diff --git a/library/ecp_invasive.h b/library/ecp_invasive.h
index 18815be..d6f6f95 100644
--- a/library/ecp_invasive.h
+++ b/library/ecp_invasive.h
@@ -61,7 +61,7 @@
  *                  This is the bit-size of the key minus 1:
  *                  254 for Curve25519 or 447 for Curve448.
  * \param d         The randomly generated key. This is a number of size
- *                  exactly \p n_bits + 1 bits, with the least significant bits
+ *                  exactly \p high_bit + 1 bits, with the least significant bits
  *                  masked as specified in [Curve25519] and in [RFC7748] §5.
  * \param f_rng     The RNG function.
  * \param p_rng     The RNG context to be passed to \p f_rng.
@@ -69,7 +69,7 @@
  * \return          \c 0 on success.
  * \return          \c MBEDTLS_ERR_ECP_xxx or MBEDTLS_ERR_MPI_xxx on failure.
  */
-int mbedtls_ecp_gen_privkey_mx(size_t n_bits,
+int mbedtls_ecp_gen_privkey_mx(size_t high_bit,
                                mbedtls_mpi *d,
                                int (*f_rng)(void *, unsigned char *, size_t),
                                void *p_rng);
diff --git a/tests/data_files/Makefile b/tests/data_files/Makefile
index b123dc9..80172e2 100644
--- a/tests/data_files/Makefile
+++ b/tests/data_files/Makefile
@@ -889,6 +889,57 @@
 all_final += ec_prv.pk8param.pem
 
 ################################################################
+#### Convert PEM keys to DER format
+################################################################
+server1.pubkey.der: server1.pubkey
+	$(OPENSSL) pkey -pubin -in $< -out $@ -outform DER
+all_final += server1.pubkey.der
+
+rsa4096_pub.der: rsa4096_pub.pem
+	$(OPENSSL) pkey -pubin -in $< -out $@ -outform DER
+all_final += rsa4096_pub.der
+
+ec_pub.der: ec_pub.pem
+	$(OPENSSL) pkey -pubin -in $< -out $@ -outform DER
+all_final += ec_pub.der
+
+ec_521_pub.der: ec_521_pub.pem
+	$(OPENSSL) pkey -pubin -in $< -out $@ -outform DER
+all_final += ec_521_pub.der
+
+ec_bp512_pub.der: ec_bp512_pub.pem
+	$(OPENSSL) pkey -pubin -in $< -out $@ -outform DER
+all_final += ec_bp512_pub.der
+
+server1.key.der: server1.key
+	$(OPENSSL) pkey -in $< -out $@ -outform DER
+all_final += server1.key.der
+
+rsa4096_prv.der: rsa4096_prv.pem
+	$(OPENSSL) pkey -in $< -out $@ -outform DER
+all_final += rsa4096_prv.der
+
+ec_prv.sec1.der: ec_prv.sec1.pem
+	$(OPENSSL) pkey -in $< -out $@ -outform DER
+all_final += ec_prv.sec1.der
+
+ec_256_long_prv.der: ec_256_long_prv.pem
+	$(OPENSSL) pkey -in $< -out $@ -outform DER
+all_final += ec_256_long_prv.der
+
+ec_521_prv.der: ec_521_prv.pem
+	$(OPENSSL) pkey -in $< -out $@ -outform DER
+all_final += ec_521_prv.der
+
+ec_521_short_prv.der: ec_521_short_prv.pem
+	$(OPENSSL) pkey -in $< -out $@ -outform DER
+all_final += ec_521_short_prv.der
+
+ec_bp512_prv.der: ec_bp512_prv.pem
+	$(OPENSSL) pkey -in $< -out $@ -outform DER
+all_final += ec_bp512_prv.der
+
+################################################################
 ### Generate CSRs for X.509 write test suite
 ################################################################
 
