Determine MBEDTLS_ECP_MAX_BITS automatically
MBEDTLS_ECP_MAX_BITS doesn't make sense as a configuration option: it
must not be smaller than the largest supported curve, and it's useless
to set it to a larger value. So unconditionally set it to the size of
the largest supported curve. Remove it from the build configuration.
Alternative implementations must no longer need define this macro.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/configs/config-suite-b.h b/configs/config-suite-b.h
index 28e6443..b62bdfa 100644
--- a/configs/config-suite-b.h
+++ b/configs/config-suite-b.h
@@ -84,8 +84,7 @@
#define MBEDTLS_AES_ROM_TABLES
/* Save RAM by adjusting to our exact needs */
-#define MBEDTLS_ECP_MAX_BITS 384
-#define MBEDTLS_MPI_MAX_SIZE 48 // 384 bits is 48 bytes
+#define MBEDTLS_MPI_MAX_SIZE 48 // 384-bit EC curve = 48 bytes
/* Save RAM at the expense of speed, see ecp.h */
#define MBEDTLS_ECP_WINDOW_SIZE 2
diff --git a/configs/config-thread.h b/configs/config-thread.h
index bce9668..c1937de 100644
--- a/configs/config-thread.h
+++ b/configs/config-thread.h
@@ -85,8 +85,7 @@
#define MBEDTLS_AES_ROM_TABLES
/* Save RAM by adjusting to our exact needs */
-#define MBEDTLS_ECP_MAX_BITS 256
-#define MBEDTLS_MPI_MAX_SIZE 32 // 256 bits is 32 bytes
+#define MBEDTLS_MPI_MAX_SIZE 32 // 256-bit EC curve = 32 bytes
/* Save ROM and a few bytes of RAM by specifying our own ciphersuite list */
#define MBEDTLS_SSL_CIPHERSUITES MBEDTLS_TLS_ECJPAKE_WITH_AES_128_CCM_8
diff --git a/include/mbedtls/config.h b/include/mbedtls/config.h
index 3139b22..57783f8 100644
--- a/include/mbedtls/config.h
+++ b/include/mbedtls/config.h
@@ -3155,7 +3155,6 @@
//#define MBEDTLS_HMAC_DRBG_MAX_SEED_INPUT 384 /**< Maximum size of (re)seed buffer */
/* ECP options */
-//#define MBEDTLS_ECP_MAX_BITS 521 /**< Maximum bit size of groups */
//#define MBEDTLS_ECP_WINDOW_SIZE 6 /**< Maximum window size used */
//#define MBEDTLS_ECP_FIXED_POINT_OPTIM 1 /**< Enable fixed-point speed-up */
diff --git a/include/mbedtls/ecp.h b/include/mbedtls/ecp.h
index 8cb4fb4..dceca3e 100644
--- a/include/mbedtls/ecp.h
+++ b/include/mbedtls/ecp.h
@@ -242,13 +242,6 @@
* \{
*/
-#if !defined(MBEDTLS_ECP_MAX_BITS)
-/**
- * The maximum size of the groups, that is, of \c N and \c P.
- */
-#define MBEDTLS_ECP_MAX_BITS 521 /**< The maximum size of groups, in bits. */
-#endif
-
#if !defined(MBEDTLS_ECP_WINDOW_SIZE)
/*
* Maximum "window" size used for point multiplication.
@@ -294,6 +287,44 @@
#include "ecp_alt.h"
#endif /* MBEDTLS_ECP_ALT */
+/**
+ * The maximum size of the groups, that is, of \c N and \c P.
+ */
+#if !defined(MBEDTLS_ECP_C)
+/* Dummy definition to help code that has optional ECP support and
+ * defines an MBEDTLS_ECP_MAX_BYTES-sized array unconditionally. */
+#define MBEDTLS_ECP_MAX_BITS 1
+/* Note: the curves must be listed in DECREASING size! */
+#elif defined(MBEDTLS_ECP_DP_SECP521R1_ENABLED)
+#define MBEDTLS_ECP_MAX_BITS 521
+#elif defined(MBEDTLS_ECP_DP_BP512R1_ENABLED)
+#define MBEDTLS_ECP_MAX_BITS 512
+#elif defined(MBEDTLS_ECP_DP_CURVE448_ENABLED)
+#define MBEDTLS_ECP_MAX_BITS 448
+#elif defined(MBEDTLS_ECP_DP_BP384R1_ENABLED)
+#define MBEDTLS_ECP_MAX_BITS 384
+#elif defined(MBEDTLS_ECP_DP_SECP384R1_ENABLED)
+#define MBEDTLS_ECP_MAX_BITS 384
+#elif defined(MBEDTLS_ECP_DP_BP256R1_ENABLED)
+#define MBEDTLS_ECP_MAX_BITS 256
+#elif defined(MBEDTLS_ECP_DP_SECP256K1_ENABLED)
+#define MBEDTLS_ECP_MAX_BITS 256
+#elif defined(MBEDTLS_ECP_DP_SECP256R1_ENABLED)
+#define MBEDTLS_ECP_MAX_BITS 256
+#elif defined(MBEDTLS_ECP_DP_CURVE25519_ENABLED)
+#define MBEDTLS_ECP_MAX_BITS 255
+#elif defined(MBEDTLS_ECP_DP_SECP224K1_ENABLED)
+#define MBEDTLS_ECP_MAX_BITS 225 // n is slightly above 2^224
+#elif defined(MBEDTLS_ECP_DP_SECP224R1_ENABLED)
+#define MBEDTLS_ECP_MAX_BITS 224
+#elif defined(MBEDTLS_ECP_DP_SECP192K1_ENABLED)
+#define MBEDTLS_ECP_MAX_BITS 192
+#elif defined(MBEDTLS_ECP_DP_SECP192R1_ENABLED)
+#define MBEDTLS_ECP_MAX_BITS 192
+#else
+#error "Missing definition of MBEDTLS_ECP_MAX_BITS"
+#endif
+
#define MBEDTLS_ECP_MAX_BYTES ( ( MBEDTLS_ECP_MAX_BITS + 7 ) / 8 )
#define MBEDTLS_ECP_MAX_PT_LEN ( 2 * MBEDTLS_ECP_MAX_BYTES + 1 )
