diff --git a/include/mbedtls/sha256.h b/include/mbedtls/sha256.h
index 8d90ca0..aac48b2 100644
--- a/include/mbedtls/sha256.h
+++ b/include/mbedtls/sha256.h
@@ -72,25 +72,24 @@
 /**
  * \brief          This function initializes a SHA-256 context.
  *
- * \param ctx      The SHA-256 context to initialize.
- *                 Must not be \c NULL.
+ * \param ctx      The SHA-256 context to initialize. This must not be \c NULL.
  */
 void mbedtls_sha256_init( mbedtls_sha256_context *ctx );
 
 /**
  * \brief          This function clears a SHA-256 context.
  *
- * \param ctx      The SHA-256 context to clear.
+ * \param ctx      The SHA-256 context to clear. This may be \c NULL, in which
+ *                 case this function returns immediately. If it is not \c NULL,
+ *                 it must point to an initialized SHA-256 context.
  */
 void mbedtls_sha256_free( mbedtls_sha256_context *ctx );
 
 /**
  * \brief          This function clones the state of a SHA-256 context.
  *
- * \param dst      The destination context.
- *                 Must not be \c NULL.
- * \param src      The context to clone.
- *                 Must not be \c NULL.
+ * \param dst      The destination context. This must be initialized.
+ * \param src      The context to clone. This must be initialized.
  */
 void mbedtls_sha256_clone( mbedtls_sha256_context *dst,
                            const mbedtls_sha256_context *src );
@@ -99,12 +98,12 @@
  * \brief          This function starts a SHA-224 or SHA-256 checksum
  *                 calculation.
  *
- * \param ctx      The context to initialize.
- *                 Must not be \c NULL.
- * \param is224    Determines which function to use:
- *                 0: Use SHA-256, or 1: Use SHA-224.
+ * \param ctx      The context to use. This must be initialized.
+ * \param is224    This determines which function to use. This must be
+ *                 either \c 0 for SHA-256, or \c 1 for SHA-224.
  *
  * \return         \c 0 on success.
+ * \return         A negative error code on failure.
  */
 int mbedtls_sha256_starts_ret( mbedtls_sha256_context *ctx, int is224 );
 
