GnuTLS | Behavioral PKCS#1 v1.5 decryption oracle (Ok/Err bit) (#1901)
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Issue created by Mark Esler: https://gitlab.com/gnutls/gnutls/-/work_item=
s/1901
Hello,
While surveying PKCS#1 v1.5 implementations [0] for the behavioral Bleich=
enbacher oracle [1] I found that GnuTLS exposes the oracle through its ca=
llable decrypt API.
`gnutls_privkey_decrypt_data` / `gnutls_privkey_decrypt_data2` returns `G=
NUTLS_E_DECRYPTION_FAILED` on a non-conforming block and succeeds otherwi=
se =E2=80=94 a distinguishable bit. `_data2` is documented constant-time =
but that is constant-time *explicit* rejection, not implicit rejection; t=
he behavioral bit remains. The oracle is closed inside the TLS key-exchan=
ge path (result discarded) but not for general callers (JOSE / CMS / PKCS=
#11 / direct decrypt). Runtime-confirmed, source-reviewed in `lib/privkey=
.c` and `lib/pk.c`.
The CFRG implementation guidance draft [2] covers remediation: OAEP as th=
e fix, implicit rejection (=C2=A77.2) as the stopgap if v1.5 must stay.
Mark Esler
[0] https://hexproof.dev/datagrams/bleichenbacher-oracle-survey/
[1] https://hexproof.dev/datagrams/ok-err-is-a-padding-oracle/
[2] https://datatracker.ietf.org/doc/draft-irtf-cfrg-rsa-guidance/
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Issue created by <a href=3D"https://gitlab.com/eslerm">Mark Esler</a>: <a=
href=3D"https://gitlab.com/gnutls/gnutls/-/work_items/1901">#1901</a>
</p>
<div class=3D"md" style=3D"position: relative; z-index: 1; color: #3a383f=
; word-wrap: break-word;">
<p dir=3D"auto" style=3D"color: #3a383f; margin: 0px 0px 1rem;" align=3D"=
initial">Hello,</p>
<p dir=3D"auto" style=3D"color: #3a383f; margin: 0px 0px 1rem;" align=3D"=
initial">While surveying PKCS#1 v1.5 implementations [0] for the behavior=
al Bleichenbacher oracle [1] I found that GnuTLS exposes the oracle throu=
gh its callable decrypt API.</p>
<p dir=3D"auto" style=3D"color: #3a383f; margin: 0px 0px 1rem;" align=3D"=
initial"><code style=3D'font-size: 90%; color: #18171d; word-wrap: break-=
word; background-color: #ececef; border-radius: .25rem; margin-top: 0px; =
font-weight: inherit; overflow-wrap: break-word; white-space: break-space=
s; font-family: "GitLab Mono", "JetBrains Mono", "Menlo", "DejaVu Sans Mo=
no", "Liberation Mono", "Consolas", "Ubuntu Mono", "Courier New", "andale=
mono", "lucida console", monospace; font-variant-ligatures: none; word-b=
reak: keep-all; padding: 0.125rem 0.25rem;'>gnutls_privkey_decrypt_data</=
code> / <code style=3D'font-size: 90%; color: #18171d; word-wrap: break-w=
ord; background-color: #ececef; border-radius: .25rem; font-weight: inher=
it; overflow-wrap: break-word; white-space: break-spaces; font-family: "G=
itLab Mono", "JetBrains Mono", "Menlo", "DejaVu Sans Mono", "Liberation M=
ono", "Consolas", "Ubuntu Mono", "Courier New", "andale mono", "lucida co=
nsole", monospace; font-variant-ligatures: none; word-break: keep-all; pa=
dding: 0.125rem 0.25rem;'>gnutls_privkey_decrypt_data2</code> returns <co=
de style=3D'font-size: 90%; color: #18171d; word-wrap: break-word; backgr=
ound-color: #ececef; border-radius: .25rem; font-weight: inherit; overflo=
w-wrap: break-word; white-space: break-spaces; font-family: "GitLab Mono"=
