ext:MBEDTLS_ECP_DP_SECP256R1:"f12a1320760270a83cbffd53f6031ef76a5d86c8a204f2c30ca9ebf51f0f0ea7":31:MBEDTLS_ERR_ECP_BUFFER_TOO_SMALL
+
+ECP write key ext: secp256r1, output_size=0
+depends_on:MBEDTLS_ECP_DP_SECP256R1_ENABLED
+ecp_write_key_ext:MBEDTLS_ECP_DP_SECP256R1:"f12a1320760270a83cbffd53f6031ef76a5d86c8a204f2c30ca9ebf51f0f0ea7":0:MBEDTLS_ERR_ECP_BUFFER_TOO_SMALL
+
+ECP write key ext: secp256r1, top byte = 0, output_size=32
+depends_on:MBEDTLS_ECP_DP_SECP256R1_ENABLED
+ecp_write_key_ext:MBEDTLS_ECP_DP_SECP256R1:"00ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff":32:0
+
+ECP write key ext: secp256r1, top byte = 0, output_size=31
+depends_on:MBEDTLS_ECP_DP_SECP256R1_ENABLED
+ecp_write_key_ext:MBEDTLS_ECP_DP_SECP256R1:"00ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff":31:MBEDTLS_ERR_ECP_BUFFER_TOO_SMALL
+
+ECP write key ext: secp256r1, top byte = 0, output_size=30
+depends_on:MBEDTLS_ECP_DP_SECP256R1_ENABLED
+ecp_write_key_ext:MBEDTLS_ECP_DP_SECP256R1:"00ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff":30:MBEDTLS_ERR_ECP_BUFFER_TOO_SMALL
+
+ECP write key ext: secp256r1, mostly-0 key, output_size=32
+depends_on:MBEDTLS_ECP_DP_SECP256R1_ENABLED
+ecp_write_key_ext:MBEDTLS_ECP_DP_SECP256R1:"0000000000000000000000000000000000000000000000000000000000000001":32:0
+
+ECP write key ext: secp256r1, mostly-0 key, output_size=1
+depends_on:MBEDTLS_ECP_DP_SECP256R1_ENABLED
+ecp_write_key_ext:MBEDTLS_ECP_DP_SECP256R1:"0000000000000000000000000000000000000000000000000000000000000001":1:MBEDTLS_ERR_ECP_BUFFER_TOO_SMALL
+
+ECP write key ext: secp256r1, private key not set
+depends_on:MBEDTLS_ECP_DP_SECP256R1_ENABLED
+ecp_write_key_ext:MBEDTLS_ECP_DP_SECP256R1:"":32:MBEDTLS_ERR_ECP_BAD_INPUT_DATA
+
+ECP write key ext: secp384r1, nominal
+depends_on:MBEDTLS_ECP_DP_SECP384R1_ENABLED
+ecp_write_key_ext:MBEDTLS_ECP_DP_SECP384R1:"d27335ea71664af244dd14e9fd1260715dfd8a7965571c48d709ee7a7962a156d706a90cbcb5df2986f05feadb9376f1":48:0
+
+ECP write key ext: secp384r1, output longer by 1
+depends_on:MBEDTLS_ECP_DP_SECP384R1_ENABLED
+ecp_write_key_ext:MBEDTLS_ECP_DP_SECP384R1:"d27335ea71664af244dd14e9fd1260715dfd8a7965571c48d709ee7a7962a156d706a90cbcb5df2986f05feadb9376f1":49:0
+
+ECP write key ext: secp384r1, output short by 1
+depends_on:MBEDTLS_ECP_DP_SECP384R1_ENABLED
+ecp_write_key_ext:MBEDTLS_ECP_DP_SECP384R1:"d27335ea71664af244dd14e9fd1260715dfd8a7965571c48d709ee7a7962a156d706a90cbcb5df2986f05feadb9376f1":47:MBEDTLS_ERR_ECP_BUFFER_TOO_SMALL
+
+ECP write key ext: Curve25519, nominal
+depends_on:MBEDTLS_ECP_DP_CURVE25519_ENABLED
+ecp_write_key_ext:MBEDTLS_ECP_DP_CURVE25519:"a046e36bf0527c9d3b16154b82465edd62144c0ac1fc5a18506a2244ba449a44":32:0
+
+ECP write key ext: Curve25519, output longer by 1
+depends_on:MBEDTLS_ECP_DP_CURVE25519_ENABLED
+ecp_write_key_ext:MBEDTLS_ECP_DP_CURVE25519:"a046e36bf0527c9d3b16154b82465edd62144c0ac1fc5a18506a2244ba449a44":33:0
+
+ECP write key ext: Curve25519, output short by 1
+depends_on:MBEDTLS_ECP_DP_CURVE25519_ENABLED
