ASN.1: Add ASN.1 SEQUENCE traversal API
diff --git a/include/mbedtls/asn1.h b/include/mbedtls/asn1.h
index 0a2727e..07fbe56 100644
--- a/include/mbedtls/asn1.h
+++ b/include/mbedtls/asn1.h
@@ -413,6 +413,100 @@
*/
void mbedtls_asn1_sequence_free( mbedtls_asn1_sequence *seq );
+/**
+ * \brief Traverse an ASN.1 SEQUENCE container and
+ * call a callback for each entry.
+ *
+ * This function checks that the input is a SEQUENCE of elements that
+ * each have a "must" tag, and calls a callback function on the elements
+ * that have a "may" tag.
+ *
+ * For example, to validate that the input is a SEQUENCE of `tag1` and call
+ * `cb` on each element, use
+ * ```
+ * mbedtls_asn1_traverse_sequence_of(&p, end, 0xff, tag1, 0, 0, cb, ctx);
+ * ```
+ *
+ * To validate that the input is a SEQUENCE of ANY and call `cb` on
+ * each element, use
+ * ```
+ * mbedtls_asn1_traverse_sequence_of(&p, end, 0, 0, 0, 0, cb, ctx);
+ * ```
+ *
+ * To validate that the input is a SEQUENCE of CHOICE {NULL, OCTET STRING}
+ * and call `cb` on each element that is an OCTET STRING, use
+ * ```
+ * mbedtls_asn1_traverse_sequence_of(&p, end, 0xfe, 0x04, 0xff, 0x04, cb, ctx);
+ * ```
+ *
+ * The callback is called on the elements with a "may" tag from left to
+ * right. If the input is not a valid SEQUENCE of elements with a "must" tag,
+ * the callback is called on the elements up to the leftmost point where
+ * the input is invalid.
+ *
+ * \warning This function is still experimental and may change
+ * at any time.
+ *
+ * \param p The address of the pointer to the beginning of
+ * the ASN.1 SEQUENCE header. This is updated to
+ * point to the end of the ASN.1 SEQUENCE container
+ * on a successful invocation.
+ * \param end The end of the ASN.1 SEQUENCE container.
+ * \param tag_must_mask A mask to be applied to the ASN.1 tags found within
+ * the SEQUENCE before comparing to \p tag_must_value.
+ * \param tag_must_val The required value of each ASN.1 tag found in the
+ * SEQUENCE, after masking with \p tag_must_mask.
+ * Mismatching tags lead to an error.
+ * For example, a value of \c 0 for both \p tag_must_mask
+ * and \p tag_must_val means that every tag is allowed,
+ * while a value of \c 0xFF for \p tag_must_mask means
+ * that \p tag_must_val is the only allowed tag.
+ * \param tag_may_mask A mask to be applied to the ASN.1 tags found within
+ * the SEQUENCE before comparing to \p tag_may_value.
+ * \param tag_may_val The desired value of each ASN.1 tag found in the
+ * SEQUENCE, after masking with \p tag_may_mask.
+ * Mismatching tags will be silently ignored.
+ * For example, a value of \c 0 for \p tag_may_mask and
+ * \p tag_may_val means that any tag will be considered,
+ * while a value of \c 0xFF for \p tag_may_mask means
+ * that all tags with value different from \p tag_may_val
+ * will be ignored.
+ * \param cb The callback to trigger for each component
+ * in the ASN.1 SEQUENCE that matches \p tag_may_val.
+ * The callback function is called with the following
+ * parameters:
+ * - \p ctx.
+ * - The tag of the current element.
+ * - A pointer to the start of the current element's
+ * content inside the input.
+ * - The length of the content of the current element.
+ * If the callback returns a non-zero value,
+ * the function stops immediately,
+ * forwarding the callback's return value.
+ * \param ctx The context to be passed to the callback \p cb.
+ *
+ * \return \c 0 if successful the entire ASN.1 SEQUENCE
+ * was traversed without parsing or callback errors.
+ * \return #MBEDTLS_ERR_ASN1_LENGTH_MISMATCH if the input
+ * contains extra data after a valid SEQUENCE
+ * of elements with an accepted tag.
+ * \return #MBEDTLS_ERR_ASN1_UNEXPECTED_TAG if the input starts
+ * with an ASN.1 SEQUENCE in which an element has a tag
+ * that is not accepted.
+ * \return An ASN.1 error code if the input does not start with
+ * a valid ASN.1 SEQUENCE.
+ * \return A non-zero error code forwarded from the callback
+ * \p cb in case the latter returns a non-zero value.
+ */
+int mbedtls_asn1_traverse_sequence_of(
+ unsigned char **p,
+ const unsigned char *end,
+ uint8_t tag_must_mask, uint8_t tag_must_val,
+ uint8_t tag_may_mask, uint8_t tag_may_val,
+ int (*cb)( void *ctx, int tag,
+ unsigned char* start, size_t len ),
+ void *ctx );
+
#if defined(MBEDTLS_BIGNUM_C)
/**
* \brief Retrieve an integer ASN.1 tag and its value.