diff --git a/tests/suites/test_suite_ssl.data b/tests/suites/test_suite_ssl.data
index 45765de..8e43852 100644
--- a/tests/suites/test_suite_ssl.data
+++ b/tests/suites/test_suite_ssl.data
@@ -1,3 +1,36 @@
+Test calback buffer sanity
+test_callback_buffer_sanity:
+
+Callback buffer test: Exercise simple write/read
+test_callback_buffer:50:25:25:25:25:0:0:0:0
+
+Callback buffer test: Filling up the buffer
+test_callback_buffer:50:50:50:50:50:0:0:0:0
+
+Callback buffer test: Filling up the buffer in two steps
+test_callback_buffer:50:20:20:0:0:30:30:50:50
+
+Callback buffer test: Reading out the buffer in two steps
+test_callback_buffer:50:50:50:30:30:0:0:20:20
+
+Callback buffer test: Data wraps in buffer
+test_callback_buffer:50:45:45:10:10:10:10:45:45
+
+Callback buffer test: Data starts at the end
+test_callback_buffer:50:50:50:49:49:10:10:11:11
+
+Callback buffer test: Can write less than requested
+test_callback_buffer:50:75:50:30:30:25:25:45:45
+
+Callback buffer test: Can read less than requested
+test_callback_buffer:50:25:25:30:25:5:5:5:5
+
+Callback buffer test: Writing to full buffer
+test_callback_buffer:50:50:50:0:0:10:0:60:50
+
+Callback buffer test: Reading from empty buffer
+test_callback_buffer:50:0:0:10:0:0:0:0:0
+
 SSL DTLS replay: initial state, seqnum 0
 ssl_dtls_replay:"":"000000000000":0
 
diff --git a/tests/suites/test_suite_ssl.function b/tests/suites/test_suite_ssl.function
index 2fa716b..ea81a8b 100644
--- a/tests/suites/test_suite_ssl.function
+++ b/tests/suites/test_suite_ssl.function
@@ -2,6 +2,141 @@
 #include <mbedtls/ssl.h>
 #include <mbedtls/ssl_internal.h>
 
+
+/*
+ * Buffer structure for custom I/O callbacks.
+ */
+
+typedef struct mbedtls_test_buffer
+{
+    size_t start;
+    size_t content_length;
+    size_t capacity;
+    unsigned char *buffer;
+} mbedtls_test_buffer;
+
+/*
+ * Initialises \p buf. After calling this function it is safe to call
+ * `mbedtls_test_buffer_free()` on \p buf.
+ */
+void mbedtls_test_buffer_init( mbedtls_test_buffer *buf )
+{
+    memset( buf, 0, sizeof( *buf ) );
+}
+
+/*
+ * Sets up \p buf. After calling this function it is safe to call
+ * `mbedtls_test_buffer_put()` and `mbedtls_test_buffer_get()` on \p buf.
+ */
+int mbedtls_test_buffer_setup( mbedtls_test_buffer *buf, size_t capacity )
+{
+    buf->buffer = (unsigned char*) mbedtls_calloc( capacity,
+                                                   sizeof(unsigned char) );
+    if( NULL == buf->buffer )
+        return MBEDTLS_ERR_SSL_ALLOC_FAILED;
+    buf->capacity = capacity;
+
+    return 0;
+}
+
+void mbedtls_test_buffer_free( mbedtls_test_buffer *buf )
+{
+    if( buf->buffer != NULL )
+        mbedtls_free( buf->buffer );
+
+    memset( buf, 0, sizeof( *buf ) );
+}
+
+/*
+ * Puts \p input_len bytes from the \p input buffer into the ring buffer \p buf.
+ *
+ * \p buf must have been initialized and set up by calling
+ * `mbedtls_test_buffer_init()` and `mbedtls_test_buffer_setup()`.
+ *
+ * \retval  \p input_len, if the data fits.
+ * \retval  0 <= value < \p input_len, if the data does not fit.
+ * \retval  -1, if \p buf is NULL, it hasn't been set up or \p input_len is not
+ *          zero and \p input is NULL.
+ */
+int mbedtls_test_buffer_put( mbedtls_test_buffer *buf,
+                             const unsigned char* input, size_t input_len )
+{
+    size_t overflow = 0;
+
+    if( ( buf == NULL ) || ( buf->buffer == NULL ) )
+        return -1;
+
+    /* Reduce input_len to a number that fits in the buffer. */
+    if ( ( buf->content_length + input_len ) > buf->capacity )
+    {
+        input_len = buf->capacity - buf->content_length;
+    }
+
+    if( input == NULL )
+    {
+        return ( input_len == 0 ) ? 0 : -1;
+    }
+
+    /* Calculate the number of bytes that need to be placed at lower memory
+     * address */
+    if( buf->start + buf->content_length + input_len
+        > buf->capacity )
+    {
+        overflow = ( buf->start + buf->content_length + input_len )
+                    % buf->capacity;
+    }
+
+    memcpy( buf->buffer + buf->start + buf->content_length, input,
+            input_len - overflow );
+    memcpy( buf->buffer, input + input_len - overflow, overflow );
+    buf->content_length += input_len;
+
+    return input_len;
+}
+
+/*
+ * Gets \p output_len bytes from the \p output buffer into the ring buffer
+ * \p buf.
