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putty/sshrsa.c

968 строки
26 KiB
C
Исходник Ответственный История

/*
* RSA implementation for PuTTY.
*/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <assert.h>
#include "ssh.h"
#include "mpint.h"
#include "misc.h"
void BinarySource_get_rsa_ssh1_pub(
BinarySource *src, RSAKey *rsa, RsaSsh1Order order)
{
unsigned bits;
mp_int *e, *m;
bits = get_uint32(src);
if (order == RSA_SSH1_EXPONENT_FIRST) {
e = get_mp_ssh1(src);
m = get_mp_ssh1(src);
} else {
m = get_mp_ssh1(src);
e = get_mp_ssh1(src);
}
if (rsa) {
rsa->bits = bits;
rsa->exponent = e;
rsa->modulus = m;
rsa->bytes = (mp_get_nbits(m) + 7) / 8;
} else {
mp_free(e);
mp_free(m);
}
}
void BinarySource_get_rsa_ssh1_priv(
BinarySource *src, RSAKey *rsa)
{
rsa->private_exponent = get_mp_ssh1(src);
}
bool rsa_ssh1_encrypt(unsigned char *data, int length, RSAKey *key)
{
mp_int *b1, *b2;
int i;
unsigned char *p;
if (key->bytes < length + 4)
return false; /* RSA key too short! */
memmove(data + key->bytes - length, data, length);
data[0] = 0;
data[1] = 2;
size_t npad = key->bytes - length - 3;
/*
* Generate a sequence of nonzero padding bytes. We do this in a
* reasonably uniform way and without having to loop round
* retrying the random number generation, by first generating an
* integer in [0,2^n) for an appropriately large n; then we
* repeatedly multiply by 255 to give an integer in [0,255*2^n),
* extract the top 8 bits to give an integer in [0,255), and mask
* those bits off before multiplying up again for the next digit.
* This gives us a sequence of numbers in [0,255), and of course
* adding 1 to each of them gives numbers in [1,256) as we wanted.
*
* (You could imagine this being a sort of fixed-point operation:
* given a uniformly random binary _fraction_, multiplying it by k
* and subtracting off the integer part will yield you a sequence
* of integers each in [0,k). I'm just doing that scaled up by a
* power of 2 to avoid the fractions.)
*/
size_t random_bits = (npad + 16) * 8;
mp_int *randval = mp_new(random_bits + 8);
mp_int *tmp = mp_random_bits(random_bits);
mp_copy_into(randval, tmp);
mp_free(tmp);
for (i = 2; i < key->bytes - length - 1; i++) {
mp_mul_integer_into(randval, randval, 255);
uint8_t byte = mp_get_byte(randval, random_bits / 8);
assert(byte != 255);
data[i] = byte + 1;
mp_reduce_mod_2to(randval, random_bits);
}
mp_free(randval);
data[key->bytes - length - 1] = 0;
b1 = mp_from_bytes_be(make_ptrlen(data, key->bytes));
b2 = mp_modpow(b1, key->exponent, key->modulus);
p = data;
for (i = key->bytes; i--;) {
*p++ = mp_get_byte(b2, i);
}
mp_free(b1);
mp_free(b2);
return true;
}
/*
* 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.
*/
mp_int *crt_modpow(mp_int *base, mp_int *exp, mp_int *mod,
mp_int *p, mp_int *q, mp_int *iqmp)
{
mp_int *pm1, *qm1, *pexp, *qexp, *presult, *qresult;
mp_int *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 = mp_copy(p);
mp_sub_integer_into(pm1, pm1, 1);
qm1 = mp_copy(q);
mp_sub_integer_into(qm1, qm1, 1);
pexp = mp_mod(exp, pm1);
qexp = mp_mod(exp, qm1);
/*
* Do the two modpows.
*/
mp_int *base_mod_p = mp_mod(base, p);
presult = mp_modpow(base_mod_p, pexp, p);
mp_free(base_mod_p);
mp_int *base_mod_q = mp_mod(base, q);
qresult = mp_modpow(base_mod_q, qexp, q);
mp_free(base_mod_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 presult-qresult < 0, we add p to it to keep it positive.)
