diff --git a/include/mbedtls/check_config.h b/include/mbedtls/check_config.h
index 48e96e5ce..8ea1c1572 100644
--- a/include/mbedtls/check_config.h
+++ b/include/mbedtls/check_config.h
@@ -107,7 +107,7 @@
 #error "MBEDTLS_ECDSA_DETERMINISTIC defined, but not all prerequisites"
 #endif
 
-#if defined(MBEDTLS_ECP_C) && ( !defined(MBEDTLS_BIGNUM_C) || (   \
+#if defined(MBEDTLS_ECP_C) && ( !defined(MBEDTLS_BIGNUM_C) || (    \
     !defined(MBEDTLS_ECP_DP_SECP192R1_ENABLED) &&                  \
     !defined(MBEDTLS_ECP_DP_SECP224R1_ENABLED) &&                  \
     !defined(MBEDTLS_ECP_DP_SECP256R1_ENABLED) &&                  \
@@ -118,7 +118,8 @@
     !defined(MBEDTLS_ECP_DP_BP512R1_ENABLED)   &&                  \
     !defined(MBEDTLS_ECP_DP_SECP192K1_ENABLED) &&                  \
     !defined(MBEDTLS_ECP_DP_SECP224K1_ENABLED) &&                  \
-    !defined(MBEDTLS_ECP_DP_SECP256K1_ENABLED) ) )
+    !defined(MBEDTLS_ECP_DP_SECP256K1_ENABLED) &&                  \
+    !defined(MBEDTLS_ECP_DP_CURVE25519_ENABLED) ) )
 #error "MBEDTLS_ECP_C defined, but not all prerequisites"
 #endif
 
diff --git a/include/mbedtls/pk.h b/include/mbedtls/pk.h
index ee06b2fd2..8beb2af41 100644
--- a/include/mbedtls/pk.h
+++ b/include/mbedtls/pk.h
@@ -343,6 +343,10 @@ int mbedtls_pk_verify_ext( mbedtls_pk_type_t type, const void *options,
  *
  * \note            For RSA, md_alg may be MBEDTLS_MD_NONE if hash_len != 0.
  *                  For ECDSA, md_alg may never be MBEDTLS_MD_NONE.
+ *
+ * \note            In order to ensure enough space for the signature, the
+ *                  \p sig buffer size must be of at least
+ *                  `max(MBEDTLS_ECDSA_MAX_LEN, MBEDTLS_MPI_MAX_SIZE)` bytes.
  */
 int mbedtls_pk_sign( mbedtls_pk_context *ctx, mbedtls_md_type_t md_alg,
              const unsigned char *hash, size_t hash_len,
diff --git a/include/mbedtls/rsa.h b/include/mbedtls/rsa.h
index 23e0ebb0c..a1298da81 100644
--- a/include/mbedtls/rsa.h
+++ b/include/mbedtls/rsa.h
@@ -819,6 +819,7 @@ int mbedtls_rsa_rsaes_oaep_decrypt( mbedtls_rsa_context *ctx,
  *
  * \note           The \p sig buffer must be as large as the size
  *                 of \p ctx->N. For example, 128 Bytes if RSA-1024 is used.
+ *                 A buffer length of #MBEDTLS_MPI_MAX_SIZE is always safe.
  *
  * \note           For PKCS#1 v2.1 encoding, see comments on
  *                 mbedtls_rsa_rsassa_pss_sign() for details on
@@ -862,6 +863,7 @@ int mbedtls_rsa_pkcs1_sign( mbedtls_rsa_context *ctx,
  *
  * \note           The \p sig buffer must be as large as the size
  *                 of \p ctx->N. For example, 128 Bytes if RSA-1024 is used.
+ *                 A buffer length of #MBEDTLS_MPI_MAX_SIZE is always safe.
  */
 int mbedtls_rsa_rsassa_pkcs1_v15_sign( mbedtls_rsa_context *ctx,
                                int (*f_rng)(void *, unsigned char *, size_t),
@@ -902,6 +904,7 @@ int mbedtls_rsa_rsassa_pkcs1_v15_sign( mbedtls_rsa_context *ctx,
  *
  * \note           The \p sig buffer must be as large as the size
  *                 of \p ctx->N. For example, 128 Bytes if RSA-1024 is used.
+ *                 A buffer length of #MBEDTLS_MPI_MAX_SIZE is always safe.
  *
  * \note           The \p hash_id in the RSA context is the one used for the
  *                 encoding. \p md_alg in the function call is the type of hash
diff --git a/library/x509write_crt.c b/library/x509write_crt.c
index 4cdb941a1..905de4cfa 100644
--- a/library/x509write_crt.c
+++ b/library/x509write_crt.c
@@ -49,6 +49,16 @@ static void mbedtls_zeroize( void *v, size_t n ) {
     volatile unsigned char *p = v; while( n-- ) *p++ = 0;
 }
 
