diff --git a/sshcrc.c b/sshcrc.c index d4a92376..b1dc8333 100644 --- a/sshcrc.c +++ b/sshcrc.c @@ -37,156 +37,56 @@ #include "ssh.h" -/* ---------------------------------------------------------------------- - * Multi-function module. Can be compiled three ways. - * - * - Compile with no special #defines. Will generate a table - * that's already initialised at compile time, and one function - * crc32_compute(buf,len) that uses it. Normal usage. - * - * - Compile with INITFUNC defined. Will generate an uninitialised - * array as the table, and as well as crc32_compute(buf,len) it - * will also generate void crc32_init(void) which sets up the - * table at run time. Useful if binary size is important. - * - * - Compile with GENPROGRAM defined. Will create a standalone - * program that does the initialisation and outputs the table as - * C code. - */ - -#define POLY (0xEDB88320L) - -#ifdef GENPROGRAM -#define INITFUNC /* the gen program needs the init func :-) */ -#endif - -#ifdef INITFUNC - /* - * This variant of the code generates the table at run-time from an - * init function. + * Multiply a CRC value by x^4. This implementation strategy avoids + * using a lookup table (which would be a side-channel hazard, since + * SSH-1 applies this CRC to decrypted session data). + * + * The basic idea is that you'd like to "multiply" the shifted-out 4 + * bits by the CRC polynomial value 0xEDB88320, or rather by that + * value shifted right 3 bits (since you want the _last_ bit shifted + * out, i.e. the one originally at the 2^3 position, to generate + * 0xEDB88320 itself). But the scare-quoted "multiply" would have to + * be a multiplication of polynomials over GF(2), which differs from + * integer multiplication in that you don't have any carries. In other + * words, you make a copy of one input shifted left by the index of + * each set bit in the other, so that adding them all together would + * give you the ordinary integer product, and then you XOR them + * together instead. + * + * With a 4-bit multiplier, the two kinds of multiplication coincide + * provided the multiplicand has no two set bits at positions + * differing by less than 4, because then no two copies of the + * multiplier can overlap to generate a carry. So I break up the + * intended multiplicand K = 0xEDB88320 >> 3 into three sub-constants + * a,b,c with that property, such that a^b^c = K. Then I can multiply + * m by each of them separately, and XOR together the results. */ -static uint32_t crc32_table[256]; - -void crc32_init(void) +static inline uint32_t crc32_shift_4(uint32_t v) { - uint32_t crcword; - int i; - - for (i = 0; i < 256; i++) { - uint32_t newbyte, x32term; - int j; - crcword = 0; - newbyte = i; - for (j = 0; j < 8; j++) { - x32term = (crcword ^ newbyte) & 1; - crcword = (crcword >> 1) ^ (x32term * POLY); - newbyte >>= 1; - } - crc32_table[i] = crcword; - } + const uint32_t a = 0x11111044, b = 0x08840020, c = 0x04220000; + uint32_t m = v & 0xF; + return (v >> 4) ^ (a*m) ^ (b*m) ^ (c*m); } -#else +/* + * The 8-bit shift you need every time you absorb an input byte, + * implemented simply by iterating the 4-bit shift twice. + */ +static inline uint32_t crc32_shift_8(uint32_t v) +{ + return crc32_shift_4(crc32_shift_4(v)); +} /* - * This variant of the code has the data already prepared. + * Update an existing hash value with extra bytes of data. */ -static const uint32_t crc32_table[256] = { - 0x00000000L, 0x77073096L, 0xEE0E612CL, 0x990951BAL, - 0x076DC419L, 0x706AF48FL, 0xE963A535L, 0x9E6495A3L, - 0x0EDB8832L, 0x79DCB8A4L, 0xE0D5E91EL, 0x97D2D988L, - 0x09B64C2BL, 0x7EB17CBDL, 0xE7B82D07L, 0x90BF1D91L, - 0x1DB71064L, 0x6AB020F2L, 0xF3B97148L, 0x84BE41DEL, - 0x1ADAD47DL, 0x6DDDE4EBL, 0xF4D4B551L, 0x83D385C7L, - 0x136C9856L, 0x646BA8C0L, 