+ecp_write_key_ext:MBEDTLS_ECP_DP_CURVE25519:"a046e36bf0527c9d3b16154b82465edd62144c0ac1fc5a18506a2244ba449a44":31:MBEDTLS_ERR_ECP_BUFFER_TOO_SMALL
+
+ECP write key ext: Curve25519, output_size=0
+depends_on:MBEDTLS_ECP_DP_CURVE25519_ENABLED
+ecp_write_key_ext:MBEDTLS_ECP_DP_CURVE25519:"a046e36bf0527c9d3b16154b82465edd62144c0ac1fc5a18506a2244ba449a44":0:MBEDTLS_ERR_ECP_BUFFER_TOO_SMALL
+
+ECP write key ext: Curve25519, mostly-0 key, output_size=32
+depends_on:MBEDTLS_ECP_DP_CURVE25519_ENABLED
+ecp_write_key_ext:MBEDTLS_ECP_DP_CURVE25519:"0000000000000000000000000000000000000000000000000000000000000040":32:0
+
+ECP write key ext: Curve25519, mostly-0 key, output_size=31
+depends_on:MBEDTLS_ECP_DP_CURVE25519_ENABLED
+ecp_write_key_ext:MBEDTLS_ECP_DP_CURVE25519:"0000000000000000000000000000000000000000000000000000000000000040":31:MBEDTLS_ERR_ECP_BUFFER_TOO_SMALL
+
+ECP write key ext: Curve25519, private key not set
+depends_on:MBEDTLS_ECP_DP_CURVE25519_ENABLED
+ecp_write_key_ext:MBEDTLS_ECP_DP_CURVE25519:"":32:MBEDTLS_ERR_ECP_BAD_INPUT_DATA
+
+ECP write key ext: Curve448, nominal
+depends_on:MBEDTLS_ECP_DP_CURVE448_ENABLED
+ecp_write_key_ext:MBEDTLS_ECP_DP_CURVE448:"3c262fddf9ec8e88495266fea19a34d28882acef045104d0d1aae121700a779c984c24f8cdd78fbff44943eba368f54b29259a4f1c600ad3":56:0
+
+ECP write key ext: Curve448, output longer by 1
+depends_on:MBEDTLS_ECP_DP_CURVE448_ENABLED
+ecp_write_key_ext:MBEDTLS_ECP_DP_CURVE448:"3c262fddf9ec8e88495266fea19a34d28882acef045104d0d1aae121700a779c984c24f8cdd78fbff44943eba368f54b29259a4f1c600ad3":57:0
+
+ECP write key ext: Curve448, output short by 1
+depends_on:MBEDTLS_ECP_DP_CURVE448_ENABLED
+ecp_write_key_ext:MBEDTLS_ECP_DP_CURVE448:"3c262fddf9ec8e88495266fea19a34d28882acef045104d0d1aae121700a779c984c24f8cdd78fbff44943eba368f54b29259a4f1c600ad3":55:MBEDTLS_ERR_ECP_BUFFER_TOO_SMALL
+
+ECP write key ext: Curve448, mostly-0 key, output_size=56
+depends_on:MBEDTLS_ECP_DP_CURVE448_ENABLED
+ecp_write_key_ext:MBEDTLS_ECP_DP_CURVE448:"0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000080":56:0
+
+ECP write key ext: Curve448, mostly-0 key, output_size=55
+depends_on:MBEDTLS_ECP_DP_CURVE448_ENABLED
+ecp_write_key_ext:MBEDTLS_ECP_DP_CURVE448:"0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000080":55:MBEDTLS_ERR_ECP_BUFFER_TOO_SMALL
+
+ECP write key ext: group not set
+ecp_write_key_ext:MBEDTLS_ECP_DP_NONE:"":32:MBEDTLS_ERR_ECP_BAD_INPUT_DATA
+
 ECP mod p192 small (more than 192 bits, less limbs than 2 * 192 bits)
 depends_on:MBEDTLS_ECP_DP_SECP192R1_ENABLED:MBEDTLS_ECP_NIST_OPTIM
 ecp_fast_mod:MBEDTLS_ECP_DP_SECP192R1:"0100000000000103010000000000010201000000000001010100000000000100"
diff --git a/tests/suites/test_suite_ecp.function b/tests/suites/test_suite_ecp.function
index 9cf0ce1..9b5c86f 100644
--- a/tests/suites/test_suite_ecp.function
+++ b/tests/suites/test_suite_ecp.function
@@ -1204,29 +1204,46 @@
         TEST_EQUAL(mbedtls_mpi_cmp_int(&key.Q.Y, 2), 0);
         TEST_EQUAL(mbedtls_mpi_cmp_int(&key.Q.Z, 3), 0);
 