diff --git a/tests/data_files/ec_256_long_prv.der b/tests/data_files/ec_256_long_prv.der
new file mode 100644
index 0000000..96e329e
--- /dev/null
+++ b/tests/data_files/ec_256_long_prv.der
Binary files differ
diff --git a/tests/data_files/ec_521_prv.der b/tests/data_files/ec_521_prv.der
new file mode 100644
index 0000000..734714a
--- /dev/null
+++ b/tests/data_files/ec_521_prv.der
Binary files differ
diff --git a/tests/data_files/ec_521_pub.der b/tests/data_files/ec_521_pub.der
new file mode 100644
index 0000000..5b685de
--- /dev/null
+++ b/tests/data_files/ec_521_pub.der
Binary files differ
diff --git a/tests/data_files/ec_521_short_prv.der b/tests/data_files/ec_521_short_prv.der
new file mode 100644
index 0000000..0a1f18c
--- /dev/null
+++ b/tests/data_files/ec_521_short_prv.der
Binary files differ
diff --git a/tests/data_files/ec_bp512_prv.der b/tests/data_files/ec_bp512_prv.der
new file mode 100644
index 0000000..2d9a3de
--- /dev/null
+++ b/tests/data_files/ec_bp512_prv.der
Binary files differ
diff --git a/tests/data_files/ec_bp512_pub.der b/tests/data_files/ec_bp512_pub.der
new file mode 100644
index 0000000..6a8c4c7
--- /dev/null
+++ b/tests/data_files/ec_bp512_pub.der
Binary files differ
diff --git a/tests/data_files/rsa4096_prv.der b/tests/data_files/rsa4096_prv.der
new file mode 100644
index 0000000..86ea818
--- /dev/null
+++ b/tests/data_files/rsa4096_prv.der
Binary files differ
diff --git a/tests/data_files/rsa4096_pub.der b/tests/data_files/rsa4096_pub.der
new file mode 100644
index 0000000..270bf3a
--- /dev/null
+++ b/tests/data_files/rsa4096_pub.der
Binary files differ
diff --git a/tests/data_files/server1.key.der b/tests/data_files/server1.key.der
new file mode 100644
index 0000000..88288d1
--- /dev/null
+++ b/tests/data_files/server1.key.der
Binary files differ
diff --git a/tests/data_files/server1.pubkey.der b/tests/data_files/server1.pubkey.der
new file mode 100644
index 0000000..1a432a4
--- /dev/null
+++ b/tests/data_files/server1.pubkey.der
Binary files differ
diff --git a/tests/suites/test_suite_pkwrite.data b/tests/suites/test_suite_pkwrite.data
index e0101cc..83bfdcb 100644
--- a/tests/suites/test_suite_pkwrite.data
+++ b/tests/suites/test_suite_pkwrite.data
@@ -1,47 +1,95 @@
 Public key write check RSA
-depends_on:MBEDTLS_RSA_C:MBEDTLS_BASE64_C
-pk_write_pubkey_check:"data_files/server1.pubkey"
+depends_on:MBEDTLS_RSA_C:MBEDTLS_PEM_PARSE_C:MBEDTLS_PEM_WRITE_C
+pk_write_pubkey_check:"data_files/server1.pubkey":TEST_PEM
+
+Public key write check RSA (DER)
+depends_on:MBEDTLS_RSA_C
+pk_write_pubkey_check:"data_files/server1.pubkey.der":TEST_DER
 
 Public key write check RSA 4096
-depends_on:MBEDTLS_RSA_C:MBEDTLS_BASE64_C
-pk_write_pubkey_check:"data_files/rsa4096_pub.pem"
+depends_on:MBEDTLS_RSA_C:MBEDTLS_PEM_PARSE_C:MBEDTLS_PEM_WRITE_C
+pk_write_pubkey_check:"data_files/rsa4096_pub.pem":TEST_PEM
+
+Public key write check RSA 4096 (DER)
+depends_on:MBEDTLS_RSA_C
+pk_write_pubkey_check:"data_files/rsa4096_pub.der":TEST_DER
 