0xFD62F97AL, 0x8A65C9ECL, - 0x14015C4FL, 0x63066CD9L, 0xFA0F3D63L, 0x8D080DF5L, - 0x3B6E20C8L, 0x4C69105EL, 0xD56041E4L, 0xA2677172L, - 0x3C03E4D1L, 0x4B04D447L, 0xD20D85FDL, 0xA50AB56BL, - 0x35B5A8FAL, 0x42B2986CL, 0xDBBBC9D6L, 0xACBCF940L, - 0x32D86CE3L, 0x45DF5C75L, 0xDCD60DCFL, 0xABD13D59L, - 0x26D930ACL, 0x51DE003AL, 0xC8D75180L, 0xBFD06116L, - 0x21B4F4B5L, 0x56B3C423L, 0xCFBA9599L, 0xB8BDA50FL, - 0x2802B89EL, 0x5F058808L, 0xC60CD9B2L, 0xB10BE924L, - 0x2F6F7C87L, 0x58684C11L, 0xC1611DABL, 0xB6662D3DL, - 0x76DC4190L, 0x01DB7106L, 0x98D220BCL, 0xEFD5102AL, - 0x71B18589L, 0x06B6B51FL, 0x9FBFE4A5L, 0xE8B8D433L, - 0x7807C9A2L, 0x0F00F934L, 0x9609A88EL, 0xE10E9818L, - 0x7F6A0DBBL, 0x086D3D2DL, 0x91646C97L, 0xE6635C01L, - 0x6B6B51F4L, 0x1C6C6162L, 0x856530D8L, 0xF262004EL, - 0x6C0695EDL, 0x1B01A57BL, 0x8208F4C1L, 0xF50FC457L, - 0x65B0D9C6L, 0x12B7E950L, 0x8BBEB8EAL, 0xFCB9887CL, - 0x62DD1DDFL, 0x15DA2D49L, 0x8CD37CF3L, 0xFBD44C65L, - 0x4DB26158L, 0x3AB551CEL, 0xA3BC0074L, 0xD4BB30E2L, - 0x4ADFA541L, 0x3DD895D7L, 0xA4D1C46DL, 0xD3D6F4FBL, - 0x4369E96AL, 0x346ED9FCL, 0xAD678846L, 0xDA60B8D0L, - 0x44042D73L, 0x33031DE5L, 0xAA0A4C5FL, 0xDD0D7CC9L, - 0x5005713CL, 0x270241AAL, 0xBE0B1010L, 0xC90C2086L, - 0x5768B525L, 0x206F85B3L, 0xB966D409L, 0xCE61E49FL, - 0x5EDEF90EL, 0x29D9C998L, 0xB0D09822L, 0xC7D7A8B4L, - 0x59B33D17L, 0x2EB40D81L, 0xB7BD5C3BL, 0xC0BA6CADL, - 0xEDB88320L, 0x9ABFB3B6L, 0x03B6E20CL, 0x74B1D29AL, - 0xEAD54739L, 0x9DD277AFL, 0x04DB2615L, 0x73DC1683L, - 0xE3630B12L, 0x94643B84L, 0x0D6D6A3EL, 0x7A6A5AA8L, - 0xE40ECF0BL, 0x9309FF9DL, 0x0A00AE27L, 0x7D079EB1L, - 0xF00F9344L, 0x8708A3D2L, 0x1E01F268L, 0x6906C2FEL, - 0xF762575DL, 0x806567CBL, 0x196C3671L, 0x6E6B06E7L, - 0xFED41B76L, 0x89D32BE0L, 0x10DA7A5AL, 0x67DD4ACCL, - 0xF9B9DF6FL, 0x8EBEEFF9L, 0x17B7BE43L, 0x60B08ED5L, - 0xD6D6A3E8L, 0xA1D1937EL, 0x38D8C2C4L, 0x4FDFF252L, - 0xD1BB67F1L, 0xA6BC5767L, 0x3FB506DDL, 0x48B2364BL, - 0xD80D2BDAL, 0xAF0A1B4CL, 0x36034AF6L, 0x41047A60L, - 0xDF60EFC3L, 0xA867DF55L, 0x316E8EEFL, 0x4669BE79L, - 0xCB61B38CL, 0xBC66831AL, 0x256FD2A0L, 0x5268E236L, - 0xCC0C7795L, 0xBB0B4703L, 0x220216B9L, 0x5505262FL, - 0xC5BA3BBEL, 0xB2BD0B28L, 0x2BB45A92L, 0x5CB36A04L, - 0xC2D7FFA7L, 0xB5D0CF31L, 0x2CD99E8BL, 0x5BDEAE1DL, - 0x9B64C2B0L, 0xEC63F226L, 0x756AA39CL, 0x026D930AL, - 0x9C0906A9L, 0xEB0E363FL, 0x72076785L, 0x05005713L, - 0x95BF4A82L, 0xE2B87A14L, 0x7BB12BAEL, 0x0CB61B38L, - 0x92D28E9BL, 0xE5D5BE0DL, 0x7CDCEFB7L, 0x0BDBDF21L, - 0x86D3D2D4L, 0xF1D4E242L, 0x68DDB3F8L, 0x1FDA836EL, - 0x81BE16CDL, 0xF6B9265BL, 0x6FB077E1L, 0x18B74777L, - 0x88085AE6L, 0xFF0F6A70L, 0x66063BCAL, 0x11010B5CL, - 0x8F659EFFL, 0xF862AE69L, 0x616BFFD3L, 0x166CCF45L, - 0xA00AE278L, 0xD70DD2EEL, 0x4E048354L, 0x3903B3C2L, - 0xA7672661L, 0xD06016F7L, 0x4969474DL, 0x3E6E77DBL, - 0xAED16A4AL, 0xD9D65ADCL, 0x40DF0B66L, 0x37D83BF0L, - 0xA9BCAE53L, 0xDEBB9EC5L, 0x47B2CF7FL, 0x30B5FFE9L, - 0xBDBDF21CL, 0xCABAC28AL, 0x53B39330L, 0x24B4A3A6L, - 0xBAD03605L, 0xCDD70693L, 0x54DE5729L, 0x23D967BFL, - 0xB3667A2EL, 0xC4614AB8L, 0x5D681B02L, 0x2A6F2B94L, - 0xB40BBE37L, 0xC30C8EA1L, 0x5A05DF1BL, 0x2D02EF8DL -}; - -#endif - -#ifdef GENPROGRAM -int main(void) -{ - int i; - - crc32_init(); - for (i = 0; i < 256; i++) { - printf("%s0x%08lXL%s", - (i % 4 == 0 ? " " : " "), - crc32_table[i], - (i % 4 == 3 ? (i == 255 ? "\n" : ",\n") : ",")); - } - - return 0; -} -#endif - -uint32_t crc32_update(uint32_t crcword, ptrlen data) +uint32_t crc32_update(uint32_t crc, ptrlen data) { const uint8_t *p = (const uint8_t *)data.ptr; - for (size_t len = data.len; len-- > 0 ;) { - uint32_t newbyte = *p++; - newbyte ^= crcword & 0xFFL; - crcword = (crcword >> 8) ^ crc32_table[newbyte]; - } - return crcword; + for (size_t len = data.len; len-- > 0 ;) + crc = crc32_shift_8(crc ^ *p++); + return crc; } /*