Add new macro to detemine ECDSA signature length
Revived from a previous PR by Gilles, see:
https://github.com/ARMmbed/mbedtls/pull/1293/files#diff-568ef321d275f2035b8b26a70ee9af0bR71
This will be useful in eliminating temporary stack buffers for transcoding the
signature: in order to do that in place we need to be able to make assumptions
about the size of the output buffer, which this macro will provide. (See next
commit.)
diff --git a/include/mbedtls/ecdsa.h b/include/mbedtls/ecdsa.h
index 4057828..5245c6e 100644
--- a/include/mbedtls/ecdsa.h
+++ b/include/mbedtls/ecdsa.h
@@ -35,25 +35,30 @@
#include "ecp.h"
#include "md.h"
-/*
- * RFC-4492 page 20:
+/**
+ * \brief Maximum ECDSA signature size for a given curve bit size
*
+ * \param bits Curve size in bits
+ * \return Maximum signature size in bytes
+ *
+ * \note This macro returns a compile-time constant if its argument
+ * is one. It may evaluate its argument multiple times.
+ */
+/*
* Ecdsa-Sig-Value ::= SEQUENCE {
* r INTEGER,
* s INTEGER
* }
*
- * Size is at most
- * 1 (tag) + 1 (len) + 1 (initial 0) + ECP_MAX_BYTES for each of r and s,
- * twice that + 1 (tag) + 2 (len) for the sequence
- * (assuming ECP_MAX_BYTES is less than 126 for r and s,
- * and less than 124 (total len <= 255) for the sequence)
+ * For each of r and s, the value (V) may include an extra initial "0" bit.
*/
-#if MBEDTLS_ECP_MAX_BYTES > 124
-#error "MBEDTLS_ECP_MAX_BYTES bigger than expected, please fix MBEDTLS_ECDSA_MAX_LEN"
-#endif
+#define MBEDTLS_ECDSA_MAX_SIG_LEN( bits ) \
+ ( /*T,L of SEQUENCE*/ ( ( bits ) >= 61 * 8 ? 3 : 2 ) + \
+ /*T,L of r,s*/ 2 * ( ( ( bits ) >= 127 * 8 ? 3 : 2 ) + \
+ /*V of r,s*/ ( ( bits ) + 8 ) / 8 ) )
+
/** The maximal size of an ECDSA signature in Bytes. */
-#define MBEDTLS_ECDSA_MAX_LEN ( 3 + 2 * ( 3 + MBEDTLS_ECP_MAX_BYTES ) )
+#define MBEDTLS_ECDSA_MAX_LEN MBEDTLS_ECDSA_MAX_SIG_LEN( MBEDTLS_ECP_MAX_BITS )
#ifdef __cplusplus
extern "C" {