@@ -112,13 +111,14 @@
  * \brief          This function feeds an input buffer into an ongoing
  *                 SHA-256 checksum calculation.
  *
- * \param ctx      The SHA-256 context.
- *                 Must not be \c NULL.
- * \param input    The buffer holding the data.
- *                 Must not be \c NULL if \p ilen is greater than 0.
- * \param ilen     The length of the input data.
+ * \param ctx      The SHA-256 context. This must be initialized
+ *                 and have a hash operation started.
+ * \param input    The buffer holding the data. This must be a readable
+ *                 buffer of length \p ilen Bytes.
+ * \param ilen     The length of the input data \p input in Bytes.
  *
  * \return         \c 0 on success.
+ * \return         A negative error code on failure.
  */
 int mbedtls_sha256_update_ret( mbedtls_sha256_context *ctx,
                                const unsigned char *input,
@@ -128,12 +128,13 @@
  * \brief          This function finishes the SHA-256 operation, and writes
  *                 the result to the output buffer.
  *
- * \param ctx      The SHA-256 context.
- *                 Must not be \c NULL.
+ * \param ctx      The SHA-256 context. This must be initialized
+ *                 and have a hash operation started.
  * \param output   The SHA-224 or SHA-256 checksum result.
- *                 Must not be \c NULL.
+ *                 This must be a writable buffer of length \c 32 Bytes.
  *
  * \return         \c 0 on success.
+ * \return         A negative error code on failure.
  */
 int mbedtls_sha256_finish_ret( mbedtls_sha256_context *ctx,
                                unsigned char output[32] );
@@ -143,12 +144,13 @@
  *                 the ongoing SHA-256 computation. This function is for
  *                 internal use only.
  *
- * \param ctx      The SHA-256 context.
- *                 Must not be \c NULL.
- * \param data     The buffer holding one block of data.
- *                 Must not be \c NULL.
+ * \param ctx      The SHA-256 context. This must be initialized
+ *                 and have a hash operation started.
+ * \param data     The buffer holding one block of data. This must
+ *                 be a readable buffer of length \c 64 Bytes.
  *
  * \return         \c 0 on success.
+ * \return         A negative error code on failure.
  */
 int mbedtls_internal_sha256_process( mbedtls_sha256_context *ctx,
                                      const unsigned char data[64] );
@@ -163,13 +165,11 @@
  * \brief          This function starts a SHA-224 or SHA-256 checksum
  *                 calculation.
  *
- *
  * \deprecated     Superseded by mbedtls_sha256_starts_ret() in 2.7.0.
  *
- * \param ctx      The context to initialize.
- *                 Must not be \c NULL.
- * \param is224    Determines which function to use:
- *                 0: Use SHA-256, or 1: Use SHA-224.
+ * \param ctx      The context to use. This must be initialized.
+ * \param is224    Determines which function to use. This must be
+ *                 either \c 0 for SHA-256, or \c 1 for SHA-224.
  */
 MBEDTLS_DEPRECATED void mbedtls_sha256_starts( mbedtls_sha256_context *ctx,
                                                int is224 );
@@ -180,11 +180,11 @@
  *
  * \deprecated     Superseded by mbedtls_sha256_update_ret() in 2.7.0.
  *
- * \param ctx      The SHA-256 context to initialize.
- *                 Must not be \c NULL.
- * \param input    The buffer holding the data.
- *                 Must not be \c NULL if \p ilen is greater than 0.
- * \param ilen     The length of the input data.
+ * \param ctx      The SHA-256 context to use. This must be
+ *                 initialized and have a hash operation started.
+ * \param input    The buffer holding the data. This must be a readable
+ *                 buffer of length \p ilen Bytes.
+ * \param ilen     The length of the input data \p input in Bytes.
  */
 MBEDTLS_DEPRECATED void mbedtls_sha256_update( mbedtls_sha256_context *ctx,
                                                const unsigned char *input,
@@ -196,10 +196,10 @@
  *
  * \deprecated     Superseded by mbedtls_sha256_finish_ret() in 2.7.0.
  *
- * \param ctx      The SHA-256 context.
- *                 Must not be \c NULL.
- * \param output   The SHA-224 or SHA-256 checksum result.
- *                 Must not be \c NULL.
+ * \param ctx      The SHA-256 context. This must be initialized and
+ *                 have a has hoperation started.
+ * \param output   The SHA-224 or SHA-256 checksum result. This must be
+ *                 a writable buffer of length \c 32 Bytes.
  */
 MBEDTLS_DEPRECATED void mbedtls_sha256_finish( mbedtls_sha256_context *ctx,
                                                unsigned char output[32] );
@@ -211,10 +211,10 @@
  *
  * \deprecated     Superseded by mbedtls_internal_sha256_process() in 2.7.0.
  *
- * \param ctx      The SHA-256 context.
- *                 Must not be \c NULL.
- * \param data     The buffer holding one block of data.
- *                 Must not be \c NULL.
+ * \param ctx      The SHA-256 context. This must be initialized and
+ *                 have a hash operation started.
+ * \param data     The buffer holding one block of data. This must be
+ *                 a readable buffer of size \c 64 Bytes.
  */
 MBEDTLS_DEPRECATED void mbedtls_sha256_process( mbedtls_sha256_context *ctx,
                                                 const unsigned char data[64] );
@@ -232,13 +232,13 @@
  *                 The SHA-256 result is calculated as
  *                 output = SHA-256(input buffer).
  *
- * \param input    The buffer holding the input data.
- *                 Must not be \c NULL if \p ilen is greater than 0.
- * \param ilen     The length of the input data.
- * \param output   The SHA-224 or SHA-256 checksum result.
- *                 Must not be \c NULL.
- * \param is224    Determines which function to use:
- *                 0: Use SHA-256, or 1: Use SHA-224.
+ * \param input    The buffer holding the data. This must be a readable
+ *                 buffer of length \p ilen Bytes.
+ * \param ilen     The length of the input data \p input in Bytes.
+ * \param output   The SHA-224 or SHA-256 checksum result. This must
+ *                 be a writable buffer of length \c 32 Bytes.
+ * \param is224    Determines which function to use. This must be
+ *                 either \c 0 for SHA-256, or \c 1 for SHA-224.
  */
 int mbedtls_sha256_ret( const unsigned char *input,
                         size_t ilen,
@@ -264,13 +264,13 @@
  *
  * \deprecated     Superseded by mbedtls_sha256_ret() in 2.7.0.
  *
- * \param input    The buffer holding the data.
- *                 Must not be \c NULL if \p ilen is greater than 0.
- * \param ilen     The length of the input data.
- * \param output   The SHA-224 or SHA-256 checksum result.
- *                 Must not be \c NULL.
- * \param is224    Determines which function to use:
- *                 0: Use SHA-256, or 1: Use SHA-224.
+ * \param input    The buffer holding the data. This must be a readable
+ *                 buffer of length \p ilen Bytes.
+ * \param ilen     The length of the input data \p input in Bytes.
+ * \param output   The SHA-224 or SHA-256 checksum result. This must be
+ *                 a writable buffer of length \c 32 Bytes.
+ * \param is224    Determines which function to use. This must be eithern
+ *                 \c 0 for SHA-256, or \c 1 for SHA-224.
  */
 MBEDTLS_DEPRECATED void mbedtls_sha256( const unsigned char *input,
                                         size_t ilen,