diff --git a/include/mbedtls/rsa.h b/include/mbedtls/rsa.h
index 4ff5bdd..489f2ed 100644
--- a/include/mbedtls/rsa.h
+++ b/include/mbedtls/rsa.h
@@ -601,7 +601,8 @@
  *                 #MBEDTLS_RSA_PUBLIC or #MBEDTLS_RSA_PRIVATE (deprecated).
  * \param ilen     The length of the plaintext in Bytes.
  * \param input    The input data to encrypt. This must be a readable
- *                 buffer of size \p ilen Bytes. This must not be \c NULL.
+ *                 buffer of size \p ilen Bytes. It may be \c NULL if
+ *                 `ilen == 0`.
  * \param output   The output buffer. This must be a writable buffer
  *                 of length \c ctx->len Bytes. For example, \c 256 Bytes
  *                 for an 2048-bit RSA modulus.
@@ -641,7 +642,8 @@
  *                 #MBEDTLS_RSA_PUBLIC or #MBEDTLS_RSA_PRIVATE (deprecated).
  * \param ilen     The length of the plaintext in Bytes.
  * \param input    The input data to encrypt. This must be a readable
- *                 buffer of size \p ilen Bytes. This must not be \c NULL.
+ *                 buffer of size \p ilen Bytes. It may be \c NULL if
+ *                 `ilen == 0`.
  * \param output   The output buffer. This must be a writable buffer
  *                 of length \c ctx->len Bytes. For example, \c 256 Bytes
  *                 for an 2048-bit RSA modulus.
@@ -685,7 +687,8 @@
  * \param label_len  The length of the label in Bytes.
  * \param ilen       The length of the plaintext buffer \p input in Bytes.
  * \param input      The input data to encrypt. This must be a readable
- *                   buffer of size \p ilen Bytes. This must not be \c NULL.
+ *                   buffer of size \p ilen Bytes. It may be \c NULL if
+ *                   `ilen == 0`.
  * \param output     The output buffer. This must be a writable buffer
  *                   of length \c ctx->len Bytes. For example, \c 256 Bytes
  *                   for an 2048-bit RSA modulus.
diff --git a/library/rsa.c b/library/rsa.c
index af1a878..389d747 100644
--- a/library/rsa.c
+++ b/library/rsa.c
@@ -1135,7 +1135,7 @@
     RSA_VALIDATE_RET( mode == MBEDTLS_RSA_PRIVATE ||
                       mode == MBEDTLS_RSA_PUBLIC );
     RSA_VALIDATE_RET( output != NULL );
-    RSA_VALIDATE_RET( input != NULL );
+    RSA_VALIDATE_RET( ilen == 0 || input != NULL );
     RSA_VALIDATE_RET( label_len == 0 || label != NULL );
 
     if( mode == MBEDTLS_RSA_PRIVATE && ctx->padding != MBEDTLS_RSA_PKCS_V21 )
@@ -1218,7 +1218,7 @@
     RSA_VALIDATE_RET( mode == MBEDTLS_RSA_PRIVATE ||
                       mode == MBEDTLS_RSA_PUBLIC );
     RSA_VALIDATE_RET( output != NULL );
-    RSA_VALIDATE_RET( input != NULL );
+    RSA_VALIDATE_RET( ilen == 0 || input != NULL );
 
     if( mode == MBEDTLS_RSA_PRIVATE && ctx->padding != MBEDTLS_RSA_PKCS_V15 )
         return( MBEDTLS_ERR_RSA_BAD_INPUT_DATA );
@@ -1285,7 +1285,7 @@
     RSA_VALIDATE_RET( mode == MBEDTLS_RSA_PRIVATE ||
                       mode == MBEDTLS_RSA_PUBLIC );
     RSA_VALIDATE_RET( output != NULL );
-    RSA_VALIDATE_RET( input != NULL );
+    RSA_VALIDATE_RET( ilen == 0 || input != NULL );
 
     switch( ctx->padding )
     {