diff --git a/sipsimple/util/_sha1.pyx b/sipsimple/util/_sha1.pyx
index ea53c78b..a6d882cd 100644
--- a/sipsimple/util/_sha1.pyx
+++ b/sipsimple/util/_sha1.pyx
@@ -1,93 +1,93 @@
 # cython: language_level=2
 
 __all__ = ['sha1']
 
 
 from libc.stddef cimport size_t
 from libc.stdint cimport uint8_t, uint32_t, uint64_t
 from libc.string cimport memcpy
 from cpython.buffer cimport PyObject_CheckBuffer, PyObject_GetBuffer, PyBuffer_Release
-from cpython.string cimport PyString_FromStringAndSize, PyString_AS_STRING
+from cpython.bytes cimport PyBytes_FromStringAndSize, PyBytes_AsString
 from cpython.unicode cimport PyUnicode_Check
 
 
 cdef extern from "_sha1.h":
     enum:
         SHA1_BLOCK_SIZE  = 64
         SHA1_DIGEST_SIZE = 20
 
     ctypedef struct sha1_context:
         uint32_t state[SHA1_DIGEST_SIZE/4]  # state variables
         uint64_t count                      # 64-bit block count
         uint8_t  block[SHA1_BLOCK_SIZE]     # data block buffer
         uint32_t index                      # index into buffer
 
     cdef void sha1_init(sha1_context *context)
     cdef void sha1_update(sha1_context *context, const uint8_t *data, size_t length)
     cdef void sha1_digest(sha1_context *context, uint8_t *digest)
 
 
 cdef class sha1(object):
     cdef sha1_context context
 
     def __cinit__(self, *args, **kw):
         sha1_init(&self.context)
 
     def __init__(self, data=''):
         self.update(data)
 
     property name:
         def __get__(self):
             return 'sha1'
 
     property block_size:
         def __get__(self):
             return SHA1_BLOCK_SIZE
 
     property digest_size:
         def __get__(self):
             return SHA1_DIGEST_SIZE
 
     def __reduce__(self):
         state_variables = [self.context.state[i] for i in range(sizeof(self.context.state)/4)]
-        block = PyString_FromStringAndSize(<char*>self.context.block, self.context.index)
+        block = PyBytes_FromStringAndSize(<char*>self.context.block, self.context.index)
         return self.__class__, (), (state_variables, self.context.count, block)
 
     def __setstate__(self, state):
         state_variables, count, block = state
         for i, number in enumerate(state_variables):
             self.context.state[i] = number
         self.context.count = count
         self.context.index = len(block)
-        memcpy(self.context.block, PyString_AS_STRING(block), self.context.index)
+        memcpy(self.context.block, PyBytes_AsString(block), self.context.index)
 
     def copy(self):
         cdef sha1 instance = self.__class__()
         instance.context = self.context
         return instance
 
     def update(self, data):
         cdef Py_buffer view
 
         if PyObject_CheckBuffer(data):
             PyObject_GetBuffer(data, &view, 0)
             if view.ndim > 1:
                 raise BufferError('Buffer must be single dimension')
             sha1_update(&self.context, <uint8_t*>view.buf, view.len)
             PyBuffer_Release(&view)
         elif PyUnicode_Check(data):
             raise TypeError('Unicode-objects must be encoded before hashing')
         else:
             raise TypeError('object supporting the buffer API required')
 
     def digest(self):
         cdef sha1_context context_copy
         cdef uint8_t digest[SHA1_DIGEST_SIZE]
 
         context_copy = self.context
         sha1_digest(&context_copy, digest)
-        return PyString_FromStringAndSize(<char*>digest, SHA1_DIGEST_SIZE)
+        return PyBytes_FromStringAndSize(<char*>digest, SHA1_DIGEST_SIZE)
 
     def hexdigest(self):
         return self.digest().encode('hex')