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Kurt Fitzner
Asked: 2018-12-17 14:40:08 +0800 CST

How to configure Dovecot to disable NIST's curves and still rertain EECDH?

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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, in some quarters there is a general distrust of NIST's curves and other cryptographic primitives after the Dual EC DRBG debacle.

From what I can tell, the following will prevent the use of NIST's curves (and some 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:

$ 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?

ssl

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