Fix potential stack underflow in mpi_read_file.
When provided with an empty line, mpi_read_file causes a numeric
underflow resulting in a stack underflow. This commit fixes this and
adds some documentation to mpi_read_file.
diff --git a/library/bignum.c b/library/bignum.c
index 199b4f2..780fa28 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -616,11 +616,11 @@
if( slen == sizeof( s ) - 2 )
return( MBEDTLS_ERR_MPI_BUFFER_TOO_SMALL );
- if( s[slen - 1] == '\n' ) { slen--; s[slen] = '\0'; }
- if( s[slen - 1] == '\r' ) { slen--; s[slen] = '\0'; }
+ if( slen > 0 && s[slen - 1] == '\n' ) { slen--; s[slen] = '\0'; }
+ if( slen > 0 && s[slen - 1] == '\r' ) { slen--; s[slen] = '\0'; }
p = s + slen;
- while( --p >= s )
+ while( p-- > s )
if( mpi_get_digit( &d, radix, *p ) != 0 )
break;