diff --git a/ChangeLog.d/psa_cipher_update_ecp.txt b/ChangeLog.d/psa_cipher_update_ecp.txt
new file mode 100644
index 000000000..1c3fbc6b1
--- /dev/null
+++ b/ChangeLog.d/psa_cipher_update_ecp.txt
@@ -0,0 +1,2 @@
+Bugfix
+   * Fix a parameter set but unused in psa_crypto_cipher.c. Fixes #4935.
diff --git a/library/psa_crypto_cipher.c b/library/psa_crypto_cipher.c
index 2213ad0c2..6c4150b4c 100644
--- a/library/psa_crypto_cipher.c
+++ b/library/psa_crypto_cipher.c
@@ -258,16 +258,34 @@ static psa_status_t cipher_set_iv( mbedtls_psa_cipher_operation_t *operation,
                                        iv, iv_length ) ) );
 }
 
-/* Process input for which the algorithm is set to ECB mode. This requires
- * manual processing, since the PSA API is defined as being able to process
- * arbitrary-length calls to psa_cipher_update() with ECB mode, but the
- * underlying mbedtls_cipher_update only takes full blocks. */
+/** Process input for which the algorithm is set to ECB mode.
+ *
+ * This requires manual processing, since the PSA API is defined as being
+ * able to process arbitrary-length calls to psa_cipher_update() with ECB mode,
+ * but the underlying mbedtls_cipher_update only takes full blocks.
+ *
+ * \param ctx           The mbedtls cipher context to use. It must have been
+ *                      set up for ECB.
+ * \param[in] input     The input plaintext or ciphertext to process.
+ * \param input_length  The number of bytes to process from \p input.
+ *                      This does not need to be aligned to a block boundary.
+ *                      If there is a partial block at the end of the input,
+ *                      it is stored in \p ctx for future processing.
+ * \param output        The buffer where the output is written. It must be
+ *                      at least `BS * floor((p + input_length) / BS)` bytes
+ *                      long, where `p` is the number of bytes in the
+ *                      unprocessed partial block in \p ctx (with
+ *                      `0 <= p <= BS - 1`) and `BS` is the block size.
+ * \param output_length On success, the number of bytes written to \p output.
+ *                      \c 0 on error.
+ *
+ * \return #PSA_SUCCESS or an error from a hardware accelerator
+ */
 static psa_status_t psa_cipher_update_ecb(
     mbedtls_cipher_context_t *ctx,
     const uint8_t *input,
     size_t input_length,
     uint8_t *output,
-    size_t output_size,
     size_t *output_length )
 {
     psa_status_t status = PSA_ERROR_CORRUPTION_DETECTED;
@@ -307,7 +325,6 @@ static psa_status_t psa_cipher_update_ecb(
                 goto exit;
 
             output += internal_output_length;
-            output_size -= internal_output_length;
             *output_length += internal_output_length;
             ctx->unprocessed_len = 0;
         }
@@ -328,7 +345,6 @@ static psa_status_t psa_cipher_update_ecb(
         input += block_size;
 
         output += internal_output_length;
-        output_size -= internal_output_length;
         *output_length += internal_output_length;
     }
 
@@ -383,7 +399,6 @@ static psa_status_t cipher_update( mbedtls_psa_cipher_operation_t *operation,
                                         input,
                                         input_length,
                                         output,
-                                        output_size,
                                         output_length );
     }
     else