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Revision as of 12:56, 4 January 2018 by Nmav (talk | contribs) (added Category:ChangeReadyForWrangler)

Strong crypto settings


This change is about updating the current system-wide crypto policy to disable legacy and unused cryptographic protocols.


Current status

  • Targeted release: Fedora 28
  • Last updated: 2018-01-04
  • Tracker bug: <will be assigned by the Wrangler>

Detailed Description

Fedora includes several cryptographic components who's security doesn't remain constant over time. Algorithms such as (cryptographic) hashing and encryption typically have a lifetime after which they are considered either too risky to use or plain insecure. That would mean we need to phase out such algorithms from the default settings, or completely disable if they could cause irreparable issue.

While in the past we did not disable algorithms in a consistent way (different applications utilized different policies), today we have a system-wide policy followed by a large part of Fedora components. That allows us to move consistently and deprecate algorithms system-wide. For rationale see RFC 7457 for a more complete list of attacks taking advantage of legacy crypto algorithms.

The propose changes for default policy are:

* Keep only TLS 1.2 (and TLS 1.3 when available) as enabled protocols and move the TLS 1.x, x<=1 to legacy level.
* Require finite field parameters (RSA, Diffie-Hellman) of 2048 and more in the default settings

That is a policy of:

 MACs: All HMAC with SHA1 or better + all modern MACs (poly1305 etc)
 Curves: all prime >= 255 bits (including bernstein curves)
 Signature algorithms: SHA-1 hash or better (not RIPEMD)
 Ciphers: all available > 112-bit key, >= 128-bit block (no rc4, but with 3DES)
 key exchange: ECDHE, RSA, DHE
 DH params size: >=1024
 RSA params size: >=1024
 TLS protocols: TLS >= 1.0
 MACs: All HMAC with SHA1 or better + all modern MACs (poly1305 etc)
 Curves: all prime >= 255 bits (including bernstein curves)
 Signature algorithms: with SHA-1 hash or better
 Ciphers: >= 128-bit key, >= 128-bit block (aes, camellia, chacha20, including aes-cbc)
 key exchange: ECDHE, RSA, DHE
 DH params size: >= 2048
 RSA params size: >= 2048
 TLS protocols: TLS >= 1.2
 MACs: All HMAC with SHA256 or better + all modern MACs (poly1305 etc)
 Curves: all prime >= 384 bits (including bernstein curves)
 Signature algorithms: SHA-384 hash or better
 Ciphers: >= 256-bit key, >= 128-bit block, only Authenticated Encryption (AE) ciphers
 key exchange: ECDHE, DHE
 DH params size: >= 3072
 RSA params size: >= 3072
 TLS protocols: TLS >= 1.2

Benefit to Fedora

With this change we protect users from relying on enabled-by-default weak cryptography, as well as reduce our maintenance cost for future attacks that rely on weak crypto for exploitation.


  • Proposal owners:

The policies include in crypto-policies package need to be updated.

  • Other developers:
* Crypto policies are updated to the settings above
* OpenSSL is updated to allow setting policies for TLS versions
* Crypto policies are updated to the settings above
* OpenSSL, NSS, GnuTLS and all applications covered under the Fedora Crypto Policies follow the new crypto settings.
  • Policies and guidelines:

No changes to packaging or other guidelines is needed.

  • Trademark approval: N/A (not needed for this Change)

Upgrade/compatibility impact

It may be that the new settings break software that connects to servers which utilize weak algorithms. Compatibility can be obtained by switching the system to legacy mode.

How To Test

Applications which follow the system-wide policy (e.g., curl,wget) should be tested:

* whether they can connect to legacy (TLS1.0, TLS1.1) servers when system is in legacy mode
* whether the previous connection breaks when system is in default mode
* whether the system can connect to TLS 1.2 servers when in default, legacy or future mode.

User Experience

Given the existing deployment of TLS 1.2 on the internet, there should not be significant user experience degradation, although that's a speculation.


* nss
* gnutls
* openssl
* crypto-policies

Contingency Plan

  • Contingency mechanism: (What to do? Who will do it?)

If we notice significant user experience degradation, e.g., due to many custom servers utilizing legacy protocols, we should consider postponing that change. The change owner will take care of this.

  • Contingency deadline: beta freeze
  • Blocks release? No



Release Notes