*/
unsigned presult_too_small = mp_cmp_hs(qresult, presult);
mp_cond_add_into(presult, presult, p, presult_too_small);
diff = mp_sub(presult, qresult);
multiplier = mp_mul(iqmp, q);
ret0 = mp_mul(multiplier, diff);
mp_add_into(ret0, ret0, qresult);
/*
* Finally, reduce the result mod n.
*/
ret = mp_mod(ret0, mod);
/*
* Free all the intermediate results before returning.
*/
mp_free(pm1);
mp_free(qm1);
mp_free(pexp);
mp_free(qexp);
mp_free(presult);
mp_free(qresult);
mp_free(diff);
mp_free(multiplier);
mp_free(ret0);
return ret;
}
/*
* Wrapper on crt_modpow that looks up all the right values from an
* RSAKey.
*/
static mp_int *rsa_privkey_op(mp_int *input, RSAKey *key)
{
return crt_modpow(input, key->private_exponent,
key->modulus, key->p, key->q, key->iqmp);
}
mp_int *rsa_ssh1_decrypt(mp_int *input, RSAKey *key)
{
return rsa_privkey_op(input, key);
}
bool rsa_ssh1_decrypt_pkcs1(mp_int *input, RSAKey *key,
strbuf *outbuf)
{
strbuf *data = strbuf_new();
bool success = false;
BinarySource src[1];
{
mp_int *b = rsa_ssh1_decrypt(input, key);
for (size_t i = (mp_get_nbits(key->modulus) + 7) / 8; i-- > 0 ;) {
put_byte(data, mp_get_byte(b, i));
}
mp_free(b);
}
BinarySource_BARE_INIT(src, data->u, data->len);
/* Check PKCS#1 formatting prefix */
if (get_byte(src) != 0) goto out;
if (get_byte(src) != 2) goto out;
while (1) {
unsigned char byte = get_byte(src);
if (get_err(src)) goto out;
if (byte == 0)
break;
}
/* Everything else is the payload */
success = true;
put_data(outbuf, get_ptr(src), get_avail(src));
out:
strbuf_free(data);
return success;
}
static void append_hex_to_strbuf(strbuf *sb, mp_int *x)
{
if (sb->len > 0)
put_byte(sb, ',');
put_data(sb, "0x", 2);
char *hex = mp_get_hex(x);
size_t hexlen = strlen(hex);
put_data(sb, hex, hexlen);
smemclr(hex, hexlen);
sfree(hex);
}
char *rsastr_fmt(RSAKey *key)
{
strbuf *sb = strbuf_new();
append_hex_to_strbuf(sb, key->exponent);
append_hex_to_strbuf(sb, key->modulus);
return strbuf_to_str(sb);
}
/*
* Generate a fingerprint string for the key. Compatible with the
* OpenSSH fingerprint code.
*/
char *rsa_ssh1_fingerprint(RSAKey *key)
{
unsigned char digest[16];
strbuf *out;
int i;
/*
* The hash preimage for SSH-1 key fingerprinting consists of the
* modulus and exponent _without_ any preceding length field -
* just the minimum number of bytes to represent each integer,
* stored big-endian, concatenated with no marker at the division
* between them.
*/
ssh_hash *hash = ssh_hash_new(&ssh_md5);
for (size_t i = (mp_get_nbits(key->modulus) + 7) / 8; i-- > 0 ;)
put_byte(hash, mp_get_byte(key->modulus, i));
for (size_t i = (mp_get_nbits(key->exponent) + 7) / 8; i-- > 0 ;)
put_byte(hash, mp_get_byte(key->exponent, i));
ssh_hash_final(hash, digest);
out = strbuf_new();
strbuf_catf(out, "%d ", mp_get_nbits(key->modulus));
for (i = 0; i < 16; i++)
strbuf_catf(out, "%s%02x", i ? ":" : "", digest[i]);
if (key->comment)
strbuf_catf(out, " %s", key->comment);
return strbuf_to_str(out);
}
/*
* 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.