+ *
+ * \p buf must have been initialized and set up by calling
+ * `mbedtls_test_buffer_init()` and `mbedtls_test_buffer_setup()`.
+ *
+ * \retval  \p output_len, if the data is available.
+ * \retval  0 <= value < \p output_len, if the data is not available.
+ * \retval  -1, if \buf is NULL, it hasn't been set up or \p output_len is not
+ *          zero and \p output is NULL
+ */
+int mbedtls_test_buffer_get( mbedtls_test_buffer *buf,
+                             unsigned char* output, size_t output_len )
+{
+    size_t overflow = 0;
+
+    if( ( buf == NULL ) || ( buf->buffer == NULL ) )
+        return -1;
+
+    if( output == NULL )
+    {
+        return ( output_len == 0 ) ? 0 : -1;
+    }
+
+    if( buf->content_length < output_len )
+        output_len = buf->content_length;
+
+    /* Calculate the number of bytes that need to be drawn from lower memory
+     * address */
+    if( buf->start + output_len > buf->capacity )
+    {
+        overflow = ( buf->start + output_len ) % buf->capacity;
+    }
+
+    memcpy( output, buf->buffer + buf->start, output_len - overflow );
+    memcpy( output + output_len - overflow, buf->buffer, overflow );
+    buf->content_length -= output_len;
+    buf->start = ( buf->start + output_len ) % buf->capacity;
+
+    return output_len;
+}
+
 /*
  * Helper function setting up inverse record transformations
  * using given cipher, hash, EtM mode, authentication tag length,
@@ -361,6 +496,175 @@
  * END_DEPENDENCIES
  */
 
+/* BEGIN_CASE */
+void test_callback_buffer_sanity()
+{
+    enum { MSGLEN = 10 };
+    mbedtls_test_buffer buf;
+    unsigned char input[MSGLEN];
+    unsigned char output[MSGLEN];
+
+    memset( input, 0, sizeof(input) );
+
+    /* Make sure calling put and get on NULL buffer results in error. */
+    TEST_ASSERT( mbedtls_test_buffer_put( NULL, input, sizeof( input ) )
+                    == -1 );
+    TEST_ASSERT( mbedtls_test_buffer_get( NULL, output, sizeof( output ) )
+                    == -1 );
+    TEST_ASSERT( mbedtls_test_buffer_put( NULL, NULL, sizeof( input ) ) == -1 );
+    TEST_ASSERT( mbedtls_test_buffer_get( NULL, NULL, sizeof( output ) )
+                    == -1 );
+    TEST_ASSERT( mbedtls_test_buffer_put( NULL, NULL, 0 ) == -1 );
+    TEST_ASSERT( mbedtls_test_buffer_get( NULL, NULL, 0 ) == -1 );
+
+    /* Make sure calling put and get on a buffer that hasn't been set up results
+     * in eror. */
+    mbedtls_test_buffer_init( &buf );
+
+    TEST_ASSERT( mbedtls_test_buffer_put( &buf, input, sizeof( input ) ) == -1 );
+    TEST_ASSERT( mbedtls_test_buffer_get( &buf, output, sizeof( output ) )
+                    == -1 );
+    TEST_ASSERT( mbedtls_test_buffer_put( &buf, NULL, sizeof( input ) ) == -1 );
+    TEST_ASSERT( mbedtls_test_buffer_get( &buf, NULL, sizeof( output ) )
+                    == -1 );
+    TEST_ASSERT( mbedtls_test_buffer_put( &buf, NULL, 0 ) == -1 );
+    TEST_ASSERT( mbedtls_test_buffer_get( &buf, NULL, 0 ) == -1 );
+
+    /* Make sure calling put end get on NULL input and output only results in
+     * error if the length is not zero. */
+
+    TEST_ASSERT( mbedtls_test_buffer_setup( &buf, sizeof( input ) ) == 0 );
+
+    TEST_ASSERT( mbedtls_test_buffer_put( &buf, NULL, sizeof( input ) ) == -1 );
+    TEST_ASSERT( mbedtls_test_buffer_get( &buf, NULL, sizeof( output ) )
+                    == -1 );
+    TEST_ASSERT( mbedtls_test_buffer_put( &buf, NULL, 0 ) == 0 );
+    TEST_ASSERT( mbedtls_test_buffer_get( &buf, NULL, 0 ) == 0 );
+
+exit:
+
+    mbedtls_test_buffer_free( &buf );
+}
+/* END_CASE */
+
+/*
+ * Test if the implementation of `mbedtls_test_buffer` related functions is
+ * correct and works as expected.
