diff --git a/library/bignum.c b/library/bignum.c
index f94d08d..e2da5a8 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -788,7 +788,7 @@
 }
 
 /*
- * Helper for mpi substraction
+ * Helper for mpi subtraction
  */
 static void mpi_sub_hlp( size_t n, t_uint *s, t_uint *d )
 {
@@ -809,7 +809,7 @@
 }
 
 /*
- * Unsigned substraction: X = |A| - |B|  (HAC 14.9)
+ * Unsigned subtraction: X = |A| - |B|  (HAC 14.9)
  */
 int mpi_sub_abs( mpi *X, const mpi *A, const mpi *B )
 {
@@ -832,7 +832,7 @@
         MPI_CHK( mpi_copy( X, A ) );
 
     /*
-     * X should always be positive as a result of unsigned substractions.
+     * X should always be positive as a result of unsigned subtractions.
      */
     X->s = 1;
 
@@ -883,7 +883,7 @@
 }
 
 /*
- * Signed substraction: X = A - B
+ * Signed subtraction: X = A - B
  */
 int mpi_sub_mpi( mpi *X, const mpi *A, const mpi *B )
 {
@@ -930,7 +930,7 @@
 }
 
 /*
- * Signed substraction: X = A - b
+ * Signed subtraction: X = A - b
  */
 int mpi_sub_int( mpi *X, const mpi *A, t_sint b )
 {