*/
bool rsa_verify(RSAKey *key)
{
mp_int *n, *ed, *pm1, *qm1;
unsigned ok = 1;
/* Preliminary checks: p,q must actually be nonzero. */
if (mp_eq_integer(key->p, 0) | mp_eq_integer(key->q, 0))
return false;
/* n must equal pq. */
n = mp_mul(key->p, key->q);
ok &= mp_cmp_eq(n, key->modulus);
mp_free(n);
/* e * d must be congruent to 1, modulo (p-1) and modulo (q-1). */
pm1 = mp_copy(key->p);
mp_sub_integer_into(pm1, pm1, 1);
ed = mp_modmul(key->exponent, key->private_exponent, pm1);
mp_free(pm1);
ok &= mp_eq_integer(ed, 1);
mp_free(ed);
qm1 = mp_copy(key->q);
mp_sub_integer_into(qm1, qm1, 1);
ed = mp_modmul(key->exponent, key->private_exponent, qm1);
mp_free(qm1);
ok &= mp_eq_integer(ed, 1);
mp_free(ed);
/*
* 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.
*/
mp_int *p_new = mp_max(key->p, key->q);
mp_int *q_new = mp_min(key->p, key->q);
mp_free(key->p);
mp_free(key->q);
key->p = p_new;
key->q = q_new;
key->iqmp = mp_invert(key->q, key->p);
return ok;
}
void rsa_ssh1_public_blob(BinarySink *bs, RSAKey *key,
RsaSsh1Order order)
{
put_uint32(bs, mp_get_nbits(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 an SSH-1 public key blob, determine its length. */
int rsa_ssh1_public_blob_len(ptrlen data)
{
BinarySource src[1];
BinarySource_BARE_INIT(src, data.ptr, data.len);
/* Expect a length word, then exponent and modulus. (It doesn't
* even matter which order.) */
get_uint32(src);
mp_free(get_mp_ssh1(src));
mp_free(get_mp_ssh1(src));
if (get_err(src))
return -1;
/* Return the number of bytes consumed. */
return src->pos;
}
void freersapriv(RSAKey *key)
{
if (key->private_exponent) {
mp_free(key->private_exponent);
key->private_exponent = NULL;
}
if (key->p) {
mp_free(key->p);
key->p = NULL;
}
if (key->q) {
mp_free(key->q);
key->q = NULL;
}
if (key->iqmp) {
mp_free(key->iqmp);
key->iqmp = NULL;
}
}
void freersakey(RSAKey *key)
{
freersapriv(key);
if (key->modulus) {
mp_free(key->modulus);
key->modulus = NULL;
}
if (key->exponent) {
mp_free(key->exponent);
key->exponent = NULL;
}
if (key->comment) {
sfree(key->comment);
key->comment = NULL;
}
}
/* ----------------------------------------------------------------------
* Implementation of the ssh-rsa signing key type.
*/
static void rsa2_freekey(ssh_key *key); /* forward reference */
static ssh_key *rsa2_new_pub(const ssh_keyalg *self, ptrlen data)
{
BinarySource src[1];
RSAKey *rsa;
BinarySource_BARE_INIT(src, data.ptr, data.len);
if (!ptrlen_eq_string(get_string(src), "ssh-rsa"))
return NULL;
rsa = snew(RSAKey);
rsa->sshk.vt = &ssh_rsa;
rsa->exponent = get_mp_ssh2(src);
rsa->modulus = get_mp_ssh2(src);
rsa->private_exponent = NULL;
rsa->p = rsa->q = rsa->iqmp = NULL;
rsa->comment = NULL;
if (get_err(src)) {
rsa2_freekey(&rsa->sshk);
return NULL;
}
return &rsa->sshk;
}
static void rsa2_freekey(ssh_key *key)
{
RSAKey *rsa = container_of(key, RSAKey, sshk);
freersakey(rsa);
sfree(rsa);
}
static char *rsa2_cache_str(ssh_key *key)
{
RSAKey *rsa = container_of(key, RSAKey, sshk);
return rsastr_fmt(rsa);
}
static void rsa2_public_blob(ssh_key *key, BinarySink *bs)
{
RSAKey *rsa = container_of(key, 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)
{
RSAKey *rsa = container_of(key, 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_new_priv(const ssh_keyalg *self,
ptrlen pub, ptrlen priv)
{
BinarySource src[1];
ssh_key *sshk;
RSAKey *rsa;
sshk = rsa2_new_pub(self, pub);
if (!sshk)
return NULL;
rsa = container_of(sshk, RSAKey, sshk);
