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putty/sshrsa.c

978 строки
25 KiB
C
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/*
* RSA implementation for PuTTY.
*/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <assert.h>
#include "ssh.h"
#include "misc.h"
int rsa_ssh1_readpub(const unsigned char *data, int len, struct RSAKey *result,
const unsigned char **keystr, RsaSsh1Order order)
{
const 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;
if (order == RSA_SSH1_EXPONENT_FIRST) {
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 == RSA_SSH1_MODULUS_FIRST) {
n = ssh1_read_bignum(p, len, result ? &result->exponent : NULL);
if (n < 0) return -1;
p += n;
len -= n;
}
return p - data;
}
int rsa_ssh1_readpriv(const unsigned char *data, int len,
struct RSAKey *result)
{
return ssh1_read_bignum(data, len, &result->private_exponent);
}
int rsa_ssh1_encrypt(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;
}
/*
* Compute (base ^ exp) % mod, provided mod == p * q, with p,q
* distinct primes, and iqmp is the multiplicative inverse of q mod p.
* Uses Chinese Remainder Theorem to speed computation up over the
* obvious implementation of a single big modpow.
*/
Bignum crt_modpow(Bignum base, Bignum exp, Bignum mod,
Bignum p, Bignum q, Bignum iqmp)
{
Bignum pm1, qm1, pexp, qexp, presult, qresult, diff, multiplier, ret0, ret;
/*
* Reduce the exponent mod phi(p) and phi(q), to save time when
* exponentiating mod p and mod q respectively. Of course, since p
* and q are prime, phi(p) == p-1 and similarly for q.
*/
pm1 = copybn(p);
decbn(pm1);
qm1 = copybn(q);
decbn(qm1);
pexp = bigmod(exp, pm1);
qexp = bigmod(exp, qm1);
/*
* Do the two modpows.
*/
presult = modpow(base, pexp, p);
qresult = modpow(base, qexp, q);
/*
* Recombine the results. We want a value which is congruent to
* qresult mod q, and to presult mod p.
*
* We know that iqmp * q is congruent to 1 * mod p (by definition
* of iqmp) and to 0 mod q (obviously). So we start with qresult
* (which is congruent to qresult mod both primes), and add on
* (presult-qresult) * (iqmp * q) which adjusts it to be congruent
* to presult mod p without affecting its value mod q.
*/
if (bignum_cmp(presult, qresult) < 0) {
/*
* Can't subtract presult from qresult without first adding on
* p.
*/
Bignum tmp = presult;
presult = bigadd(presult, p);
freebn(tmp);
}
diff = bigsub(presult, qresult);
multiplier = bigmul(iqmp, q);
ret0 = bigmuladd(multiplier, diff, qresult);
/*
* Finally, reduce the result mod n.
*/
ret = bigmod(ret0, mod);
/*
* Free all the intermediate results before returning.
*/
freebn(pm1);
freebn(qm1);
freebn(pexp);
freebn(qexp);
freebn(presult);
freebn(qresult);
freebn(diff);
freebn(multiplier);
freebn(ret0);
return ret;
}
/*
* This function is a wrapper on modpow(). It has the same effect as
* modpow(), but employs RSA blinding to protect against timing
* attacks and also uses the Chinese Remainder Theorem (implemented
* above, in crt_modpow()) to speed up the main operation.
*/
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)) {
SHA512_Init(&ss);
put_data(&ss, "RSA deterministic blinding", 26);
put_uint32(&ss, hashseq);
put_mp_ssh2(&ss, key->private_exponent);
SHA512_Final(&ss, digest512);
hashseq++;
/*
* Now hash that digest plus the signature
* input.
*/
SHA512_Init(&ss);
put_data(&ss, digest512, sizeof(digest512));
put_mp_ssh2(&ss, input);
SHA512_Final(&ss, digest512);
digestused = 0;
}
byte = digest512[digestused++];
}
v = byte & 1;
byte >>= 1;
bitsleft--;
bignum_set_bit(random, bits, v);
}
bn_restore_invariant(random);
/*
* 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;
}
/*
* Also, make sure it has an inverse mod modulus.