-        if (canonical) {
+        if (canonical && in_key->len == (key.grp.nbits + 7) / 8) {
             unsigned char buf[MBEDTLS_ECP_MAX_BYTES];
+            size_t length = 0xdeadbeef;
 
-            ret = mbedtls_ecp_write_key(&key, buf, in_key->len);
-            TEST_ASSERT(ret == 0);
+            TEST_EQUAL(mbedtls_ecp_write_key_ext(&key,
+                                                 &length, buf, in_key->len), 0);
+            TEST_MEMORY_COMPARE(in_key->x, in_key->len,
+                                buf, length);
 
+#if defined(MBEDTLS_TEST_DEPRECATED)
+            memset(buf, 0, sizeof(buf));
+            TEST_EQUAL(mbedtls_ecp_write_key(&key, buf, in_key->len), 0);
             TEST_MEMORY_COMPARE(in_key->x, in_key->len,
                                 buf, in_key->len);
+#endif /* MBEDTLS_TEST_DEPRECATED */
         } else {
             unsigned char export1[MBEDTLS_ECP_MAX_BYTES];
             unsigned char export2[MBEDTLS_ECP_MAX_BYTES];
 
-            ret = mbedtls_ecp_write_key(&key, export1, in_key->len);
-            TEST_ASSERT(ret == 0);
+            size_t length1 = 0xdeadbeef;
+            TEST_EQUAL(mbedtls_ecp_write_key_ext(&key, &length1,
+                                                 export1, sizeof(export1)), 0);
+            TEST_EQUAL(mbedtls_ecp_read_key(grp_id, &key2, export1, length1),
+                       expected);
+            size_t length2 = 0xdeadbeef;
+            TEST_EQUAL(mbedtls_ecp_write_key_ext(&key2, &length2,
+                                                 export2, sizeof(export2)), 0);
+            TEST_MEMORY_COMPARE(export1, length1,
+                                export2, length2);
 
-            ret = mbedtls_ecp_read_key(grp_id, &key2, export1, in_key->len);
-            TEST_ASSERT(ret == expected);
-
-            ret = mbedtls_ecp_write_key(&key2, export2, in_key->len);
-            TEST_ASSERT(ret == 0);
-
+#if defined(MBEDTLS_TEST_DEPRECATED)
+            memset(export1, 0, sizeof(export1));
+            memset(export2, 0, sizeof(export2));
+            TEST_EQUAL(mbedtls_ecp_write_key(&key, export1, in_key->len), 0);
+            TEST_EQUAL(mbedtls_ecp_read_key(grp_id, &key2, export1, in_key->len),
+                       expected);
+            TEST_EQUAL(mbedtls_ecp_write_key(&key2, export2, in_key->len), 0);
             TEST_MEMORY_COMPARE(export1, in_key->len,
                                 export2, in_key->len);
+#endif /* MBEDTLS_TEST_DEPRECATED */
         }
     }
 
@@ -1236,7 +1253,7 @@
 }
 /* END_CASE */
 
-/* BEGIN_CASE */
+/* BEGIN_CASE depends_on:MBEDTLS_TEST_DEPRECATED */
 void ecp_write_key(int grp_id, data_t *in_key,
                    int exported_size, int expected_ret)
 {
@@ -1296,6 +1313,42 @@
 }
 /* END_CASE */
 
+/* BEGIN_CASE */
+void ecp_write_key_ext(int grp_id, data_t *in_key,
+                       int exported_size, int expected_ret)
+{
+    mbedtls_ecp_keypair key;
+    mbedtls_ecp_keypair_init(&key);
+    unsigned char *exported = NULL;
+
+    if (in_key->len != 0) {
+        TEST_EQUAL(mbedtls_ecp_read_key(grp_id, &key, in_key->x, in_key->len), 0);
+    } else if (grp_id != MBEDTLS_ECP_DP_NONE) {
+        TEST_EQUAL(mbedtls_ecp_group_load(&key.grp, grp_id), 0);
+    }
+
+    TEST_CALLOC(exported, exported_size);
+    size_t olen = 0xdeadbeef;
+    TEST_EQUAL(mbedtls_ecp_write_key_ext(&key, &olen, exported, exported_size),
+               expected_ret);
+
+    if (expected_ret == 0) {
+        TEST_EQUAL(olen, (key.grp.nbits + 7) / 8);
+        TEST_LE_U(olen, MBEDTLS_ECP_MAX_BYTES);
+        TEST_MEMORY_COMPARE(in_key->x, in_key->len,
+                            exported, olen);
+    } else {
+        /* Robustness check: even in the error case, insist that olen is less
+         * than the buffer size. */
+        TEST_LE_U(olen, exported_size);
+    }
+
+exit:
+    mbedtls_ecp_keypair_free(&key);
+    mbedtls_free(exported);
+}
+/* END_CASE */
+
 /* BEGIN_CASE depends_on:MBEDTLS_TEST_HOOKS:MBEDTLS_ECP_MONTGOMERY_ENABLED:MBEDTLS_ECP_LIGHT */
 void genkey_mx_known_answer(int bits, data_t *seed, data_t *expected)
 {