BinarySource_BARE_INIT(src, priv.ptr, priv.len);
rsa->private_exponent = get_mp_ssh2(src);
rsa->p = get_mp_ssh2(src);
rsa->q = get_mp_ssh2(src);
rsa->iqmp = get_mp_ssh2(src);
if (get_err(src) || !rsa_verify(rsa)) {
rsa2_freekey(&rsa->sshk);
return NULL;
}
return &rsa->sshk;
}
static ssh_key *rsa2_new_priv_openssh(const ssh_keyalg *self,
BinarySource *src)
{
RSAKey *rsa;
rsa = snew(RSAKey);
rsa->sshk.vt = &ssh_rsa;
rsa->comment = NULL;
rsa->modulus = get_mp_ssh2(src);
rsa->exponent = get_mp_ssh2(src);
rsa->private_exponent = get_mp_ssh2(src);
rsa->iqmp = get_mp_ssh2(src);
rsa->p = get_mp_ssh2(src);
rsa->q = get_mp_ssh2(src);
if (get_err(src) || !rsa_verify(rsa)) {
rsa2_freekey(&rsa->sshk);
return NULL;
}
return &rsa->sshk;
}
static void rsa2_openssh_blob(ssh_key *key, BinarySink *bs)
{
RSAKey *rsa = container_of(key, 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, ptrlen pub)
{
ssh_key *sshk;
RSAKey *rsa;
int ret;
sshk = rsa2_new_pub(self, pub);
if (!sshk)
return -1;
rsa = container_of(sshk, RSAKey, sshk);
ret = mp_get_nbits(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 sha1_asn1_prefix[] = {
0x00, 0x30, 0x21, 0x30, 0x09, 0x06, 0x05, 0x2B,
0x0E, 0x03, 0x02, 0x1A, 0x05, 0x00, 0x04, 0x14,
};
/*
* Two more similar pieces of ASN.1 used for signatures using SHA-256
* and SHA-512, in the same format but differing only in various
* length fields and OID.
*/
static const unsigned char sha256_asn1_prefix[] = {
0x00, 0x30, 0x31, 0x30, 0x0d, 0x06, 0x09, 0x60,
0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x01,
0x05, 0x00, 0x04, 0x20,
};
static const unsigned char sha512_asn1_prefix[] = {
0x00, 0x30, 0x51, 0x30, 0x0d, 0x06, 0x09, 0x60,
0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x03,
0x05, 0x00, 0x04, 0x40,
};
#define SHA1_ASN1_PREFIX_LEN sizeof(sha1_asn1_prefix)
static unsigned char *rsa_pkcs1_signature_string(
size_t nbytes, const ssh_hashalg *halg, ptrlen data)
{
const unsigned char *asn1_prefix;
unsigned asn1_prefix_size;
if (halg == &ssh_sha256) {
asn1_prefix = sha256_asn1_prefix;
asn1_prefix_size = sizeof(sha256_asn1_prefix);
} else if (halg == &ssh_sha512) {
asn1_prefix = sha512_asn1_prefix;
asn1_prefix_size = sizeof(sha512_asn1_prefix);
} else {
assert(halg == &ssh_sha1);
asn1_prefix = sha1_asn1_prefix;
asn1_prefix_size = sizeof(sha1_asn1_prefix);
}
size_t fixed_parts = halg->hlen + asn1_prefix_size + 2;
assert(nbytes >= fixed_parts);
size_t padding = nbytes - fixed_parts;
unsigned char *bytes = snewn(nbytes, unsigned char);
bytes[0] = 0;
bytes[1] = 1;
memset(bytes + 2, 0xFF, padding);
memcpy(bytes + 2 + padding, asn1_prefix, asn1_prefix_size);
ssh_hash *h = ssh_hash_new(halg);
put_datapl(h, data);
ssh_hash_final(h, bytes + 2 + padding + asn1_prefix_size);
return bytes;
}
static bool rsa2_verify(ssh_key *key, ptrlen sig, ptrlen data)
{
RSAKey *rsa = container_of(key, RSAKey, sshk);
BinarySource src[1];
ptrlen type, in_pl;
mp_int *in, *out;
BinarySource_BARE_INIT(src, sig.ptr, sig.len);
type = get_string(src);
/*
* RFC 4253 section 6.6: the signature integer in an ssh-rsa
* signature is 'without lengths or padding'. That is, we _don't_
* expect the usual leading zero byte if the topmost bit of the
* first byte is set. (However, because of the possibility of
* BUG_SSH2_RSA_PADDING at the other end, we tolerate it if it's
* there.) So we can't use get_mp_ssh2, which enforces that
* leading-byte scheme; instead we use get_string and
* mp_from_bytes_be, which will tolerate anything.
