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pki_crypto: pad RSA signature blobs

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Pad RSA signature blobs to the expected RSA signature length
when processing via 'pki_signature_to_blob'.

Some clients, notably PuTTY, may send unpadded RSA signatures
during the public key exchange: before this change, one can
sometimes observe failure in signature validation when using
PuTTY's 'plink' client, along these lines:

   ssh_packet_process: ssh_packet_process: Dispatching handler for packet type 50
   ssh_packet_userauth_request: ssh_packet_userauth_request: Auth request for service ssh-connection, method publickey for user 'foo'
   ssh_pki_signature_verify_blob: ssh_pki_signature_verify_blob: Going to verify a ssh-rsa type signature
   pki_signature_verify: pki_signature_verify: RSA error: error:04091077:rsa routines:INT_RSA_VERIFY:wrong signature length
   ssh_packet_userauth_request: ssh_packet_userauth_request: Received an invalid  signature from peer

For cross-reference this issue once also existed between
PuTTY and OpenSSH:

  http://www.chiark.greenend.org.uk/~sgtatham/putty/wishlist/rsa-verify-failed.html

  http://www.openbsd.org/cgi-bin/cvsweb/src/usr.bin/ssh/ssh-rsa.c?rev=1.19;content-type=text%2Fx-cvsweb-markup

With the fix I am unable to reproduce the above failure mode when
testing with 'plink'.

Reviewed-by: Andreas Schneider <asn@cryptomilk.org>
This commit is contained in:
Jon Simons
2014-01-19 16:37:57 -08:00
committed by Andreas Schneider
parent 9fff70fa41
commit 465816f4a0
1 changed files with 56 additions and 24 deletions
Download Patch File Download Diff File Expand all files Collapse all files

80
src/pki_crypto.c
View File

@@ -1188,6 +1188,61 @@ ssh_string pki_signature_to_blob(const ssh_signature sig)
return sig_blob;
}
static ssh_signature pki_signature_from_rsa_blob(const ssh_key pubkey,
const ssh_string sig_blob,
ssh_signature sig)
{
uint32_t pad_len = 0;
char *blob_orig;
char *blob_padded_data;
ssh_string sig_blob_padded;
size_t len = ssh_string_len(sig_blob);
size_t rsalen= RSA_size(pubkey->rsa);
if (len > rsalen) {
ssh_pki_log("Signature is too big: %lu > %lu",
(unsigned long)len, (unsigned long)rsalen);
goto errout;
}
#ifdef DEBUG_CRYPTO
ssh_pki_log("RSA signature len: %lu", (unsigned long)len);
ssh_print_hexa("RSA signature", ssh_string_data(sig_blob), len);
#endif
if (len == rsalen) {
sig->rsa_sig = ssh_string_copy(sig_blob);
} else {
/* pad the blob to the expected rsalen size */
ssh_pki_log("RSA signature len %lu < %lu",
(unsigned long)len, (unsigned long)rsalen);
pad_len = rsalen - len;
sig_blob_padded = ssh_string_new(rsalen);
if (sig_blob_padded == NULL) {
goto errout;
}
blob_padded_data = (char *) ssh_string_data(sig_blob_padded);
blob_orig = (char *) ssh_string_data(sig_blob);
/* front-pad the buffer with zeroes */
BURN_BUFFER(blob_padded_data, pad_len);
/* fill the rest with the actual signature blob */
memcpy(blob_padded_data + pad_len, blob_orig, len);
sig->rsa_sig = sig_blob_padded;
}
return sig;
errout:
ssh_signature_free(sig);
return NULL;
}
ssh_signature pki_signature_from_blob(const ssh_key pubkey,
const ssh_string sig_blob,
enum ssh_keytypes_e type)
@@ -1196,7 +1251,6 @@ ssh_signature pki_signature_from_blob(const ssh_key pubkey,
ssh_string r;
ssh_string s;
size_t len;
size_t rsalen;
sig = ssh_signature_new();
if (sig == NULL) {
@@ -1260,29 +1314,7 @@ ssh_signature pki_signature_from_blob(const ssh_key pubkey,
break;
case SSH_KEYTYPE_RSA:
case SSH_KEYTYPE_RSA1:
rsalen = RSA_size(pubkey->rsa);
if (len > rsalen) {
ssh_pki_log("Signature is to big size: %lu",
(unsigned long)len);
ssh_signature_free(sig);
return NULL;
}
if (len < rsalen) {
ssh_pki_log("RSA signature len %lu < %lu",
(unsigned long)len, (unsigned long)rsalen);
}
#ifdef DEBUG_CRYPTO
ssh_pki_log("RSA signature len: %lu", (unsigned long)len);
ssh_print_hexa("RSA signature", ssh_string_data(sig_blob), len);
#endif
sig->rsa_sig = ssh_string_copy(sig_blob);
if (sig->rsa_sig == NULL) {
ssh_signature_free(sig);
return NULL;
}
sig = pki_signature_from_rsa_blob(pubkey, sig_blob, sig);
break;
case SSH_KEYTYPE_ECDSA:
#ifdef HAVE_OPENSSL_ECC