linux/lib/asn1_encoder.c
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   1// SPDX-License-Identifier: GPL-2.0-only
   2/*
   3 * Simple encoder primitives for ASN.1 BER/DER/CER
   4 *
   5 * Copyright (C) 2019 James.Bottomley@HansenPartnership.com
   6 */
   7
   8#include <linux/asn1_encoder.h>
   9#include <linux/bug.h>
  10#include <linux/string.h>
  11#include <linux/module.h>
  12
  13/**
  14 * asn1_encode_integer() - encode positive integer to ASN.1
  15 * @data:       pointer to the pointer to the data
  16 * @end_data:   end of data pointer, points one beyond last usable byte in @data
  17 * @integer:    integer to be encoded
  18 *
  19 * This is a simplified encoder: it only currently does
  20 * positive integers, but it should be simple enough to add the
  21 * negative case if a use comes along.
  22 */
  23unsigned char *
  24asn1_encode_integer(unsigned char *data, const unsigned char *end_data,
  25                    s64 integer)
  26{
  27        int data_len = end_data - data;
  28        unsigned char *d = &data[2];
  29        bool found = false;
  30        int i;
  31
  32        if (WARN(integer < 0,
  33                 "BUG: integer encode only supports positive integers"))
  34                return ERR_PTR(-EINVAL);
  35
  36        if (IS_ERR(data))
  37                return data;
  38
  39        /* need at least 3 bytes for tag, length and integer encoding */
  40        if (data_len < 3)
  41                return ERR_PTR(-EINVAL);
  42
  43        /* remaining length where at d (the start of the integer encoding) */
  44        data_len -= 2;
  45
  46        data[0] = _tag(UNIV, PRIM, INT);
  47        if (integer == 0) {
  48                *d++ = 0;
  49                goto out;
  50        }
  51
  52        for (i = sizeof(integer); i > 0 ; i--) {
  53                int byte = integer >> (8 * (i - 1));
  54
  55                if (!found && byte == 0)
  56                        continue;
  57
  58                /*
  59                 * for a positive number the first byte must have bit
  60                 * 7 clear in two's complement (otherwise it's a
  61                 * negative number) so prepend a leading zero if
  62                 * that's not the case
  63                 */
  64                if (!found && (byte & 0x80)) {
  65                        /*
  66                         * no check needed here, we already know we
  67                         * have len >= 1
  68                         */
  69                        *d++ = 0;
  70                        data_len--;
  71                }
  72
  73                found = true;
  74                if (data_len == 0)
  75                        return ERR_PTR(-EINVAL);
  76
  77                *d++ = byte;
  78                data_len--;
  79        }
  80
  81 out:
  82        data[1] = d - data - 2;
  83
  84        return d;
  85}
  86EXPORT_SYMBOL_GPL(asn1_encode_integer);
  87
  88/* calculate the base 128 digit values setting the top bit of the first octet */
  89static int asn1_encode_oid_digit(unsigned char **_data, int *data_len, u32 oid)
  90{
  91        unsigned char *data = *_data;
  92        int start = 7 + 7 + 7 + 7;
  93        int ret = 0;
  94
  95        if (*data_len < 1)
  96                return -EINVAL;
  97
  98        /* quick case */
  99        if (oid == 0) {
 100                *data++ = 0x80;
 101                (*data_len)--;
 102                goto out;
 103        }
 104
 105        while (oid >> start == 0)
 106                start -= 7;
 107
 108        while (start > 0 && *data_len > 0) {
 109                u8 byte;
 110
 111                byte = oid >> start;
 112                oid = oid - (byte << start);
 113                start -= 7;
 114                byte |= 0x80;
 115                *data++ = byte;
 116                (*data_len)--;
 117        }
 118
 119        if (*data_len > 0) {
 120                *data++ = oid;
 121                (*data_len)--;
 122        } else {
 123                ret = -EINVAL;
 124        }
 125
 126 out:
 127        *_data = data;
 128        return ret;
 129}
 130
 131/**
 132 * asn1_encode_oid() - encode an oid to ASN.1
 133 * @data:       position to begin encoding at
 134 * @end_data:   end of data pointer, points one beyond last usable byte in @data
 135 * @oid:        array of oids
 136 * @oid_len:    length of oid array
 137 *
 138 * this encodes an OID up to ASN.1 when presented as an array of OID values
 139 */
 140unsigned char *
 141asn1_encode_oid(unsigned char *data, const unsigned char *end_data,
 142                u32 oid[], int oid_len)
 143{
 144        int data_len = end_data - data;
 145        unsigned char *d = data + 2;
 146        int i, ret;
 147
 148        if (WARN(oid_len < 2, "OID must have at least two elements"))
 149                return ERR_PTR(-EINVAL);
 150
 151        if (WARN(oid_len > 32, "OID is too large"))
 152                return ERR_PTR(-EINVAL);
 153
 154        if (IS_ERR(data))
 155                return data;
 156
 157
 158        /* need at least 3 bytes for tag, length and OID encoding */
 159        if (data_len < 3)
 160                return ERR_PTR(-EINVAL);
 161
 162        data[0] = _tag(UNIV, PRIM, OID);
 163        *d++ = oid[0] * 40 + oid[1];
 164
 165        data_len -= 3;
 166
 167        ret = 0;
 168
 169        for (i = 2; i < oid_len; i++) {
 170                ret = asn1_encode_oid_digit(&d, &data_len, oid[i]);
 171                if (ret < 0)
 172                        return ERR_PTR(ret);
 173        }
 174
 175        data[1] = d - data - 2;
 176
 177        return d;
 178}
 179EXPORT_SYMBOL_GPL(asn1_encode_oid);
 180
 181/**
 182 * asn1_encode_length() - encode a length to follow an ASN.1 tag
 183 * @data: pointer to encode at
 184 * @data_len: pointer to remaning length (adjusted by routine)
 185 * @len: length to encode
 186 *
 187 * This routine can encode lengths up to 65535 using the ASN.1 rules.
