How to configure Dovecot to disable NIST's curves and still rertain EECDH?
I am interested in configuring Dovecot's TLS so as to retain forward secrecy, but eliminate all of NIST's elliptic curves.
Besides being subject to side channel attacks [1], in some quarters there is a general distrust of NIST's curves and any of their other cryptographic primitives after the Dual EC DRBG debacle.
From what I can tell, the following will prevent the use of NIST's curves (along with other dangerous primitives) in Dovecot, but this is accomplished by simply disabling EECDH entirely.
ssl_cipher_list = HIGH:!DSS:!EECDH:!ECDH:!SHA1:!aNULL:!eNULL:@STRENGTH
This should still retain forward secrecy through the use of EDH, but this doesn't leave much in the way of allowable algorithms on my server:
$ openssl ciphers -V 'HIGH:!DSS:!EECDH:!ECDH:!SHA1:!aNULL:!eNULL:@STRENGTH' 0x00,0x9F - DHE-RSA-AES256-GCM-SHA384 TLSv1.2 Kx=DH Au=RSA Enc=AESGCM(256) Mac=AEAD 0x00,0x6B - DHE-RSA-AES256-SHA256 TLSv1.2 Kx=DH Au=RSA Enc=AES(256) Mac=SHA256 0x00,0x9D - AES256-GCM-SHA384 TLSv1.2 Kx=RSA Au=RSA Enc=AESGCM(256) Mac=AEAD 0x00,0x3D - AES256-SHA256 TLSv1.2 Kx=RSA Au=RSA Enc=AES(256) Mac=SHA256 0x00,0x9E - DHE-RSA-AES128-GCM-SHA256 TLSv1.2 Kx=DH Au=RSA Enc=AESGCM(128) Mac=AEAD 0x00,0x67 - DHE-RSA-AES128-SHA256 TLSv1.2 Kx=DH Au=RSA Enc=AES(128) Mac=SHA256 0x00,0x9C - AES128-GCM-SHA256 TLSv1.2 Kx=RSA Au=RSA Enc=AESGCM(128) Mac=AEAD 0x00,0x3C - AES128-SHA256 TLSv1.2 Kx=RSA Au=RSA Enc=AES(128) Mac=SHA256
Is there a better way to do this? Is there a way to disable only the suspect NIST curves and still retain EECDH but with side-channel safe curves like X25519?
Thanks,
Kurt Fitzner Links:
Am 19.12.18 um 04:39 schrieb Kurt Fitzner:
I am interested in configuring Dovecot's TLS so as to retain forward secrecy, but eliminate all of NIST's elliptic curves.
Besides being subject to side channel attacks https://blog.cr.yp.to/20140323-ecdsa.html, in some quarters there is a general distrust of NIST's curves and any of their other cryptographic primitives after the Dual EC DRBG debacle.
From what I can tell, the following will prevent the use of NIST's curves (along with other dangerous primitives) in Dovecot, but this is accomplished by simply disabling EECDH entirely.
|ssl_cipher_list = HIGH:!DSS:!EECDH:!ECDH:!SHA1:!aNULL:!eNULL:@STRENGTH |
This should still retain forward secrecy through the use of EDH, but this doesn't leave much in the way of allowable algorithms on my server:
|$ openssl ciphers -V 'HIGH:!DSS:!EECDH:!ECDH:!SHA1:!aNULL:!eNULL:@STRENGTH' 0x00,0x9F - DHE-RSA-AES256-GCM-SHA384 TLSv1.2 Kx=DH Au=RSA Enc=AESGCM(256) Mac=AEAD 0x00,0x6B - DHE-RSA-AES256-SHA256 TLSv1.2 Kx=DH Au=RSA Enc=AES(256) Mac=SHA256 0x00,0x9D - AES256-GCM-SHA384 TLSv1.2 Kx=RSA Au=RSA Enc=AESGCM(256) Mac=AEAD 0x00,0x3D - AES256-SHA256 TLSv1.2 Kx=RSA Au=RSA Enc=AES(256) Mac=SHA256 0x00,0x9E - DHE-RSA-AES128-GCM-SHA256 TLSv1.2 Kx=DH Au=RSA Enc=AESGCM(128) Mac=AEAD 0x00,0x67 - DHE-RSA-AES128-SHA256 TLSv1.2 Kx=DH Au=RSA Enc=AES(128) Mac=SHA256 0x00,0x9C - AES128-GCM-SHA256 TLSv1.2 Kx=RSA Au=RSA Enc=AESGCM(128) Mac=AEAD 0x00,0x3C - AES128-SHA256 TLSv1.2 Kx=RSA Au=RSA Enc=AES(128) Mac=SHA256 |
Is there a better way to do this? Is there a way to disable only the suspect NIST curves and still retain EECDH but with side-channel safe curves like X25519?
Thanks,
Kurt Fitzner
Do you really plan to do this? RFC 8446 section 9.1: A TLS-compliant application MUST support key exchange with secp256r1 (NIST P-256) and SHOULD support key exchange with X25519
I think your idea could be not future proved.
Beside that, how many mail-clients will remain usable with this cipher selection?
Torsten
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Kurt Fitzner
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Tributh