+ *
+ * That is
+ *  - If we try to put in \p put1 bytes then we can put in \p put1_ret bytes.
+ *  - Afterwards if we try to get \p get1 bytes then we can get \get1_ret bytes.
+ *  - Next, if we try to put in \p put1 bytes then we can put in \p put1_ret
+ *    bytes.
+ *  - Afterwards if we try to get \p get1 bytes then we can get \get1_ret bytes.
+ *  - All of the bytes we got match the bytes we put in in a FIFO manner.
+ */
+
+/* BEGIN_CASE */
+void test_callback_buffer( int size, int put1, int put1_ret,
+                           int get1, int get1_ret, int put2, int put2_ret,
+                           int get2, int get2_ret )
+{
+    enum { ROUNDS = 2 };
+    size_t put[ROUNDS];
+    int put_ret[ROUNDS];
+    size_t get[ROUNDS];
+    int get_ret[ROUNDS];
+    mbedtls_test_buffer buf;
+    unsigned char* input = NULL;
+    size_t input_len;
+    unsigned char* output = NULL;
+    size_t output_len;
+    size_t i, written, read;
+    int j;
+
+    mbedtls_test_buffer_init( &buf );
+    TEST_ASSERT( mbedtls_test_buffer_setup( &buf, size ) == 0 );
+
+    /* Check the sanity of input parameters and initialise local variables. That
+     * is, ensure that the amount of data is not negative and that we are not
+     * expecting more to put or get than we actually asked for. */
+    TEST_ASSERT( put1 >= 0 );
+    put[0] = put1;
+    put_ret[0] = put1_ret;
+    TEST_ASSERT( put1_ret <= put1 );
+    TEST_ASSERT( put2 >= 0 );
+    put[1] = put2;
+    put_ret[1] = put2_ret;
+    TEST_ASSERT( put2_ret <= put2 );
+
+    TEST_ASSERT( get1 >= 0 );
+    get[0] = get1;
+    get_ret[0] = get1_ret;
+    TEST_ASSERT( get1_ret <= get1 );
+    TEST_ASSERT( get2 >= 0 );
+    get[1] = get2;
+    get_ret[1] = get2_ret;
+    TEST_ASSERT( get2_ret <= get2 );
+
+    input_len = 0;
+    /* Calculate actual input and output lengths */
+    for( j = 0; j < ROUNDS; j++ )
+    {
+        if( put_ret[j] > 0 )
+        {
+            input_len += put_ret[j];
+        }
+    }
+    /* In order to always have a valid pointer we always allocate at least 1
+     * byte. */
+    if( input_len == 0 )
+        input_len = 1;
+    ASSERT_ALLOC( input, input_len );
+
+    output_len = 0;
+    for( j = 0; j < ROUNDS; j++ )
+    {
+        if( get_ret[j] > 0 )
+        {
+            output_len += get_ret[j];
+        }
+    }
+    TEST_ASSERT( output_len <= input_len );
+    /* In order to always have a valid pointer we always allocate at least 1
+     * byte. */
+    if( output_len == 0 )
+        output_len = 1;
+    ASSERT_ALLOC( output, output_len );
+
+    /* Fill up the buffer with structured data so that unwanted changes
+     * can be detected */
+    for( i = 0; i < input_len; i++ )
+    {
+        input[i] = i & 0xFF;
+    }
+
+    written = read = 0;
+    for( j = 0; j < ROUNDS; j++ )
+    {
+        TEST_ASSERT( put_ret[j] == mbedtls_test_buffer_put( &buf,
+                                        input + written, put[j] ) );
+        written += put_ret[j];
+        TEST_ASSERT( get_ret[j] == mbedtls_test_buffer_get( &buf,
+                                        output + read, get[j] ) );
+        read += get_ret[j];
+        TEST_ASSERT( read <= written );
+        if( get_ret[j] > 0 )
+        {
+            TEST_ASSERT( memcmp( output + read - get_ret[j],
+                                 input + read - get_ret[j], get_ret[j] )
+                         == 0 );
+        }
+    }
+
+exit:
+
+    mbedtls_free( input );
+    mbedtls_free( output );
+    mbedtls_test_buffer_free( &buf );
+}
+/* END_CASE */
+
 /* BEGIN_CASE depends_on:MBEDTLS_SSL_DTLS_ANTI_REPLAY */
 void ssl_dtls_replay( data_t * prevs, data_t * new, int ret )
 {