*/
random_inverse = modinv(random, key->modulus);
if (!random_inverse) {
freebn(random);
continue;
}
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 = crt_modpow(random, key->exponent,
key->modulus, key->p, key->q, key->iqmp);
input_blinded = modmul(input, random_encrypted, key->modulus);
ret_blinded = crt_modpow(input_blinded, key->private_exponent,
key->modulus, key->p, key->q, key->iqmp);
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 rsa_ssh1_decrypt(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 slen, i;
MD5Init(&md5c);
put_mp_ssh1(&md5c, key->modulus);
put_mp_ssh1(&md5c, key->exponent);
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);
freebn(pm1);
cmp = bignum_cmp(ed, One);
freebn(ed);
if (cmp != 0)
return 0;
qm1 = copybn(key->q);
decbn(qm1);
ed = modmul(key->exponent, key->private_exponent, qm1);
freebn(qm1);
cmp = bignum_cmp(ed, One);
freebn(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);
if (!key->iqmp)
return 0;
}
/*
* Ensure iqmp * q is congruent to 1, modulo p.
*/
n = modmul(key->iqmp, key->q, key->p);
cmp = bignum_cmp(n, One);
freebn(n);
if (cmp != 0)
return 0;
return 1;
}
void rsa_ssh1_public_blob(BinarySink *bs, struct RSAKey *key,
RsaSsh1Order order)
{
put_uint32(bs, bignum_bitcount(key->modulus));
if (order == RSA_SSH1_EXPONENT_FIRST) {
put_mp_ssh1(bs, key->exponent);
put_mp_ssh1(bs, key->modulus);
} else {
put_mp_ssh1(bs, key->modulus);
put_mp_ssh1(bs, key->exponent);
}
}
/* 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(const char **data, int *datalen,
const char **p, int *length)
{
*p = NULL;
if (*datalen < 4)
return;
*length = toint(GET_32BIT(*data));
if (*length < 0)
return;
*datalen -= 4;
*data += 4;
if (*datalen < *length)
return;
*p = *data;
*data += *length;
*datalen -= *length;
}
static Bignum getmp(const char **data, int *datalen)
{
const char *p;
int length;
Bignum b;
getstring(data, datalen, &p, &length);
if (!p)
return NULL;
b = bignum_from_bytes(p, length);
return b;
}
static void rsa2_freekey(ssh_key *key); /* forward reference */
static ssh_key *rsa2_newkey(const ssh_keyalg *self,
const void *vdata, int len)
{
const char *p;
const char *data = (const char *)vdata;
int slen;
struct RSAKey *rsa;
rsa = snew(struct RSAKey);
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;
if (!rsa->exponent || !rsa->modulus) {
rsa2_freekey(&rsa->sshk);
return NULL;
}
return &rsa->sshk;
}
static void rsa2_freekey(ssh_key *key)
{
struct RSAKey *rsa = FROMFIELD(key, struct RSAKey, sshk);
freersakey(rsa);
sfree(rsa);
}
static char *rsa2_fmtkey(ssh_key *key)
{
struct RSAKey *rsa = FROMFIELD(key, struct RSAKey, sshk);
char *p;
int len;
len = rsastr_len(rsa);
p = snewn(len, char);
rsastr_fmt(p, rsa);
return p;
}
static void rsa2_public_blob(ssh_key *key, BinarySink *bs)
{
struct RSAKey *rsa = FROMFIELD(key, struct RSAKey, sshk);
put_stringz(bs, "ssh-rsa");
put_mp_ssh2(bs, rsa->exponent);
put_mp_ssh2(bs, rsa->modulus);
}
static void rsa2_private_blob(ssh_key *key, BinarySink *bs)
{
struct RSAKey *rsa = FROMFIELD(key, struct RSAKey, sshk);
put_mp_ssh2(bs, rsa->private_exponent);
put_mp_ssh2(bs, rsa->p);
put_mp_ssh2(bs, rsa->q);
put_mp_ssh2(bs, rsa->iqmp);
}
static ssh_key *rsa2_createkey(const ssh_keyalg *self,
const void *pub_blob, int pub_len,