 Public key write check EC 192 bits
-depends_on:MBEDTLS_ECP_C:MBEDTLS_BASE64_C:MBEDTLS_ECP_DP_SECP192R1_ENABLED
-pk_write_pubkey_check:"data_files/ec_pub.pem"
+depends_on:MBEDTLS_ECP_C:MBEDTLS_PEM_PARSE_C:MBEDTLS_PEM_WRITE_C:MBEDTLS_ECP_DP_SECP192R1_ENABLED
+pk_write_pubkey_check:"data_files/ec_pub.pem":TEST_PEM
+
+Public key write check EC 192 bits (DER)
+depends_on:MBEDTLS_ECP_C:MBEDTLS_ECP_DP_SECP192R1_ENABLED
+pk_write_pubkey_check:"data_files/ec_pub.der":TEST_DER
 
 Public key write check EC 521 bits
-depends_on:MBEDTLS_ECP_C:MBEDTLS_BASE64_C:MBEDTLS_ECP_DP_SECP521R1_ENABLED
-pk_write_pubkey_check:"data_files/ec_521_pub.pem"
+depends_on:MBEDTLS_ECP_C:MBEDTLS_PEM_PARSE_C:MBEDTLS_PEM_WRITE_C:MBEDTLS_ECP_DP_SECP521R1_ENABLED
+pk_write_pubkey_check:"data_files/ec_521_pub.pem":TEST_PEM
+
+Public key write check EC 521 bits (DER)
+depends_on:MBEDTLS_ECP_C:MBEDTLS_ECP_DP_SECP521R1_ENABLED
+pk_write_pubkey_check:"data_files/ec_521_pub.der":TEST_DER
 
 Public key write check EC Brainpool 512 bits
-depends_on:MBEDTLS_ECP_C:MBEDTLS_BASE64_C:MBEDTLS_ECP_DP_BP512R1_ENABLED
-pk_write_pubkey_check:"data_files/ec_bp512_pub.pem"
+depends_on:MBEDTLS_ECP_C:MBEDTLS_PEM_PARSE_C:MBEDTLS_PEM_WRITE_C:MBEDTLS_ECP_DP_BP512R1_ENABLED
+pk_write_pubkey_check:"data_files/ec_bp512_pub.pem":TEST_PEM
+
+Public key write check EC Brainpool 512 bits (DER)
+depends_on:MBEDTLS_ECP_C:MBEDTLS_ECP_DP_BP512R1_ENABLED
+pk_write_pubkey_check:"data_files/ec_bp512_pub.der":TEST_DER
 
 Private key write check RSA
-depends_on:MBEDTLS_RSA_C:MBEDTLS_BASE64_C
-pk_write_key_check:"data_files/server1.key"
+depends_on:MBEDTLS_RSA_C:MBEDTLS_PEM_PARSE_C:MBEDTLS_PEM_WRITE_C
+pk_write_key_check:"data_files/server1.key":TEST_PEM
+
+Private key write check RSA (DER)
+depends_on:MBEDTLS_RSA_C
+pk_write_key_check:"data_files/server1.key.der":TEST_DER
 
 Private key write check RSA 4096
-depends_on:MBEDTLS_RSA_C:MBEDTLS_BASE64_C
-pk_write_key_check:"data_files/rsa4096_prv.pem"
+depends_on:MBEDTLS_RSA_C:MBEDTLS_PEM_PARSE_C:MBEDTLS_PEM_WRITE_C
+pk_write_key_check:"data_files/rsa4096_prv.pem":TEST_PEM
+
+Private key write check RSA 4096 (DER)
+depends_on:MBEDTLS_RSA_C
+pk_write_key_check:"data_files/rsa4096_prv.der":TEST_DER
 
