diff --git a/ChangeLog.d/issue4093.txt b/ChangeLog.d/issue4093.txt
new file mode 100644
index 0000000..f6985cf
--- /dev/null
+++ b/ChangeLog.d/issue4093.txt
@@ -0,0 +1,6 @@
+Security
+   * Fix an errorneous estimation for an internal buffer in
+     mbedtls_pk_write_key_pem(). If MBEDTLS_MPI_MAX_SIZE is set to an odd
+     value the function might fail to write a private RSA keys of the largest
+     supported size.
+     Found by Daniel Otte, reported in #4093 and fixed in #4094.
diff --git a/ChangeLog.d/mpi_sub_abs.txt b/ChangeLog.d/mpi_sub_abs.txt
new file mode 100644
index 0000000..9f34ee7
--- /dev/null
+++ b/ChangeLog.d/mpi_sub_abs.txt
@@ -0,0 +1,7 @@
+Security
+   * Fix a buffer overflow in mbedtls_mpi_sub_abs() when calculating
+     |A| - |B| where |B| is larger than |A| and has more limbs (so the
+     function should return MBEDTLS_ERR_MPI_NEGATIVE_VALUE). Only
+     applications calling mbedtls_mpi_sub_abs() directly are affected:
+     all calls inside the library were safe since this function is
+     only called with |A| >= |B|. Reported by Guido Vranken in #4042.
diff --git a/library/bignum.c b/library/bignum.c
index 9cc5d66..e74a1ad 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -1401,6 +1401,12 @@
     for( n = B->n; n > 0; n-- )
         if( B->p[n - 1] != 0 )
             break;
+    if( n > A->n )
+    {
+        /* B >= (2^ciL)^n > A */
+        ret = MBEDTLS_ERR_MPI_NEGATIVE_VALUE;
+        goto cleanup;
+    }
 
     carry = mpi_sub_hlp( n, X->p, B->p );
     if( carry != 0 )
diff --git a/library/pkwrite.c b/library/pkwrite.c
index 0da3698..566153d 100644
--- a/library/pkwrite.c
+++ b/library/pkwrite.c
@@ -487,7 +487,7 @@
  *      publicExponent    INTEGER   -- e            1 + 3 + MPI_MAX + 1
  *  }
  */
-#define RSA_PUB_DER_MAX_BYTES   38 + 2 * MBEDTLS_MPI_MAX_SIZE
+#define RSA_PUB_DER_MAX_BYTES   ( 38 + 2 * MBEDTLS_MPI_MAX_SIZE )
 
 /*
  * RSA private keys:
@@ -504,10 +504,10 @@
  *      otherPrimeInfos   OtherPrimeInfos OPTIONAL  0 (not supported)
  *  }
  */
-#define MPI_MAX_SIZE_2          MBEDTLS_MPI_MAX_SIZE / 2 + \
-                                MBEDTLS_MPI_MAX_SIZE % 2
-#define RSA_PRV_DER_MAX_BYTES   47 + 3 * MBEDTLS_MPI_MAX_SIZE \
-                                   + 5 * MPI_MAX_SIZE_2
+#define MPI_MAX_SIZE_2          ( MBEDTLS_MPI_MAX_SIZE / 2 + \
+                                  MBEDTLS_MPI_MAX_SIZE % 2 )
+#define RSA_PRV_DER_MAX_BYTES   ( 47 + 3 * MBEDTLS_MPI_MAX_SIZE \
+                                   + 5 * MPI_MAX_SIZE_2 )
 
 #else /* MBEDTLS_RSA_C */
 
@@ -528,7 +528,7 @@
  *                                            + 2 * ECP_MAX (coords)    [1]
  *  }
  */
-#define ECP_PUB_DER_MAX_BYTES   30 + 2 * MBEDTLS_ECP_MAX_BYTES
+#define ECP_PUB_DER_MAX_BYTES   ( 30 + 2 * MBEDTLS_ECP_MAX_BYTES )
 
 /*
  * EC private keys:
@@ -539,7 +539,7 @@
  *      publicKey  [1] BIT STRING OPTIONAL      1 + 2 + [1] above
  *    }
  */
-#define ECP_PRV_DER_MAX_BYTES   29 + 3 * MBEDTLS_ECP_MAX_BYTES
+#define ECP_PRV_DER_MAX_BYTES   ( 29 + 3 * MBEDTLS_ECP_MAX_BYTES )
 
