/* * RSA implementation for PuTTY. */ #include <stdio.h> #include <stdlib.h> #include <string.h> #include <assert.h> #include "ssh.h" #include "misc.h" int makekey(unsigned char *data, int len, struct RSAKey *result, unsigned char **keystr, int order) { unsigned char *p = data; int i, n; if (len < 4) return -1; if (result) { result->bits = 0; for (i = 0; i < 4; i++) result->bits = (result->bits << 8) + *p++; } else p += 4; len -= 4; /* * order=0 means exponent then modulus (the keys sent by the * server). order=1 means modulus then exponent (the keys * stored in a keyfile). */ if (order == 0) { n = ssh1_read_bignum(p, len, result ? &result->exponent : NULL); if (n < 0) return -1; p += n; len -= n; } n = ssh1_read_bignum(p, len, result ? &result->modulus : NULL); if (n < 0 || (result && bignum_bitcount(result->modulus) == 0)) return -1; if (result) result->bytes = n - 2; if (keystr) *keystr = p + 2; p += n; len -= n; if (order == 1) { n = ssh1_read_bignum(p, len, result ? &result->exponent : NULL); if (n < 0) return -1; p += n; len -= n; } return p - data; } int makeprivate(unsigned char *data, int len, struct RSAKey *result) { return ssh1_read_bignum(data, len, &result->private_exponent); } int rsaencrypt(unsigned char *data, int length, struct RSAKey *key) { Bignum b1, b2; int i; unsigned char *p; if (key->bytes < length + 4) return 0; /* RSA key too short! */ memmove(data + key->bytes - length, data, length); data[0] = 0; data[1] = 2; for (i = 2; i < key->bytes - length - 1; i++) { do { data[i] = random_byte(); } while (data[i] == 0); } data[key->bytes - length - 1] = 0; b1 = bignum_from_bytes(data, key->bytes); b2 = modpow(b1, key->exponent, key->modulus); p = data; for (i = key->bytes; i--;) { *p++ = bignum_byte(b2, i); } freebn(b1); freebn(b2); return 1; } static void sha512_mpint(SHA512_State * s, Bignum b) { unsigned char lenbuf[4]; int len; len = (bignum_bitcount(b) + 8) / 8; PUT_32BIT(lenbuf, len); SHA512_Bytes(s, lenbuf, 4); while (len-- > 0) { lenbuf[0] = bignum_byte(b, len); SHA512_Bytes(s, lenbuf, 1); } memset(lenbuf, 0, sizeof(lenbuf)); } /* * This function is a wrapper on modpow(). It has the same effect * as modpow(), but employs RSA blinding to protect against timing * attacks. */ static Bignum rsa_privkey_op(Bignum input, struct RSAKey *key) { Bignum random, random_encrypted, random_inverse; Bignum input_blinded, ret_blinded; Bignum ret; SHA512_State ss; unsigned char digest512[64]; int digestused = lenof(digest512); int hashseq = 0; /* * Start by inventing a random number chosen uniformly from the * range 2..modulus-1. (We do this by preparing a random number * of the right length and retrying if it's greater than the * modulus, to prevent any potential Bleichenbacher-like * attacks making use of the uneven distribution within the * range that would arise from just reducing our number mod n. * There are timing implications to the potential retries, of * course, but all they tell you is the modulus, which you * already knew.) * * To preserve determinism and avoid Pageant needing to share * the random number pool, we actually generate this `random' * number by hashing stuff with the private key. */ while (1) { int bits, byte, bitsleft, v; random = copybn(key->modulus); /* * Find