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Introduce ASN.1 API for traversing ASN.1 SEQUENCEs

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This commit adds a new function `mbedtls_asn1_traverse_sequence_of()`
which traverses an ASN.1 SEQUENCE and calls a user-provided callback
for each entry.

It allows to put the following constraints on the tags allowed
in the SEQUENCE:
- A tag mask and mandatory tag value w.r.t. that mask.
  A non-matching tag leads to an MBEDTLS_ERR_ASN1_UNEXPECTED_TAG error.
  For example, it the mask if 0xFF, this means that only
  a single tag will be allowed in the SEQUENCE.
- A tag mask and optional tag value w.r.t. that mask.
  A non-matching tag is silently ignored.

The main use for this flexibility is the traversal of the
`SubjectAlternativeNames` extension, where some parts of the
tag are fixed but some are flexible to indicate which type
of name the entry describes.
This commit is contained in:
Hanno Becker 2019-02-21 20:52:09 +00:00
parent 5984d30f4b
commit 8730610ae0
2 changed files with 144 additions and 38 deletions
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52
include/mbedtls/asn1.h
View file

@ -291,6 +291,58 @@ int mbedtls_asn1_get_sequence_of( unsigned char **p,
mbedtls_asn1_sequence *cur,
int tag);
/**
* \brief Traverse an ASN.1 SEQUENCE container and
* call a callback for each entry.
*
* \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. 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 A negative ASN.1 error code on a parsing failure.
* \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 a MPI value from an integer ASN.1 tag.

130
library/asn1parse.c
View file

@ -230,6 +230,93 @@ int mbedtls_asn1_get_bitstring_null( unsigned char **p, const unsigned char *end
}
/*
* Traverse an ASN.1 "SEQUENCE OF <tag>"
* and call a callback for each entry found.
*/
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 )
{
int ret;
size_t len;
/* Get main sequence tag */
if( ( ret = mbedtls_asn1_get_tag( p, end, &len,
MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE ) ) != 0 )
{
return( ret );
}
if( *p + len != end )
return( MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
while( *p < end )
{
unsigned char const tag = *(*p)++;
if( ( tag & tag_must_mask ) != tag_must_val )
return( MBEDTLS_ERR_ASN1_UNEXPECTED_TAG );
if( ( tag & tag_may_mask ) == tag_may_val )
{
if( ( ret = mbedtls_asn1_get_len( p, end, &len ) ) != 0 )
return( ret );
if( cb != NULL )
{
ret = cb( ctx, tag, *p, len );
if( ret != 0 )
return( ret );
}
}
*p += len;
}
return( 0 );
}
typedef struct
{
int tag;
mbedtls_asn1_sequence *cur;
} asn1_get_sequence_of_cb_ctx_t;
static int asn1_get_sequence_of_cb( void *ctx,
int tag,
unsigned char *start,
size_t len )
{
asn1_get_sequence_of_cb_ctx_t *cb_ctx =
(asn1_get_sequence_of_cb_ctx_t *) ctx;
mbedtls_asn1_sequence *cur =
cb_ctx->cur;
if( cur->buf.p != NULL )
{
cur->next =
mbedtls_calloc( 1, sizeof( mbedtls_asn1_sequence ) );
if( cur->next == NULL )
return( MBEDTLS_ERR_ASN1_ALLOC_FAILED );
cur = cur->next;
}
cur->buf.p = start;
cur->buf.len = len;
cur->buf.tag = tag;
cb_ctx->cur = cur;
return( 0 );
}
/*
* Parses and splits an ASN.1 "SEQUENCE OF <tag>"
@ -239,44 +326,11 @@ int mbedtls_asn1_get_sequence_of( unsigned char **p,
mbedtls_asn1_sequence *cur,
int tag)
{
int ret;
size_t len;
mbedtls_asn1_buf *buf;
/* Get main sequence tag */
if( ( ret = mbedtls_asn1_get_tag( p, end, &len,
MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE ) ) != 0 )
return( ret );
if( *p + len != end )
return( MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
while( 1 )
{
buf = &(cur->buf);
buf->tag = **p;
if( ( ret = mbedtls_asn1_get_tag( p, end, &buf->len, tag ) ) != 0 )
return( ret );
buf->p = *p;
*p += buf->len;
if( *p == end )
{
cur->next = NULL;
break;
}
cur->next = (mbedtls_asn1_sequence*)mbedtls_calloc( 1,
sizeof( mbedtls_asn1_sequence ) );
if( cur->next == NULL )
return( MBEDTLS_ERR_ASN1_ALLOC_FAILED );
cur = cur->next;
}
return( 0 );
asn1_get_sequence_of_cb_ctx_t cb_ctx = { tag, cur };
memset( cur, 0, sizeof( mbedtls_asn1_sequence ) );
return( mbedtls_asn1_traverse_sequence_of(
p, end, 0xFF, tag, 0, 0,
asn1_get_sequence_of_cb, &cb_ctx ) );
}
int mbedtls_asn1_get_alg( unsigned char **p,