 188 * It will accept a negative length and place a zero length tag
 189 * instead (to keep the ASN.1 valid).  This convention allows other
 190 * encoder primitives to accept negative lengths as singalling the
 191 * sequence will be re-encoded when the length is known.
 192 */
 193static int asn1_encode_length(unsigned char **data, int *data_len, int len)
 194{
 195        if (*data_len < 1)
 196                return -EINVAL;
 197
 198        if (len < 0) {
 199                *((*data)++) = 0;
 200                (*data_len)--;
 201                return 0;
 202        }
 203
 204        if (len <= 0x7f) {
 205                *((*data)++) = len;
 206                (*data_len)--;
 207                return 0;
 208        }
 209
 210        if (*data_len < 2)
 211                return -EINVAL;
 212
 213        if (len <= 0xff) {
 214                *((*data)++) = 0x81;
 215                *((*data)++) = len & 0xff;
 216                *data_len -= 2;
 217                return 0;
 218        }
 219
 220        if (*data_len < 3)
 221                return -EINVAL;
 222
 223        if (len <= 0xffff) {
 224                *((*data)++) = 0x82;
 225                *((*data)++) = (len >> 8) & 0xff;
 226                *((*data)++) = len & 0xff;
 227                *data_len -= 3;
 228                return 0;
 229        }
 230
 231        if (WARN(len > 0xffffff, "ASN.1 length can't be > 0xffffff"))
 232                return -EINVAL;
 233
 234        if (*data_len < 4)
 235                return -EINVAL;
 236        *((*data)++) = 0x83;
 237        *((*data)++) = (len >> 16) & 0xff;
 238        *((*data)++) = (len >> 8) & 0xff;
 239        *((*data)++) = len & 0xff;
 240        *data_len -= 4;
 241
 242        return 0;
 243}
 244
 245/**
 246 * asn1_encode_tag() - add a tag for optional or explicit value
 247 * @data:       pointer to place tag at
 248 * @end_data:   end of data pointer, points one beyond last usable byte in @data
 249 * @tag:        tag to be placed
 250 * @string:     the data to be tagged
 251 * @len:        the length of the data to be tagged
 252 *
 253 * Note this currently only handles short form tags < 31.
 254 *
 255 * Standard usage is to pass in a @tag, @string and @length and the
 256 * @string will be ASN.1 encoded with @tag and placed into @data.  If
 257 * the encoding would put data past @end_data then an error is
 258 * returned, otherwise a pointer to a position one beyond the encoding
 259 * is returned.
 260 *
 261 * To encode in place pass a NULL @string and -1 for @len and the
 262 * maximum allowable beginning and end of the data; all this will do
 263 * is add the current maximum length and update the data pointer to
 264 * the place where the tag contents should be placed is returned.  The
 265 * data should be copied in by the calling routine which should then
 266 * repeat the prior statement but now with the known length.  In order
 267 * to avoid having to keep both before and after pointers, the repeat
 268 * expects to be called with @data pointing to where the first encode
 269 * returned it and still NULL for @string but the real length in @len.
 270 */
 271unsigned char *
 272asn1_encode_tag(unsigned char *data, const unsigned char *end_data,
 273                u32 tag, const unsigned char *string, int len)
 274{
 275        int data_len = end_data - data;
 276        int ret;
 277
 278        if (WARN(tag > 30, "ASN.1 tag can't be > 30"))
 279                return ERR_PTR(-EINVAL);
 280
 281        if (!string && WARN(len > 127,
 282                            "BUG: recode tag is too big (>127)"))
 283                return ERR_PTR(-EINVAL);
 284
 285        if (IS_ERR(data))
 286                return data;
 287
 288        if (!string && len > 0) {
 289                /*
 290                 * we're recoding, so move back to the start of the
 291                 * tag and install a dummy length because the real
 292                 * data_len should be NULL
 293                 */
 294                data -= 2;
 295                data_len = 2;
 296        }
 297
 298        if (data_len < 2)
 299                return ERR_PTR(-EINVAL);
 300
 301        *(data++) = _tagn(CONT, CONS, tag);
 302        data_len--;
 303        ret = asn1_encode_length(&data, &data_len, len);
 304        if (ret < 0)
 305                return ERR_PTR(ret);
 306
 307        if (!string)
 308                return data;
 309
 310        if (data_len < len)
 311                return ERR_PTR(-EINVAL);
 312
 313        memcpy(data, string, len);
 314        data += len;
 315
 316        return data;
 317}
 318EXPORT_SYMBOL_GPL(asn1_encode_tag);
 319
 320/**
 321 * asn1_encode_octet_string() - encode an ASN.1 OCTET STRING
 322 * @data:       pointer to encode at
 323 * @end_data:   end of data pointer, points one beyond last usable byte in @data
 324 * @string:     string to be encoded
 325 * @len:        length of string
 326 *
 327 * Note ASN.1 octet strings may contain zeros, so the length is obligatory.