+/*
+ * For the currently used signature algorithms the buffer to store any signature
+ * must be at least of size MAX(MBEDTLS_ECDSA_MAX_LEN, MBEDTLS_MPI_MAX_SIZE)
+ */
+#if MBEDTLS_ECDSA_MAX_LEN > MBEDTLS_MPI_MAX_SIZE
+#define SIGNATURE_MAX_SIZE MBEDTLS_ECDSA_MAX_LEN
+#else
+#define SIGNATURE_MAX_SIZE MBEDTLS_MPI_MAX_SIZE
+#endif
+
 void mbedtls_x509write_crt_init( mbedtls_x509write_cert *ctx )
 {
     memset( ctx, 0, sizeof( mbedtls_x509write_cert ) );
@@ -338,7 +348,7 @@ int mbedtls_x509write_crt_der( mbedtls_x509write_cert *ctx, unsigned char *buf,
     size_t sig_oid_len = 0;
     unsigned char *c, *c2;
     unsigned char hash[64];
-    unsigned char sig[MBEDTLS_MPI_MAX_SIZE];
+    unsigned char sig[SIGNATURE_MAX_SIZE];
     unsigned char tmp_buf[2048];
     size_t sub_len = 0, pub_len = 0, sig_and_oid_len = 0, sig_len;
     size_t len = 0;
diff --git a/library/x509write_csr.c b/library/x509write_csr.c
index d59354dc4..d6e8c1306 100644
--- a/library/x509write_csr.c
+++ b/library/x509write_csr.c
@@ -48,6 +48,16 @@ static void mbedtls_zeroize( void *v, size_t n ) {
     volatile unsigned char *p = v; while( n-- ) *p++ = 0;
 }
 
+/*
+ * For the currently used signature algorithms the buffer to store any signature
+ * must be at least of size MAX(MBEDTLS_ECDSA_MAX_LEN, MBEDTLS_MPI_MAX_SIZE)
+ */
+#if MBEDTLS_ECDSA_MAX_LEN > MBEDTLS_MPI_MAX_SIZE
+#define SIGNATURE_MAX_SIZE MBEDTLS_ECDSA_MAX_LEN
+#else
+#define SIGNATURE_MAX_SIZE MBEDTLS_MPI_MAX_SIZE
+#endif
+
 void mbedtls_x509write_csr_init( mbedtls_x509write_csr *ctx )
 {
     memset( ctx, 0, sizeof( mbedtls_x509write_csr ) );
@@ -163,7 +173,7 @@ int mbedtls_x509write_csr_der( mbedtls_x509write_csr *ctx, unsigned char *buf, s
     size_t sig_oid_len = 0;
     unsigned char *c, *c2;
     unsigned char hash[64];
-    unsigned char sig[MBEDTLS_MPI_MAX_SIZE];
+    unsigned char sig[SIGNATURE_MAX_SIZE];
     unsigned char tmp_buf[2048];
     size_t pub_len = 0, sig_and_oid_len = 0, sig_len;
     size_t len = 0;
diff --git a/programs/pkey/pk_sign.c b/programs/pkey/pk_sign.c
index 7ec46752a..0f08db972 100644
--- a/programs/pkey/pk_sign.c
+++ b/programs/pkey/pk_sign.c
@@ -59,6 +59,16 @@ int main( void )
 #include <stdio.h>
 #include <string.h>
 