const void *priv_blob, int priv_len)
{
ssh_key *sshk;
struct RSAKey *rsa;
const char *pb = (const char *) priv_blob;
sshk = rsa2_newkey(self, pub_blob, pub_len);
if (!sshk)
return NULL;
rsa = FROMFIELD(sshk, struct RSAKey, sshk);
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->sshk);
return NULL;
}
return &rsa->sshk;
}
static ssh_key *rsa2_openssh_createkey(const ssh_keyalg *self,
const unsigned char **blob, int *len)
{
const char **b = (const char **) blob;
struct RSAKey *rsa;
rsa = snew(struct RSAKey);
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) {
rsa2_freekey(&rsa->sshk);
return NULL;
}
if (!rsa_verify(rsa)) {
rsa2_freekey(&rsa->sshk);
return NULL;
}
return &rsa->sshk;
}
static void rsa2_openssh_fmtkey(ssh_key *key, BinarySink *bs)
{
struct RSAKey *rsa = FROMFIELD(key, struct RSAKey, sshk);
put_mp_ssh2(bs, rsa->modulus);
put_mp_ssh2(bs, rsa->exponent);
put_mp_ssh2(bs, rsa->private_exponent);
put_mp_ssh2(bs, rsa->iqmp);
put_mp_ssh2(bs, rsa->p);
put_mp_ssh2(bs, rsa->q);
}
static int rsa2_pubkey_bits(const ssh_keyalg *self,
const void *blob, int len)
{
ssh_key *sshk;
struct RSAKey *rsa;
int ret;
sshk = rsa2_newkey(self, blob, len);
if (!sshk)
return -1;
rsa = FROMFIELD(sshk, struct RSAKey, sshk);
ret = bignum_bitcount(rsa->modulus);
rsa2_freekey(&rsa->sshk);
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(ssh_key *key, const void *vsig, int siglen,
const void *data, int datalen)
{
struct RSAKey *rsa = FROMFIELD(key, struct RSAKey, sshk);
const char *sig = (const char *)vsig;
Bignum in, out;
const 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);
if (!in)
return 0;
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 void rsa2_sign(ssh_key *key, const void *data, int datalen,
BinarySink *bs)
{
struct RSAKey *rsa = FROMFIELD(key, struct RSAKey, sshk);
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);
put_stringz(bs, "ssh-rsa");
nbytes = (bignum_bitcount(out) + 7) / 8;
put_uint32(bs, nbytes);
for (i = 0; i < nbytes; i++)
put_byte(bs, bignum_byte(out, nbytes - 1 - i));
freebn(out);
}
const ssh_keyalg ssh_rsa = {
rsa2_newkey,
rsa2_freekey,
rsa2_fmtkey,
rsa2_public_blob,
rsa2_private_blob,
rsa2_createkey,
rsa2_openssh_createkey,
rsa2_openssh_fmtkey,
6 /* n,e,d,iqmp,q,p */,
rsa2_pubkey_bits,
rsa2_verifysig,
rsa2_sign,
"ssh-rsa",
"rsa2",
NULL,
};
struct RSAKey *ssh_rsakex_newkey(const void *data, int len)
{
ssh_key *sshk = rsa2_newkey(&ssh_rsa, data, len);
if (!sshk)
return NULL;
return FROMFIELD(sshk, struct RSAKey, sshk);
}
void ssh_rsakex_freekey(struct RSAKey *key)
{
rsa2_freekey(&key->sshk);
}
int ssh_rsakex_klen(struct RSAKey *rsa)
{
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;
BinarySink *bs;
unsigned char hash[SSH2_KEX_MAX_HASH_LEN];
assert(h->hlen <= SSH2_KEX_MAX_HASH_LEN);
s = h->init();
bs = h->sink(s);
put_data(bs, seed, seedlen);
put_uint32(bs, count);
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, struct RSAKey *rsa)
{
Bignum b1, b2;
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, &ssh_sha1, NULL,
};
static const struct ssh_kex ssh_rsa_kex_sha256 = {
"rsa2048-sha256", NULL, KEXTYPE_RSA, &ssh_sha256, NULL,
};
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
};
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