the topmost set bit. (This function will return its * index plus one.) Then we'll set all bits from that one * downwards randomly. */ bits = bignum_bitcount(random); byte = 0; bitsleft = 0; while (bits--) { if (bitsleft <= 0) { bitsleft = 8; /* * Conceptually the following few lines are equivalent to * byte = random_byte(); */ if (digestused >= lenof(digest512)) { unsigned char seqbuf[4]; PUT_32BIT(seqbuf, hashseq); SHA512_Init(&ss); SHA512_Bytes(&ss, "RSA deterministic blinding", 26); SHA512_Bytes(&ss, seqbuf, sizeof(seqbuf)); sha512_mpint(&ss, key->private_exponent); SHA512_Final(&ss, digest512); hashseq++; /* * Now hash that digest plus the signature * input. */ SHA512_Init(&ss); SHA512_Bytes(&ss, digest512, sizeof(digest512)); sha512_mpint(&ss, input); SHA512_Final(&ss, digest512); digestused = 0; } byte = digest512[digestused++]; } v = byte & 1; byte >>= 1; bitsleft--; bignum_set_bit(random, bits, v); } /* * Now check that this number is strictly greater than * zero, and strictly less than modulus. */ if (bignum_cmp(random, Zero) <= 0 || bignum_cmp(random, key->modulus) >= 0) { freebn(random); continue; } else { break; } } /* * RSA blinding relies on the fact that (xy)^d mod n is equal * to (x^d mod n) * (y^d mod n) mod n. We invent a random pair * y and y^d; then we multiply x by y, raise to the power d mod * n as usual, and divide by y^d to recover x^d. Thus an * attacker can't correlate the timing of the modpow with the * input, because they don't know anything about the number * that was input to the actual modpow. * * The clever bit is that we don't have to do a huge modpow to * get y and y^d; we will use the number we just invented as * _y^d_, and use the _public_ exponent to compute (y^d)^e = y * from it, which is much faster to do. */ random_encrypted = modpow(random, key->exponent, key->modulus); random_inverse = modinv(random, key->modulus); input_blinded = modmul(input, random_encrypted, key->modulus); ret_blinded = modpow(input_blinded, key->private_exponent, key->modulus); ret = modmul(ret_blinded, random_inverse, key->modulus); freebn(ret_blinded); freebn(input_blinded); freebn(random_inverse); freebn(random_encrypted); freebn(random); return ret; } Bignum rsadecrypt(Bignum input, struct RSAKey *key) { return rsa_privkey_op(input, key); } int rsastr_len(struct RSAKey *key) { Bignum md, ex; int mdlen, exlen; md = key->modulus; ex = key->exponent; mdlen = (bignum_bitcount(md) + 15) / 16; exlen = (bignum_bitcount(ex) + 15) / 16; return 4 * (mdlen + exlen) + 20; } void rsastr_fmt(char *str, struct RSAKey *key) { Bignum md, ex; int len = 0, i, nibbles; static const char hex[] = "0123456789abcdef"; md = key->modulus; ex = key->exponent; len += sprintf(str + len, "0x"); nibbles = (3 + bignum_bitcount(ex)) / 4; if (nibbles < 1) nibbles = 1; for (i = nibbles; i--;) str[len++] = hex[(bignum_byte(ex, i / 2) >> (4 * (i % 2))) & 0xF]; len += sprintf(str + len, ",0x"); nibbles = (3 + bignum_bitcount(md)) / 4; if (nibbles < 1) nibbles = 1; for (i = nibbles; i--;) str[len++] = hex[(bignum_byte(md, i / 2) >> (4 * (i % 2))) & 0xF]; str[len] = '\0'; } /* * Generate a fingerprint string for the key. Compatible with the * OpenSSH fingerprint code. */ void rsa_fingerprint(char *str, int len, struct