 Private key write check EC 192 bits
-depends_on:MBEDTLS_ECP_C:MBEDTLS_BASE64_C:MBEDTLS_ECP_DP_SECP192R1_ENABLED
-pk_write_key_check:"data_files/ec_prv.sec1.pem"
+depends_on:MBEDTLS_ECP_C:MBEDTLS_PEM_PARSE_C:MBEDTLS_PEM_WRITE_C:MBEDTLS_ECP_DP_SECP192R1_ENABLED
+pk_write_key_check:"data_files/ec_prv.sec1.pem":TEST_PEM
+
+Private key write check EC 192 bits (DER)
+depends_on:MBEDTLS_ECP_C:MBEDTLS_ECP_DP_SECP192R1_ENABLED
+pk_write_key_check:"data_files/ec_prv.sec1.der":TEST_DER
 
 Private key write check EC 256 bits (top bit set)
-depends_on:MBEDTLS_ECP_C:MBEDTLS_BASE64_C:MBEDTLS_ECP_DP_SECP256R1_ENABLED
-pk_write_key_check:"data_files/ec_256_long_prv.pem"
+depends_on:MBEDTLS_ECP_C:MBEDTLS_PEM_PARSE_C:MBEDTLS_PEM_WRITE_C:MBEDTLS_ECP_DP_SECP256R1_ENABLED
+pk_write_key_check:"data_files/ec_256_long_prv.pem":TEST_PEM
+
+Private key write check EC 256 bits (top bit set) (DER)
+depends_on:MBEDTLS_ECP_C:MBEDTLS_ECP_DP_SECP256R1_ENABLED
+pk_write_key_check:"data_files/ec_256_long_prv.der":TEST_DER
 
 Private key write check EC 521 bits
-depends_on:MBEDTLS_ECP_C:MBEDTLS_BASE64_C:MBEDTLS_ECP_DP_SECP521R1_ENABLED
-pk_write_key_check:"data_files/ec_521_prv.pem"
+depends_on:MBEDTLS_ECP_C:MBEDTLS_PEM_PARSE_C:MBEDTLS_PEM_WRITE_C:MBEDTLS_ECP_DP_SECP521R1_ENABLED
+pk_write_key_check:"data_files/ec_521_prv.pem":TEST_PEM
+
+Private key write check EC 521 bits (DER)
+depends_on:MBEDTLS_ECP_C:MBEDTLS_ECP_DP_SECP521R1_ENABLED
+pk_write_key_check:"data_files/ec_521_prv.der":TEST_DER
 
 Private key write check EC 521 bits (top byte is 0)
-depends_on:MBEDTLS_ECP_C:MBEDTLS_BASE64_C:MBEDTLS_ECP_DP_SECP521R1_ENABLED
-pk_write_key_check:"data_files/ec_521_short_prv.pem"
+depends_on:MBEDTLS_ECP_C:MBEDTLS_PEM_PARSE_C:MBEDTLS_PEM_WRITE_C:MBEDTLS_ECP_DP_SECP521R1_ENABLED
+pk_write_key_check:"data_files/ec_521_short_prv.pem":TEST_PEM
+
+Private key write check EC 521 bits (top byte is 0) (DER)
+depends_on:MBEDTLS_ECP_C:MBEDTLS_ECP_DP_SECP521R1_ENABLED
+pk_write_key_check:"data_files/ec_521_short_prv.der":TEST_DER
 
