Clarify the use of ECDSA API
In the ecdsa.c sample application we don't use hashing, we use ecdsa
directly on a buffer containing plain text. Although the text explains
that it should be the message hash it still can be confusing.
Any misunderstandings here are potentially very dangerous, because ECDSA
truncates the message hash if necessary and this can lead to trivial
signature forgeries if the API is misused and the message is passed
directly to the function without hashing.
This commit adds a hash computation step to the ecdsa.c sample
application and clarification to the doxygen documentation of the
ECDSA functions involved.
diff --git a/include/polarssl/ecdsa.h b/include/polarssl/ecdsa.h
index 5eb83d9..47b644e 100644
--- a/include/polarssl/ecdsa.h
+++ b/include/polarssl/ecdsa.h
@@ -26,6 +26,23 @@
#include "ecp.h"
#include "md.h"
+/*
+ * RFC 4492 page 20:
+ *
+ * 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)
+ *
+ */
+/** Maximum size of an ECDSA signature in bytes */
+#define POLARSSL_ECDSA_MAX_LEN ( 3 + 2 * ( 3 + POLARSSL_ECP_MAX_BYTES ) )
/**
* \brief ECDSA context structure
@@ -58,6 +75,10 @@
* \param f_rng RNG function
* \param p_rng RNG parameter
*
+ * \note If the bitlength of the message hash is larger than the
+ * bitlength of the group order, then the hash is truncated as
+ * prescribed by SEC1 4.1.3 step 5.
+ *
* \return 0 if successful,
* or a POLARSSL_ERR_ECP_XXX or POLARSSL_MPI_XXX error code
*/
@@ -78,6 +99,10 @@
* \param blen Length of buf
* \param md_alg MD algorithm used to hash the message
*
+ * \note If the bitlength of the message hash is larger than the
+ * bitlength of the group order, then the hash is truncated as
+ * prescribed by SEC1 4.1.3 step 5.
+ *
* \return 0 if successful,
* or a POLARSSL_ERR_ECP_XXX or POLARSSL_MPI_XXX error code
*/
@@ -96,6 +121,10 @@
* \param r First integer of the signature
* \param s Second integer of the signature
*
+ * \note If the bitlength of the message hash is larger than the
+ * bitlength of the group order, then the hash is truncated as
+ * prescribed by SEC1 4.1.4 step 3.
+ *
* \return 0 if successful,
* POLARSSL_ERR_ECP_BAD_INPUT_DATA if signature is invalid
* or a POLARSSL_ERR_ECP_XXX or POLARSSL_MPI_XXX error code
@@ -121,6 +150,10 @@
* size of the curve used, plus 7 (eg. 71 bytes if a 256-bit
* curve is used).
*
+ * \note If the bitlength of the message hash is larger than the
+ * bitlength of the group order, then the hash is truncated as
+ * prescribed by SEC1 4.1.3 step 5.
+ *
* \return 0 if successful,
* or a POLARSSL_ERR_ECP, POLARSSL_ERR_MPI or
* POLARSSL_ERR_ASN1 error code
@@ -168,6 +201,10 @@
* \param sig Signature to read and verify
* \param slen Size of sig
*
+ * \note If the bitlength of the message hash is larger than the
+ * bitlength of the group order, then the hash is truncated as
+ * prescribed by SEC1 4.1.4 step 3.
+ *
* \return 0 if successful,
* POLARSSL_ERR_ECP_BAD_INPUT_DATA if signature is invalid,
* POLARSSL_ERR_ECP_SIG_LEN_MISTMATCH if the signature is