RSAKey *key) { struct MD5Context md5c; unsigned char digest[16]; char buffer[16 * 3 + 40]; int numlen, slen, i; MD5Init(&md5c); numlen = ssh1_bignum_length(key->modulus) - 2; for (i = numlen; i--;) { unsigned char c = bignum_byte(key->modulus, i); MD5Update(&md5c, &c, 1); } numlen = ssh1_bignum_length(key->exponent) - 2; for (i = numlen; i--;) { unsigned char c = bignum_byte(key->exponent, i); MD5Update(&md5c, &c, 1); } MD5Final(digest, &md5c); sprintf(buffer, "%d ", bignum_bitcount(key->modulus)); for (i = 0; i < 16; i++) sprintf(buffer + strlen(buffer), "%s%02x", i ? ":" : "", digest[i]); strncpy(str, buffer, len); str[len - 1] = '\0'; slen = strlen(str); if (key->comment && slen < len - 1) { str[slen] = ' '; strncpy(str + slen + 1, key->comment, len - slen - 1); str[len - 1] = '\0'; } } /* * Verify that the public data in an RSA key matches the private * data. We also check the private data itself: we ensure that p > * q and that iqmp really is the inverse of q mod p. */ int rsa_verify(struct RSAKey *key) { Bignum n, ed, pm1, qm1; int cmp; /* n must equal pq. */ n = bigmul(key->p, key->q); cmp = bignum_cmp(n, key->modulus); freebn(n); if (cmp != 0) return 0; /* e * d must be congruent to 1, modulo (p-1) and modulo (q-1). */ pm1 = copybn(key->p); decbn(pm1); ed = modmul(key->exponent, key->private_exponent, pm1); cmp = bignum_cmp(ed, One); sfree(ed); if (cmp != 0) return 0; qm1 = copybn(key->q); decbn(qm1); ed = modmul(key->exponent, key->private_exponent, qm1); cmp = bignum_cmp(ed, One); sfree(ed); if (cmp != 0) return 0; /* * Ensure p > q. * * I have seen key blobs in the wild which were generated with * p < q, so instead of rejecting the key in this case we * should instead flip them round into the canonical order of * p > q. This also involves regenerating iqmp. */ if (bignum_cmp(key->p, key->q) <= 0) { Bignum tmp = key->p; key->p = key->q; key->q = tmp; freebn(key->iqmp); key->iqmp = modinv(key->q, key->p); } /* * Ensure iqmp * q is congruent to 1, modulo p. */ n = modmul(key->iqmp, key->q, key->p); cmp = bignum_cmp(n, One); sfree(n); if (cmp != 0) return 0; return 1; } /* Public key blob as used by Pageant: exponent before modulus. */ unsigned char *rsa_public_blob(struct RSAKey *key, int *len) { int length, pos; unsigned char *ret; length = (ssh1_bignum_length(key->modulus) + ssh1_bignum_length(key->exponent) + 4); ret = snewn(length, unsigned char); PUT_32BIT(ret, bignum_bitcount(key->modulus)); pos = 4; pos += ssh1_write_bignum(ret + pos, key->exponent); pos += ssh1_write_bignum(ret + pos, key->modulus); *len = length; return ret; } /* Given a public blob, determine its length. */ int rsa_public_blob_len(void *data, int maxlen) { unsigned char *p = (unsigned char *)data; int n; if (maxlen < 4) return -1; p += 4; /* length word */ maxlen -= 4; n = ssh1_read_bignum(p, maxlen, NULL); /* exponent */ if (n < 0) return -1; p += n; n = ssh1_read_bignum(p, maxlen, NULL); /* modulus */ if (n < 0) return -1; p += n; return p - (unsigned char *)data; } void freersakey(struct RSAKey *key) { if (key->modulus) freebn(key->modulus); if (key->exponent) freebn(key->exponent); if (key->private_exponent) freebn(key->private_exponent); if (key->p) freebn(key->p); if (key->q) freebn(key->q); if (key->iqmp) freebn(key->iqmp); if (key->comment) sfree(key->comment); } /* ---------------------------------------------------------------------- * Implementation of the ssh-rsa signing key type. */ static void getstring(char **data, int *datalen, char **p, int *length) { *p = NULL; if (*datalen < 4) return; *length = GET_32BIT(*data); *datalen -= 4; *data += 4; if (*datalen < *length) return; *p = *data; *data += *length; *datalen -= *length; } static Bignum getmp(char **data, int *datalen) { char *p; int length; Bignum b; getstring(data, datalen, &p, &length); if (!p) return NULL; b = bignum_from_bytes((unsigned char *)p, length); return b; } static void *rsa2_newkey(char *data, int len) { char *p; int slen; struct RSAKey *rsa; rsa = snew(struct RSAKey); if (!rsa) return NULL; getstring(&data, &len, &p, &slen); if (!p || slen != 7 || memcmp(p, "ssh-rsa", 7)) { sfree(rsa); return NULL; } rsa->exponent = getmp(&data, &len); rsa->modulus = getmp(&data, &len); rsa->private_exponent = NULL; rsa->p = rsa->q = rsa->iqmp = NULL; rsa->comment = NULL; return rsa; } static void rsa2_freekey(void *key) { struct RSAKey *rsa = (struct RSAKey *) key; freersakey(rsa); sfree(rsa); } static char *rsa2_fmtkey(void *key) { struct RSAKey *rsa = (struct RSAKey *) key; char *p; int len; len = rsastr_len(rsa); p = snewn(len, char); rsastr_fmt(p, rsa); return p; } static unsigned char *rsa2_public_blob(void *key, int *len) { struct RSAKey *rsa = (struct RSAKey *) key; int elen, mlen, bloblen; int i; unsigned char *blob, *p; elen = (bignum_bitcount(rsa->exponent) + 8) / 8; mlen = (bignum_bitcount(rsa->modulus) + 8) / 8; /* * string "ssh-rsa", mpint exp, mpint mod. Total 19+elen+mlen. * (three length fields, 12+7=19). */ bloblen = 19 + elen + mlen; blob = snewn(bloblen, unsigned char); p = blob; PUT_32BIT(p, 7); p += 4; memcpy(p, "ssh-rsa", 7); p += 7; PUT_32BIT(p, elen); p += 4; for (i = elen; i--;) *p++ = bignum_byte(rsa->exponent, i); PUT_32BIT(p, mlen); p += 4; for (i = mlen; i--;) *p++ = bignum_byte(rsa->modulus, i); assert(p == blob + bloblen); *len = bloblen; return blob; } static unsigned char *rsa2_private_blob(void *key, int *len) { struct RSAKey *rsa = (struct RSAKey *) key; int dlen, plen, qlen, ulen, bloblen; int i; unsigned char *blob, *p; dlen = (bignum_bitcount(rsa->private_exponent) + 8) / 8; plen = (bignum_bitcount(rsa->p) + 8) / 8; qlen = (bignum_bitcount(rsa->q) + 8) / 8; ulen = (bignum_bitcount(rsa->iqmp) + 8) / 8; /* * mpint private_exp, mpint p, mpint q, mpint iqmp. Total 16 + * sum of lengths. */ bloblen = 16 + dlen + plen + qlen + ulen; blob = snewn(bloblen, unsigned char); p = blob; PUT_32BIT(p, dlen); p += 4; for (i = dlen; i--;) *p++ = bignum_byte(rsa->private_exponent, i); PUT_32BIT(p, plen); p += 4; for (i = plen; i--;) *p++ = bignum_byte(rsa->p, i); PUT_32BIT(p, qlen); p += 4; for (i = qlen; i--;) *p++ = bignum_byte(rsa->q, i); PUT_32BIT(p, ulen); p += 4; for (i = ulen; i--;) *p++ = bignum_byte(rsa->iqmp, i); assert(p == blob + bloblen); *len = bloblen; return blob; } static void *rsa2_createkey(unsigned char *pub_blob, int pub_len, unsigned char *priv_blob, int priv_len) { struct RSAKey *rsa; char *pb = (char *) priv_blob; rsa = rsa2_newkey((char *) pub_blob, pub_len); rsa->private_exponent = getmp(&pb, &priv_len); rsa->p = getmp(&pb, &priv_len); rsa->q = getmp(&pb, &priv_len); rsa->iqmp = getmp(&pb, &priv_len); if (!rsa_verify(rsa)) { rsa2_freekey(rsa); return NULL; } return rsa; } static void *rsa2_openssh_createkey(unsigned char **blob, int *len) { char **b = (char **) blob; struct RSAKey *rsa; rsa = snew(struct RSAKey); if (!rsa) return NULL; rsa->comment = NULL; rsa->modulus = getmp(b, len); rsa->exponent = getmp(b, len); rsa->private_exponent = getmp(b, len); rsa->iqmp = getmp(b, len); rsa->p = getmp(b, len); rsa->q = getmp(b, len); if (!rsa->modulus || !rsa->exponent || !rsa->private_exponent || !rsa->iqmp || !rsa->p || !rsa->q) { sfree(rsa->modulus); sfree(rsa->exponent); sfree(rsa->private_exponent); sfree(rsa->iqmp); sfree(rsa->p); sfree(rsa->q); sfree(rsa); return NULL; } return rsa; } static int rsa2_openssh_fmtkey(void *key, unsigned char *blob, int len) { struct RSAKey *rsa = (struct RSAKey *) key; int bloblen, i; bloblen = ssh2_bignum_length(rsa->modulus) + ssh2_bignum_length(rsa->exponent) + ssh2_bignum_length(rsa->private_exponent) + ssh2_bignum_length(rsa->iqmp) + ssh2_bignum_length(rsa->p) + ssh2_bignum_length(rsa->q); if (bloblen > len) return bloblen; bloblen = 0; #define ENC(x) \ PUT_32BIT(blob+bloblen, ssh2_bignum_length((x))-4); bloblen += 4; \ for (i = ssh2_bignum_length((x))-4; i-- ;) blob[bloblen++]=bignum_byte((x),i); ENC(rsa->modulus); ENC(rsa->exponent); ENC(rsa->private_exponent); ENC(rsa->iqmp); ENC(rsa->p); ENC(rsa->q); return bloblen; } static int rsa2_pubkey_bits(void *blob, int len) { struct RSAKey *rsa; int ret; rsa = rsa2_newkey((char *) blob, len); ret = bignum_bitcount(rsa->modulus); rsa2_freekey(rsa); return ret; } static char *rsa2_fingerprint(void *key) { struct RSAKey *rsa = (struct RSAKey *) key; struct MD5Context md5c; unsigned char digest[16], lenbuf[4]; char buffer[16 * 3 + 40]; char *ret; int numlen, i; MD5Init(&md5c); MD5Update(&md5c, (unsigned char *)"\0\0\0\7ssh-rsa", 11); #define ADD_BIGNUM(bignum) \ numlen = (bignum_bitcount(bignum)+8)/8; \ PUT_32BIT(lenbuf, numlen); MD5Update(&md5c, lenbuf, 4); \ for (i = numlen; i-- ;) { \ unsigned char c = bignum_byte(bignum, i); \ MD5Update(&md5c, &c, 1); \ } ADD_BIGNUM(rsa->exponent); ADD_BIGNUM(rsa->modulus); #undef ADD_BIGNUM MD5Final(digest, &md5c); sprintf(buffer, "ssh-rsa %d ", bignum_bitcount(rsa->modulus)); for (i = 0; i < 16; i++) sprintf(buffer + strlen(buffer), "%s%02x", i ? ":" : "", digest[i]); ret = snewn(strlen(buffer) + 1, char); if (ret) strcpy(ret, buffer); return ret; } /* * This is the magic ASN.1/DER prefix that goes in the decoded * signature, between the string of FFs and the actual SHA hash * value. The meaning of it is: * * 00 -- this marks the end of the FFs; not part of the ASN.1 bit itself * * 30 21 -- a constructed SEQUENCE of length 0x21 * 30 09 -- a constructed sub-SEQUENCE of length 9 * 06 05 -- an object identifier, length 5 * 2B 0E 03 02 1A -- object id { 1 3 14 3 2 26 } * (the 1,3 comes from 0x2B = 43 = 40*1+3) * 05 00 -- NULL * 04 14 -- a primitive OCTET STRING of length 0x14 * [0x14 bytes of hash data follows] * * The object id in the middle there is listed as `id-sha1' in * ftp://ftp.rsasecurity.com/pub/pkcs/pkcs-1/pkcs-1v2-1d2.asn (the * ASN module for PKCS #1) and its expanded form is as follows: * * id-sha1 OBJECT IDENTIFIER ::= { * iso(1) identified-organization(3) oiw(14) secsig(3) * algorithms(2) 26 } */ static const unsigned char asn1_weird_stuff[] = { 0x00, 0x30, 0x21, 0x30, 0x09, 0x06, 0x05, 0x2B, 0x0E, 0x03, 0x02, 0x1A, 0x05, 0x00, 0x04, 0x14, }; #define ASN1_LEN ( (int) sizeof(asn1_weird_stuff) ) static int rsa2_verifysig(void *key, char *sig, int siglen, char *data, int datalen) { struct RSAKey *rsa = (struct RSAKey *) key; Bignum in, out; char *p; int slen; int bytes, i, j, ret; unsigned char hash[20]; getstring(&sig, &siglen, &p, &slen); if (!p || slen != 7 || memcmp(p, "ssh-rsa", 7)) { return 0; } in = getmp(&sig, &siglen); out = modpow(in, rsa->exponent, rsa->modulus); freebn(in); ret = 1; bytes = (bignum_bitcount(rsa->modulus)+7) / 8; /* Top (partial) byte should be zero. */ if (bignum_byte(out, bytes - 1) != 0) ret = 0; /* First whole byte should be 1. */ if (bignum_byte(out, bytes - 2) != 1) ret = 0; /* Most of the rest should be FF. */ for (i = bytes - 3; i >= 20 + ASN1_LEN; i--) { if (bignum_byte(out, i) != 0xFF) ret = 0; } /* Then we expect to see the asn1_weird_stuff. */ for (i = 20 + ASN1_LEN - 1, j = 0; i >= 20; i--, j++) { if (bignum_byte(out, i) != asn1_weird_stuff[j]) ret = 0; } /* Finally, we expect to see the SHA-1 hash of the signed data. */ SHA_Simple(data, datalen, hash); for (i = 19, j = 0; i >= 0; i--, j++) { if (bignum_byte(out, i) != hash[j]) ret = 0; } freebn(out); return ret; } static unsigned char *rsa2_sign(void *key, char *data, int datalen, int *siglen) { struct RSAKey *rsa = (struct RSAKey *) key; unsigned char *bytes; int nbytes; unsigned char hash[20]; Bignum in, out; int i, j; SHA_Simple(data, datalen, hash); nbytes = (bignum_bitcount(rsa->modulus) - 1) / 8; assert(1 <= nbytes - 20 - ASN1_LEN); bytes = snewn(nbytes, unsigned char); bytes[0] = 1; for (i = 1; i < nbytes - 20 - ASN1_LEN; i++) bytes[i] = 0xFF; for (i = nbytes - 20 - ASN1_LEN, j = 0; i < nbytes - 20; i++, j++) bytes[i] = asn1_weird_stuff[j]; for (i = nbytes - 20, j = 0; i < nbytes; i++, j++) bytes[i] = hash[j]; in = bignum_from_bytes(bytes, nbytes); sfree(bytes); out = rsa_privkey_op(in, rsa); freebn(in); nbytes = (bignum_bitcount(out) + 7) / 8; bytes = snewn(4 + 7 + 4 + nbytes, unsigned char); PUT_32BIT(bytes, 7); memcpy(bytes + 4, "ssh-rsa", 7); PUT_32BIT(bytes + 4 + 7, nbytes); for (i = 0; i < nbytes; i++) bytes[4 + 7 + 4 + i] = bignum_byte(out, nbytes - 1 - i); freebn(out); *siglen = 4 + 7 + 4 + nbytes; return bytes; } const struct ssh_signkey ssh_rsa = { rsa2_newkey, rsa2_freekey, rsa2_fmtkey, rsa2_public_blob, rsa2_private_blob, rsa2_createkey, rsa2_openssh_createkey, rsa2_openssh_fmtkey, rsa2_pubkey_bits, rsa2_fingerprint, rsa2_verifysig, rsa2_sign, "ssh-rsa", "rsa2" }; void *ssh_rsakex_newkey(char *data, int len) { return rsa2_newkey(data, len); } void ssh_rsakex_freekey(void *key) { rsa2_freekey(key); } int ssh_rsakex_klen(void *key) { struct RSAKey *rsa = (struct RSAKey *) key; return bignum_bitcount(rsa->modulus); } static void oaep_mask(const struct ssh_hash *h, void *seed, int seedlen, void *vdata, int datalen) { unsigned char *data = (unsigned char *)vdata; unsigned count = 0; while (datalen > 0) { int i, max = (datalen > h->hlen ? h->hlen : datalen); void *s; unsigned char counter[4], hash[SSH2_KEX_MAX_HASH_LEN]; assert(h->hlen <= SSH2_KEX_MAX_HASH_LEN); PUT_32BIT(counter, count); s = h->init(); h->bytes(s, seed, seedlen); h->bytes(s, counter, 4); h->final(s, hash); count++; for (i = 0; i < max; i++) data[i] ^= hash[i]; data += max; datalen -= max; } } void ssh_rsakex_encrypt(const struct ssh_hash *h, unsigned char *in, int inlen, unsigned char *out, int outlen, void *key) { Bignum b1, b2; struct RSAKey *rsa = (struct RSAKey *) key; int k, i; char *p; const int HLEN = h->hlen; /* * Here we encrypt using RSAES-OAEP. Essentially this means: * * - we have a SHA-based `mask generation function' which * creates a pseudo-random stream of mask data * deterministically from an input chunk of data. * * - we have a random chunk of data called a seed. * * - we use the seed to generate a mask which we XOR with our * plaintext. * * - then we use _the masked plaintext_ to generate a mask * which we XOR with the seed. * * - then we concatenate the masked seed and the masked * plaintext, and RSA-encrypt that lot. * * The result is that the data input to the encryption function * is random-looking and (hopefully) contains no exploitable * structure such as PKCS1-v1_5 does. * * For a precise specification, see RFC 3447, section 7.1.1. * Some of the variable names below are derived from that, so * it'd probably help to read it anyway. */ /* k denotes the length in octets of the RSA modulus. */ k = (7 + bignum_bitcount(rsa->modulus)) / 8; /* The length of the input data must be at most k - 2hLen - 2. */ assert(inlen > 0 && inlen <= k - 2*HLEN - 2); /* The length of the output data wants to be precisely k. */ assert(outlen == k); /* * Now perform EME-OAEP encoding. First set up all the unmasked * output data. */ /* Leading byte zero. */ out[0] = 0; /* At position 1, the seed: HLEN bytes of random data. */ for (i = 0; i < HLEN; i++) out[i + 1] = random_byte(); /* At position 1+HLEN, the data block DB, consisting of: */ /* The hash of the label (we only support an empty label here) */ h->final(h->init(), out + HLEN + 1); /* A bunch of zero octets */ memset(out + 2*HLEN + 1, 0, outlen - (2*HLEN + 1)); /* A single 1 octet, followed by the input message data. */ out[outlen - inlen - 1] = 1; memcpy(out + outlen - inlen, in, inlen); /* * Now use the seed data to mask the block DB. */ oaep_mask(h, out+1, HLEN, out+HLEN+1, outlen-HLEN-1); /* * And now use the masked DB to mask the seed itself. */ oaep_mask(h, out+HLEN+1, outlen-HLEN-1, out+1, HLEN); /* * Now `out' contains precisely the data we want to * RSA-encrypt. */ b1 = bignum_from_bytes(out, outlen); b2 = modpow(b1, rsa->exponent, rsa->modulus); p = (char *)out; for (i = outlen; i--;) { *p++ = bignum_byte(b2, i); } freebn(b1); freebn(b2); /* * And we're done. */ } static const struct ssh_kex ssh_rsa_kex_sha1 = { "rsa1024-sha1", NULL, KEXTYPE_RSA, NULL, NULL, 0, 0, &ssh_sha1 }; static const struct ssh_kex ssh_rsa_kex_sha256 = { "rsa2048-sha256", NULL, KEXTYPE_RSA, NULL, NULL, 0, 0, &ssh_sha256 }; static const struct ssh_kex *const rsa_kex_list[] = { &ssh_rsa_kex_sha256, &ssh_rsa_kex_sha1 }; const struct ssh_kexes ssh_rsa_kex = { sizeof(rsa_kex_list) / sizeof(*rsa_kex_list), rsa_kex_list };