Improve documentation of mbedtls_mpi_write_string()
@@ -582,15 +582,20 @@ int mbedtls_mpi_write_string( const mbedtls_mpi *X, int radix,
if( radix < 2 || radix > 16 )
return( MBEDTLS_ERR_MPI_BAD_INPUT_DATA );
- n = mbedtls_mpi_bitlen( X );
- if( radix >= 4 ) n >>= 1;
- if( radix >= 16 ) n >>= 1;
- /*
- * Round up the buffer length to an even value to ensure that there is
- * enough room for hexadecimal values that can be represented in an odd
- * number of digits.
- */
- n += 3 + ( ( n + 1 ) & 1 );
+ n = mbedtls_mpi_bitlen( X ); /* Number of bits necessary to present `n`. */
+ if( radix >= 4 ) n >>= 1; /* Number of 4-adic digits necessary to present
+ * `n`. If radix > 4, this might be a strict
+ * overapproximation of the number of
+ * radix-adic digits needed to present `n`. */
+ if( radix >= 16 ) n >>= 1; /* Number of hexadecimal digits necessary to
+ * present `n`. */
+
+ n += 1; /* NULL termination */
+ n += 1; /* Compensate for the divisions above, which round down `n`
+ * in case it's not even. */
+ n += 1; /* Potential '-'-sign. */
+ n += ( n & 1 ); /* Make n even to have enough space for hexadecimal writing,
+ * which always uses an even number of hex-digits. */
if( buflen < n )
{