*/
in_pl = get_string(src);
if (get_err(src) || !ptrlen_eq_string(type, "ssh-rsa"))
return false;
in = mp_from_bytes_be(in_pl);
out = mp_modpow(in, rsa->exponent, rsa->modulus);
mp_free(in);
unsigned diff = 0;
size_t nbytes = (mp_get_nbits(rsa->modulus) + 7) / 8;
unsigned char *bytes = rsa_pkcs1_signature_string(nbytes, &ssh_sha1, data);
for (size_t i = 0; i < nbytes; i++)
diff |= bytes[nbytes-1 - i] ^ mp_get_byte(out, i);
smemclr(bytes, nbytes);
sfree(bytes);
mp_free(out);
return diff == 0;
}
static void rsa2_sign(ssh_key *key, ptrlen data,
unsigned flags, BinarySink *bs)
{
RSAKey *rsa = container_of(key, RSAKey, sshk);
unsigned char *bytes;
size_t nbytes;
mp_int *in, *out;
const ssh_hashalg *halg;
const char *sign_alg_name;
if (flags & SSH_AGENT_RSA_SHA2_256) {
halg = &ssh_sha256;
sign_alg_name = "rsa-sha2-256";
} else if (flags & SSH_AGENT_RSA_SHA2_512) {
halg = &ssh_sha512;
sign_alg_name = "rsa-sha2-512";
} else {
halg = &ssh_sha1;
sign_alg_name = "ssh-rsa";
}
nbytes = (mp_get_nbits(rsa->modulus) + 7) / 8;
bytes = rsa_pkcs1_signature_string(nbytes, halg, data);
in = mp_from_bytes_be(make_ptrlen(bytes, nbytes));
smemclr(bytes, nbytes);
sfree(bytes);
out = rsa_privkey_op(in, rsa);
mp_free(in);
put_stringz(bs, sign_alg_name);
nbytes = (mp_get_nbits(out) + 7) / 8;
put_uint32(bs, nbytes);
for (size_t i = 0; i < nbytes; i++)
put_byte(bs, mp_get_byte(out, nbytes - 1 - i));
mp_free(out);
}
const ssh_keyalg ssh_rsa = {
rsa2_new_pub,
rsa2_new_priv,
rsa2_new_priv_openssh,
rsa2_freekey,
rsa2_sign,
rsa2_verify,
rsa2_public_blob,
rsa2_private_blob,
rsa2_openssh_blob,
rsa2_cache_str,
rsa2_pubkey_bits,
"ssh-rsa",
"rsa2",
NULL,
SSH_AGENT_RSA_SHA2_256 | SSH_AGENT_RSA_SHA2_512,
};
RSAKey *ssh_rsakex_newkey(ptrlen data)
{
ssh_key *sshk = rsa2_new_pub(&ssh_rsa, data);
if (!sshk)
return NULL;
return container_of(sshk, RSAKey, sshk);
}
void ssh_rsakex_freekey(RSAKey *key)
{
rsa2_freekey(&key->sshk);
}
int ssh_rsakex_klen(RSAKey *rsa)
{
return mp_get_nbits(rsa->modulus);
}
static void oaep_mask(const ssh_hashalg *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);
ssh_hash *s;
unsigned char hash[MAX_HASH_LEN];
assert(h->hlen <= MAX_HASH_LEN);
s = ssh_hash_new(h);
put_data(s, seed, seedlen);
put_uint32(s, count);
ssh_hash_final(s, hash);
count++;
for (i = 0; i < max; i++)
data[i] ^= hash[i];
data += max;
datalen -= max;
}
}
strbuf *ssh_rsakex_encrypt(RSAKey *rsa, const ssh_hashalg *h, ptrlen in)
{
mp_int *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 + mp_get_nbits(rsa->modulus)) / 8;
/* The length of the input data must be at most k - 2hLen - 2. */
assert(in.len > 0 && in.len <= k - 2*HLEN - 2);
/* The length of the output data wants to be precisely k. */
strbuf *toret = strbuf_new();
int outlen = k;
unsigned char *out = strbuf_append(toret, outlen);
/*
* 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. */
random_read(out + 1, HLEN);
/* At position 1+HLEN, the data block DB, consisting of: */
/* The hash of the label (we only support an empty label here) */
{
ssh_hash *s = ssh_hash_new(h);
ssh_hash_final(s, 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 - in.len - 1] = 1;
memcpy(out + outlen - in.len, in.ptr, in.len);
/*
* 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 = mp_from_bytes_be(make_ptrlen(out, outlen));
b2 = mp_modpow(b1, rsa->exponent, rsa->modulus);
p = (char *)out;
for (i = outlen; i--;) {
*p++ = mp_get_byte(b2, i);
}
mp_free(b1);
mp_free(b2);
/*
* And we're done.