 Private key write check EC Brainpool 512 bits
-depends_on:MBEDTLS_ECP_C:MBEDTLS_BASE64_C:MBEDTLS_ECP_DP_BP512R1_ENABLED
-pk_write_key_check:"data_files/ec_bp512_prv.pem"
+depends_on:MBEDTLS_ECP_C:MBEDTLS_PEM_PARSE_C:MBEDTLS_PEM_WRITE_C:MBEDTLS_ECP_DP_BP512R1_ENABLED
+pk_write_key_check:"data_files/ec_bp512_prv.pem":TEST_PEM
+
+Private key write check EC Brainpool 512 bits (DER)
+depends_on:MBEDTLS_ECP_C:MBEDTLS_ECP_DP_BP512R1_ENABLED
+pk_write_key_check:"data_files/ec_bp512_prv.der":TEST_DER
diff --git a/tests/suites/test_suite_pkwrite.function b/tests/suites/test_suite_pkwrite.function
index a49846b..08ec835 100644
--- a/tests/suites/test_suite_pkwrite.function
+++ b/tests/suites/test_suite_pkwrite.function
@@ -2,6 +2,102 @@
 #include "mbedtls/pk.h"
 #include "mbedtls/pem.h"
 #include "mbedtls/oid.h"
+
+typedef enum {
+    TEST_PEM,
+    TEST_DER
+} pkwrite_file_format_t;
+
+/* Helper function for removing "\r" chars from a buffer. */
+static void fix_new_lines(unsigned char *in_str, size_t *len)
+{
+    size_t chars_left;
+    unsigned int i;
+
+    for (i = 0; (i < *len) && (*len > 0); i++) {
+        if (in_str[i] == '\r') {
+            if (i < (*len - 1)) {
+                chars_left = *len - i - 1;
+                memmove(&in_str[i], &in_str[i+1], chars_left);
+            } else {
+                in_str[i] = '\0';
+            }
+            *len = *len - 1;
+        }
+    }
+}
+
+static void pk_write_check_common(char *key_file, int is_public_key, int is_der)
+{
+    mbedtls_pk_context key;
+    unsigned char *buf = NULL;
+    unsigned char *check_buf = NULL;
+    unsigned char *start_buf;
+    size_t buf_len, check_buf_len;
+    int ret;
+
+    /* Note: if mbedtls_pk_load_file() successfully reads the file, then
+       it also allocates check_buf, which should be freed on exit */
+    TEST_EQUAL(mbedtls_pk_load_file(key_file, &check_buf, &check_buf_len), 0);
+    TEST_ASSERT(check_buf_len > 0);
+
+    /* Windows' line ending is different from the Linux's one ("\r\n" vs "\n").
+     * Git treats PEM files as text, so when on Windows, it replaces new lines
+     * with "\r\n" on checkout.
+     * Unfortunately mbedtls_pk_load_file() loads files in binary format,
+     * while mbedtls_pk_write_pubkey_pem() goes through the I/O layer which
+     * uses "\n" for newlines in both Windows and Linux.
+     * Here we remove the extra "\r" so that "buf" and "check_buf" can be
+     * easily compared later. */
+    if (!is_der) {
+        fix_new_lines(check_buf, &check_buf_len);
+    }
+    TEST_ASSERT(check_buf_len > 0);
+
+    ASSERT_ALLOC(buf, check_buf_len);
+
+    mbedtls_pk_init(&key);
+    if (is_public_key) {
+        TEST_EQUAL(mbedtls_pk_parse_public_keyfile(&key, key_file), 0);
+        if (is_der) {
+            ret = mbedtls_pk_write_pubkey_der(&key, buf, check_buf_len);
+        } else {
+#if defined(MBEDTLS_PEM_WRITE_C)
+            ret = mbedtls_pk_write_pubkey_pem(&key, buf, check_buf_len);
+#else
+            ret = MBEDTLS_ERR_PK_FEATURE_UNAVAILABLE;
+#endif
+        }
+    } else {
+        TEST_EQUAL(mbedtls_pk_parse_keyfile(&key, key_file, NULL), 0);
+        if (is_der) {
+            ret = mbedtls_pk_write_key_der(&key, buf, check_buf_len);
+        } else {
+#if defined(MBEDTLS_PEM_WRITE_C)
+            ret = mbedtls_pk_write_key_pem(&key, buf, check_buf_len);
+#else
+            ret = MBEDTLS_ERR_PK_FEATURE_UNAVAILABLE;
+#endif
+        }
+    }
+
+    if (is_der) {
+        TEST_LE_U(1, ret);
+        buf_len = ret;
+        start_buf = buf + check_buf_len - buf_len;
+    } else {
+        TEST_EQUAL(ret, 0);
+        buf_len = strlen((char *) buf) + 1; /* +1 takes the string terminator into account */
+        start_buf = buf;
+    }
+
+    ASSERT_COMPARE(start_buf, buf_len, check_buf, check_buf_len);
+
+exit:
+    mbedtls_free(buf);
+    mbedtls_free(check_buf);
+    mbedtls_pk_free(&key);
+}
 /* END_HEADER */
 