+/*
+ * For the currently used signature algorithms the buffer to store any signature
+ * must be at least of size MAX(MBEDTLS_ECDSA_MAX_LEN, MBEDTLS_MPI_MAX_SIZE)
+ */
+#if MBEDTLS_ECDSA_MAX_LEN > MBEDTLS_MPI_MAX_SIZE
+#define SIGNATURE_MAX_SIZE MBEDTLS_ECDSA_MAX_LEN
+#else
+#define SIGNATURE_MAX_SIZE MBEDTLS_MPI_MAX_SIZE
+#endif
+
 int main( int argc, char *argv[] )
 {
     FILE *f;
@@ -68,7 +78,7 @@ int main( int argc, char *argv[] )
     mbedtls_entropy_context entropy;
     mbedtls_ctr_drbg_context ctr_drbg;
     unsigned char hash[32];
-    unsigned char buf[MBEDTLS_MPI_MAX_SIZE];
+    unsigned char buf[SIGNATURE_MAX_SIZE];
     char filename[512];
     const char *pers = "mbedtls_pk_sign";
     size_t olen = 0;
diff --git a/tests/data_files/Makefile b/tests/data_files/Makefile
index c10020c04..8fc4f76e6 100644
--- a/tests/data_files/Makefile
+++ b/tests/data_files/Makefile
@@ -733,6 +733,14 @@ server1.req.cert_type_empty: server1.key
 	$(MBEDTLS_CERT_REQ) output_file=$@ filename=$< subject_name="C=NL,O=PolarSSL,CN=PolarSSL Server 1" md=SHA1 force_ns_cert_type=1
 all_final += server1.req.cert_type_empty
 
+###
+### A generic SECP521R1 private key
+###
+
+secp521r1_prv.der:
+	$(OPENSSL) ecparam -genkey -name secp521r1 -noout -out secp521r1_prv.der
+all_final += secp521r1_prv.der
+
 ################################################################
 ### Generate certificates for CRT write check tests
 ################################################################
diff --git a/tests/data_files/secp521r1_prv.der b/tests/data_files/secp521r1_prv.der
new file mode 100644
index 000000000..4d342bdc2
Binary files /dev/null and b/tests/data_files/secp521r1_prv.der differ
diff --git a/tests/scripts/all.sh b/tests/scripts/all.sh
index 7130abcd6..5e5ae86b5 100755
--- a/tests/scripts/all.sh
+++ b/tests/scripts/all.sh
@@ -591,6 +591,23 @@ component_check_doxygen_warnings () {
 #### Build and test many configurations and targets
 ################################################################
 
+component_test_large_ecdsa_key_signature () {
+
+    SMALL_MPI_MAX_SIZE=136 # Small enough to interfere with the EC signatures
+
+    msg "build: cmake + MBEDTLS_MPI_MAX_SIZE=${SMALL_MPI_MAX_SIZE}, gcc, ASan" # ~ 1 min 50s
+    scripts/config.pl set MBEDTLS_MPI_MAX_SIZE $SMALL_MPI_MAX_SIZE
+    CC=gcc cmake -D CMAKE_BUILD_TYPE:String=Asan .
+    make
+
+    INEVITABLY_PRESENT_FILE=Makefile
+    SIGNATURE_FILE="${INEVITABLY_PRESENT_FILE}.sig" # Warning, this is rm -f'ed below
+
+    msg "test: pk_sign secp521r1_prv.der for MBEDTLS_MPI_MAX_SIZE=${SMALL_MPI_MAX_SIZE} (ASan build)" # ~ 5s
+    if_build_succeeded programs/pkey/pk_sign tests/data_files/secp521r1_prv.der $INEVITABLY_PRESENT_FILE
+    rm -f $SIGNATURE_FILE
+}
+
 component_test_default_out_of_box () {
     msg "build: make, default config (out-of-box)" # ~1min
     make