*/
return toret;
}
mp_int *ssh_rsakex_decrypt(
RSAKey *rsa, const ssh_hashalg *h, ptrlen ciphertext)
{
mp_int *b1, *b2;
int outlen, i;
unsigned char *out;
unsigned char labelhash[64];
ssh_hash *hash;
BinarySource src[1];
const int HLEN = h->hlen;
/*
* Decryption side of the RSA key exchange operation.
*/
/* The length of the encrypted data should be exactly the length
* in octets of the RSA modulus.. */
outlen = (7 + mp_get_nbits(rsa->modulus)) / 8;
if (ciphertext.len != outlen)
return NULL;
/* Do the RSA decryption, and extract the result into a byte array. */
b1 = mp_from_bytes_be(ciphertext);
b2 = rsa_privkey_op(b1, rsa);
out = snewn(outlen, unsigned char);
for (i = 0; i < outlen; i++)
out[i] = mp_get_byte(b2, outlen-1-i);
mp_free(b1);
mp_free(b2);
/* Do the OAEP masking operations, in the reverse order from encryption */
oaep_mask(h, out+HLEN+1, outlen-HLEN-1, out+1, HLEN);
oaep_mask(h, out+1, HLEN, out+HLEN+1, outlen-HLEN-1);
/* Check the leading byte is zero. */
if (out[0] != 0) {
sfree(out);
return NULL;
}
/* Check the label hash at position 1+HLEN */
assert(HLEN <= lenof(labelhash));
hash = ssh_hash_new(h);
ssh_hash_final(hash, labelhash);
if (memcmp(out + HLEN + 1, labelhash, HLEN)) {
sfree(out);
return NULL;
}
/* Expect zero bytes followed by a 1 byte */
for (i = 1 + 2 * HLEN; i < outlen; i++) {
if (out[i] == 1) {
i++; /* skip over the 1 byte */
break;
} else if (out[i] != 1) {
sfree(out);
return NULL;
}
}
/* And what's left is the input message data, which should be
* encoded as an ordinary SSH-2 mpint. */
BinarySource_BARE_INIT(src, out + i, outlen - i);
b1 = get_mp_ssh2(src);
sfree(out);
if (get_err(src) || get_avail(src) != 0) {
mp_free(b1);
return NULL;
}
/* Success! */
return b1;
}
static const struct ssh_rsa_kex_extra ssh_rsa_kex_extra_sha1 = { 1024 };
static const struct ssh_rsa_kex_extra ssh_rsa_kex_extra_sha256 = { 2048 };
static const ssh_kex ssh_rsa_kex_sha1 = {
"rsa1024-sha1", NULL, KEXTYPE_RSA,
&ssh_sha1, &ssh_rsa_kex_extra_sha1,
};
static const ssh_kex ssh_rsa_kex_sha256 = {
"rsa2048-sha256", NULL, KEXTYPE_RSA,
&ssh_sha256, &ssh_rsa_kex_extra_sha256,
};
static const ssh_kex *const rsa_kex_list[] = {
&ssh_rsa_kex_sha256,
&ssh_rsa_kex_sha1
};
const ssh_kexes ssh_rsa_kex = { lenof(rsa_kex_list), rsa_kex_list };
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