Clarify the use of MBEDTLS_ERR_PK_SIG_LEN_MISMATCH
Clarify what MBEDTLS_ERR_ECP_SIG_LEN_MISMATCH and
MBEDTLS_ERR_PK_SIG_LEN_MISMATCH mean. Add comments to highlight that
this indicates that a valid signature is present, unlike other error
codes. See
https://github.com/ARMmbed/mbedtls/pull/1149#discussion_r178130705
diff --git a/include/mbedtls/pk.h b/include/mbedtls/pk.h
index 1059bda..ee06b2f 100644
--- a/include/mbedtls/pk.h
+++ b/include/mbedtls/pk.h
@@ -63,7 +63,7 @@
#define MBEDTLS_ERR_PK_INVALID_ALG -0x3A80 /**< The algorithm tag or value is invalid. */
#define MBEDTLS_ERR_PK_UNKNOWN_NAMED_CURVE -0x3A00 /**< Elliptic curve is unsupported (only NIST curves are supported). */
#define MBEDTLS_ERR_PK_FEATURE_UNAVAILABLE -0x3980 /**< Unavailable feature, e.g. RSA disabled for RSA key. */
-#define MBEDTLS_ERR_PK_SIG_LEN_MISMATCH -0x3900 /**< The signature is valid but its length is less than expected. */
+#define MBEDTLS_ERR_PK_SIG_LEN_MISMATCH -0x3900 /**< The buffer contains a valid signature followed by more data. */
#define MBEDTLS_ERR_PK_HW_ACCEL_FAILED -0x3880 /**< PK hardware accelerator failed. */
#ifdef __cplusplus
@@ -269,8 +269,8 @@
* \param sig_len Signature length
*
* \return 0 on success (signature is valid),
- * MBEDTLS_ERR_PK_SIG_LEN_MISMATCH if the signature is
- * valid but its actual length is less than sig_len,
+ * #MBEDTLS_ERR_PK_SIG_LEN_MISMATCH if there is a valid
+ * signature in sig but its length is less than \p siglen,
* or a specific error code.
*
* \note For RSA keys, the default padding type is PKCS#1 v1.5.
@@ -300,10 +300,10 @@
* \param sig_len Signature length
*
* \return 0 on success (signature is valid),
- * MBEDTLS_ERR_PK_TYPE_MISMATCH if the PK context can't be
+ * #MBEDTLS_ERR_PK_TYPE_MISMATCH if the PK context can't be
* used for this type of signatures,
- * MBEDTLS_ERR_PK_SIG_LEN_MISMATCH if the signature is
- * valid but its actual length is less than sig_len,
+ * #MBEDTLS_ERR_PK_SIG_LEN_MISMATCH if there is a valid
+ * signature in sig but its length is less than \p siglen,
* or a specific error code.
*
* \note If hash_len is 0, then the length associated with md_alg