, "JetBrains Mono", "Menlo", "DejaVu Sans Mono", "Liberation Mono", "Cons=
olas", "Ubuntu Mono", "Courier New", "andale mono", "lucida console", mon=
ospace; font-variant-ligatures: none; word-break: keep-all; padding: 0.12=
5rem 0.25rem;'>GNUTLS_E_DECRYPTION_FAILED</code> on a non-conforming bloc=
k and succeeds otherwise =E2=80=94 a distinguishable bit. <code style=3D'=
font-size: 90%; color: #18171d; word-wrap: break-word; background-color: =
#ececef; border-radius: .25rem; font-weight: inherit; overflow-wrap: brea=
k-word; white-space: break-spaces; font-family: "GitLab Mono", "JetBrains=
Mono", "Menlo", "DejaVu Sans Mono", "Liberation Mono", "Consolas", "Ubun=
tu Mono", "Courier New", "andale mono", "lucida console", monospace; font=
-variant-ligatures: none; word-break: keep-all; padding: 0.125rem 0.25rem=
;'>_data2</code> is documented constant-time but that is constant-time <e=
m>explicit</em> rejection, not implicit rejection; the behavioral bit rem=
ains. The oracle is closed inside the TLS key-exchange path (result disca=
rded) but not for general callers (JOSE / CMS / PKCS#11 / direct decrypt)=
. Runtime-confirmed, source-reviewed in <code style=3D'font-size: 90%; co=
lor: #18171d; word-wrap: break-word; background-color: #ececef; border-ra=
dius: .25rem; font-weight: inherit; overflow-wrap: break-word; white-spac=
e: break-spaces; font-family: "GitLab Mono", "JetBrains Mono", "Menlo", "=
DejaVu Sans Mono", "Liberation Mono", "Consolas", "Ubuntu Mono", "Courier=
New", "andale mono", "lucida console", monospace; font-variant-ligatures=
: none; word-break: keep-all; padding: 0.125rem 0.25rem;'>lib/privkey.c</=
code> and <code style=3D'font-size: 90%; color: #18171d; word-wrap: break=
-word; background-color: #ececef; border-radius: .25rem; font-weight: inh=
erit; overflow-wrap: break-word; white-space: break-spaces; font-family: =
"GitLab Mono", "JetBrains Mono", "Menlo", "DejaVu Sans Mono", "Liberation=
Mono", "Consolas", "Ubuntu Mono", "Courier New", "andale mono", "lucida =
console", monospace; font-variant-ligatures: none; word-break: keep-all; =
padding: 0.125rem 0.25rem;'>lib/pk.c</code>.</p>
<p dir=3D"auto" style=3D"color: #3a383f; margin: 0px 0px 1rem;" align=3D"=
initial">The CFRG implementation guidance draft [2] covers remediation: O=
AEP as the fix, implicit rejection (=C2=A77.2) as the stopgap if v1.5 mus=
t stay.</p>
<p dir=3D"auto" style=3D"color: #3a383f; margin: 0px 0px 1rem;" align=3D"=
initial">Mark Esler</p>
<p dir=3D"auto" style=3D"color: #3a383f; margin: 0px;" align=3D"initial">=
[0] <a href=3D"https://hexproof.dev/datagrams/bleichenbacher-oracle-surve=
y/" rel=3D"nofollow noreferrer noopener" target=3D"_blank" style=3D"margi=
n-top: 0px;">https://hexproof.dev/datagrams/bleichenbacher-oracle-survey/=
</a>
[1] <a href=3D"https://hexproof.dev/datagrams/ok-err-is-a-padding-oracle/=
" rel=3D"nofollow noreferrer noopener" target=3D"_blank">https://hexproof=
.dev/datagrams/ok-err-is-a-padding-oracle/</a>
[2] <a href=3D"https://datatracker.ietf.org/doc/draft-irtf-cfrg-rsa-guida=
nce/" rel=3D"nofollow noreferrer noopener" target=3D"_blank">https://data=
tracker.ietf.org/doc/draft-irtf-cfrg-rsa-guidance/</a></p>
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