 #else /* MBEDTLS_ECP_C */
 
@@ -548,10 +548,10 @@
 
 #endif /* MBEDTLS_ECP_C */
 
-#define PUB_DER_MAX_BYTES   RSA_PUB_DER_MAX_BYTES > ECP_PUB_DER_MAX_BYTES ? \
-                            RSA_PUB_DER_MAX_BYTES : ECP_PUB_DER_MAX_BYTES
-#define PRV_DER_MAX_BYTES   RSA_PRV_DER_MAX_BYTES > ECP_PRV_DER_MAX_BYTES ? \
-                            RSA_PRV_DER_MAX_BYTES : ECP_PRV_DER_MAX_BYTES
+#define PUB_DER_MAX_BYTES   ( RSA_PUB_DER_MAX_BYTES > ECP_PUB_DER_MAX_BYTES ? \
+                              RSA_PUB_DER_MAX_BYTES : ECP_PUB_DER_MAX_BYTES )
+#define PRV_DER_MAX_BYTES   ( RSA_PRV_DER_MAX_BYTES > ECP_PRV_DER_MAX_BYTES ? \
+                              RSA_PRV_DER_MAX_BYTES : ECP_PRV_DER_MAX_BYTES )
 
 int mbedtls_pk_write_pubkey_pem( mbedtls_pk_context *key, unsigned char *buf, size_t size )
 {
diff --git a/tests/suites/test_suite_mpi.data b/tests/suites/test_suite_mpi.data
index 0966393..b5f6844 100644
--- a/tests/suites/test_suite_mpi.data
+++ b/tests/suites/test_suite_mpi.data
@@ -484,18 +484,30 @@
 Test mbedtls_mpi_add_int #2
 mbedtls_mpi_add_int:10:"2039568783564019774057658669290345772801939933143482630947726464532830627227012776329":-9871232:10:"2039568783564019774057658669290345772801939933143482630947726464532830627227002905097"
 
-Base test mbedtls_mpi_sub_abs #1 (Test with larger second input)
+Base test mbedtls_mpi_sub_abs #1 (|B| > |A|)
 mbedtls_mpi_sub_abs:10:"5":10:"7":10:"0":MBEDTLS_ERR_MPI_NEGATIVE_VALUE
 
-Base test mbedtls_mpi_sub_abs #2 (Test with larger second input)
+Base test mbedtls_mpi_sub_abs #2 (|B| > |A|)
 mbedtls_mpi_sub_abs:10:"-5":10:"-7":10:"0":MBEDTLS_ERR_MPI_NEGATIVE_VALUE
 
-Base test mbedtls_mpi_sub_abs #3 (Test with larger second input)
+Base test mbedtls_mpi_sub_abs #3 (|B| > |A|)
 mbedtls_mpi_sub_abs:10:"-5":10:"7":10:"0":MBEDTLS_ERR_MPI_NEGATIVE_VALUE
 
-Base test mbedtls_mpi_sub_abs #4 (Test with larger second input)
+Base test mbedtls_mpi_sub_abs #4 (|B| > |A|)
 mbedtls_mpi_sub_abs:10:"5":10:"-7":10:"0":MBEDTLS_ERR_MPI_NEGATIVE_VALUE
 
+Base test mbedtls_mpi_sub_abs #1 (|B| >> |A| with more limbs)
+mbedtls_mpi_sub_abs:10:"5":16:"123456789abcdef01":10:"0":MBEDTLS_ERR_MPI_NEGATIVE_VALUE
+
+Base test mbedtls_mpi_sub_abs #2 (|B| >> |A| with more limbs)
+mbedtls_mpi_sub_abs:10:"-5":16:"-123456789abcdef01":10:"0":MBEDTLS_ERR_MPI_NEGATIVE_VALUE
+
+Base test mbedtls_mpi_sub_abs #3 (|B| >> |A| with more limbs)
+mbedtls_mpi_sub_abs:10:"-5":16:"123456789abcdef01":10:"0":MBEDTLS_ERR_MPI_NEGATIVE_VALUE
+
+Base test mbedtls_mpi_sub_abs #4 (|B| >> |A| with more limbs)
+mbedtls_mpi_sub_abs:10:"5":16:"-123456789abcdef01":10:"0":MBEDTLS_ERR_MPI_NEGATIVE_VALUE
+
 Base test mbedtls_mpi_sub_abs #1
 mbedtls_mpi_sub_abs:10:"7":10:"5":10:"2":0