GnuTLS | srptool:possible stack buffer overflow with large SRP groups (#1777)

Mikhail created an issue: https://gitlab.com/gnutls/gnutls/-/issues/1777



Hello!

I used SAST tool Svace to analyze gnutls **3.8.11** and encountered a possible buffer overflow in function `_srp_crypt`. In particular, the static buffer `result` in _srp_crypt() is only 1024 bytes, while the 8192-bit SRP group code produces base64-encoded verifier of 1366 characters. 

I've checked it manually by creating tpasswd.conf file and then tried to create verifier with 8192-bit SRP group by specifying `--index=7`:

```
~$ srptool --passwd-conf ~/tpasswd.conf --passwd ~/tpasswd --username testuser --index=7
```

gdb showed this:

```
(gdb) break srptool.c:468
Breakpoint 1 at 0x2dce: file srptool.c, line 468.
(gdb) run --passwd-conf ~/tpasswd.conf --passwd ~/tpasswd --username testuser --index=7
Starting program: /usr/local/bin/srptool --passwd-conf ~/tpasswd.conf --passwd ~/tpasswd --username testuser --index=7

<...>

Enter password: 

Breakpoint 1, _srp_crypt (username=0x7fffffffe21e "testuser", 
    passwd=0x55555557f130 "admin", salt_size=16, g=0x7fffffffb050, 
    n=0x7fffffffb040) at srptool.c:468
468    sprintf(result, "%s:%s", txt_verifier.data, txt_salt.data);
(gdb) print result
$1 = '\000' <repeats 113 times>
(gdb) print txt_verifier.size
$2 = 1366
(gdb)
```

However, program doesn't crash in this case in my env. But I've noticed, that I cannot then verify such password:

```
~$ srptool --verify --passwd-conf ~/tpasswd.conf --passwd ~/tpasswd --username testuser
Enter password: 
Encoding error
```

At the current moment I didn't find precise cause of such behavior because I'm newbie to gnutls srptool source code but I think it may be connected with out-of-bounds write, explained above.. 

At least, for addressing buffer overflow we could, f.e:

- increase the static buffer size to 2048 bytes (sufficient for all
  currently supported groups),
- replace sprintf() with snprintf() to prevent overflow even if the
  buffer were accidentally too small.

But unfortunately it doesn't help with `Encoding error` -- error remains. I wanted to open MR at first but for now I don't know how to deal with `Encoding error`, so fix would be incomplete or even wrong. Probably, I may misunderstand something important here, so sorry if I'm mistaking.

Thank you upfront for your time and expertise!

-- 
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