 328 */
 329unsigned char *
 330asn1_encode_octet_string(unsigned char *data,
 331                         const unsigned char *end_data,
 332                         const unsigned char *string, u32 len)
 333{
 334        int data_len = end_data - data;
 335        int ret;
 336
 337        if (IS_ERR(data))
 338                return data;
 339
 340        /* need minimum of 2 bytes for tag and length of zero length string */
 341        if (data_len < 2)
 342                return ERR_PTR(-EINVAL);
 343
 344        *(data++) = _tag(UNIV, PRIM, OTS);
 345        data_len--;
 346
 347        ret = asn1_encode_length(&data, &data_len, len);
 348        if (ret)
 349                return ERR_PTR(ret);
 350
 351        if (data_len < len)
 352                return ERR_PTR(-EINVAL);
 353
 354        memcpy(data, string, len);
 355        data += len;
 356
 357        return data;
 358}
 359EXPORT_SYMBOL_GPL(asn1_encode_octet_string);
 360
 361/**
 362 * asn1_encode_sequence() - wrap a byte stream in an ASN.1 SEQUENCE
 363 * @data:       pointer to encode at
 364 * @end_data:   end of data pointer, points one beyond last usable byte in @data
 365 * @seq:        data to be encoded as a sequence
 366 * @len:        length of the data to be encoded as a sequence
 367 *
 368 * Fill in a sequence.  To encode in place, pass NULL for @seq and -1
 369 * for @len; then call again once the length is known (still with NULL
 370 * for @seq). In order to avoid having to keep both before and after
 371 * pointers, the repeat expects to be called with @data pointing to
 372 * where the first encode placed it.
 373 */
 374unsigned char *
 375asn1_encode_sequence(unsigned char *data, const unsigned char *end_data,
 376                     const unsigned char *seq, int len)
 377{
 378        int data_len = end_data - data;
 379        int ret;
 380
 381        if (!seq && WARN(len > 127,
 382                         "BUG: recode sequence is too big (>127)"))
 383                return ERR_PTR(-EINVAL);
 384
 385        if (IS_ERR(data))
 386                return data;
 387
 388        if (!seq && len >= 0) {
 389                /*
 390                 * we're recoding, so move back to the start of the
 391                 * sequence and install a dummy length because the
 392                 * real length should be NULL
 393                 */
 394                data -= 2;
 395                data_len = 2;
 396        }
 397
 398        if (data_len < 2)
 399                return ERR_PTR(-EINVAL);
 400
 401        *(data++) = _tag(UNIV, CONS, SEQ);
 402        data_len--;
 403
 404        ret = asn1_encode_length(&data, &data_len, len);
 405        if (ret)
 406                return ERR_PTR(ret);
 407
 408        if (!seq)
 409                return data;
 410
 411        if (data_len < len)
 412                return ERR_PTR(-EINVAL);
 413
 414        memcpy(data, seq, len);
 415        data += len;
 416
 417        return data;
 418}
 419EXPORT_SYMBOL_GPL(asn1_encode_sequence);
 420
 421/**
 422 * asn1_encode_boolean() - encode a boolean value to ASN.1
 423 * @data:       pointer to encode at
 424 * @end_data:   end of data pointer, points one beyond last usable byte in @data
 425 * @val:        the boolean true/false value
 426 */
 427unsigned char *
 428asn1_encode_boolean(unsigned char *data, const unsigned char *end_data,
 429                    bool val)
 430{
 431        int data_len = end_data - data;
 432
 433        if (IS_ERR(data))
 434                return data;
 435
 436        /* booleans are 3 bytes: tag, length == 1 and value == 0 or 1 */
 437        if (data_len < 3)
 438                return ERR_PTR(-EINVAL);
 439
 440        *(data++) = _tag(UNIV, PRIM, BOOL);
 441        data_len--;
 442
 443        asn1_encode_length(&data, &data_len, 1);
 444
 445        if (val)
 446                *(data++) = 1;
 447        else
 448                *(data++) = 0;
 449
 450        return data;
 451}
 452EXPORT_SYMBOL_GPL(asn1_encode_boolean);
 453
 454MODULE_LICENSE("GPL");
 455