 /* BEGIN_DEPENDENCIES
@@ -9,80 +105,18 @@
  * END_DEPENDENCIES
  */
 
-/* BEGIN_CASE depends_on:MBEDTLS_PEM_WRITE_C */
-void pk_write_pubkey_check(char *key_file)
+/* BEGIN_CASE */
+void pk_write_pubkey_check(char *key_file, int is_der)
 {
-    mbedtls_pk_context key;
-    unsigned char buf[5000];
-    unsigned char check_buf[5000];
-    int ret;
-    FILE *f;
-    size_t ilen, pem_len, buf_index;
-
-    memset(buf, 0, sizeof(buf));
-    memset(check_buf, 0, sizeof(check_buf));
-
-    mbedtls_pk_init(&key);
-    TEST_ASSERT(mbedtls_pk_parse_public_keyfile(&key, key_file) == 0);
-
-    ret = mbedtls_pk_write_pubkey_pem(&key, buf, sizeof(buf));
-    TEST_ASSERT(ret == 0);
-
-    pem_len = strlen((char *) buf);
-
-    // check that the rest of the buffer remains clear
-    for (buf_index = pem_len; buf_index < sizeof(buf); ++buf_index) {
-        TEST_ASSERT(buf[buf_index] == 0);
-    }
-
-    f = fopen(key_file, "r");
-    TEST_ASSERT(f != NULL);
-    ilen = fread(check_buf, 1, sizeof(check_buf), f);
-    fclose(f);
-
-    TEST_ASSERT(ilen == pem_len);
-    TEST_ASSERT(memcmp((char *) buf, (char *) check_buf, ilen) == 0);
-
-exit:
-    mbedtls_pk_free(&key);
+    pk_write_check_common(key_file, 1, is_der);
+    goto exit; /* make the compiler happy */
 }
 /* END_CASE */
 
-/* BEGIN_CASE depends_on:MBEDTLS_PEM_WRITE_C */
-void pk_write_key_check(char *key_file)
+/* BEGIN_CASE */
+void pk_write_key_check(char *key_file, int is_der)
 {
-    mbedtls_pk_context key;
-    unsigned char buf[5000];
-    unsigned char check_buf[5000];
-    int ret;
-    FILE *f;
-    size_t ilen, pem_len, buf_index;
-
-    memset(buf, 0, sizeof(buf));
-    memset(check_buf, 0, sizeof(check_buf));
-
-    mbedtls_pk_init(&key);
-    TEST_ASSERT(mbedtls_pk_parse_keyfile(&key, key_file, NULL) == 0);
-
-    ret = mbedtls_pk_write_key_pem(&key, buf, sizeof(buf));
-    TEST_ASSERT(ret == 0);
-
-    pem_len = strlen((char *) buf);
-
-    // check that the rest of the buffer remains clear
-    for (buf_index = pem_len; buf_index < sizeof(buf); ++buf_index) {
-        TEST_ASSERT(buf[buf_index] == 0);
-    }
-
-    f = fopen(key_file, "r");
-    TEST_ASSERT(f != NULL);
-    ilen = fread(check_buf, 1, sizeof(check_buf), f);
-    fclose(f);
-
-    TEST_ASSERT(ilen == strlen((char *) buf));
-    TEST_ASSERT(memcmp((char *) buf, (char *) check_buf, ilen) == 0);
-
-exit:
-    mbedtls_pk_free(&key);
+    pk_write_check_common(key_file, 0, is_der);
+    goto exit; /* make the compiler happy */
 }
 /* END_CASE */