Changes/StrongCryptoSettings3Forewarning1

Strong crypto settings: phase 3, forewarning 1/3

Summary

TL;DR: Cryptographic policies will be tightened in Fedora 38-39, SHA-1 signatures will no longer be trusted by default. This is a forewarning filed for extra visibility. Test your setup with FUTURE today and file bugs so you won't get bit by F38-39.

As the next installment of our periodic tightening of cryptographic defaults, we're intending to bringing several changes to Fedora 39. The impact of them, notably distrusting SHA-1 signatures, might be so profound we're smoothing the rollout in time to give developers and maintainers ample time to react:

Fedora 36 StrongCryptoSettings3Forewarning1:

• SHA-1 signatures are distrusted in FUTURE policy (opt-in)
• TEST-FEDORA39 policy is provided

Fedora 37 StrongCryptoSettings3Forewarning2:

• creating and verifying SHA-1 signatures is logged to ease reporting bugs

Fedora 38 StrongCryptoSettings3Forewarning3:

• policies are updated, most notably
• SHA-1 signatures are distrusted in DEFAULT policy
• changes are reverted in branched f38 in time for Beta and do not reach users

Fedora 39 StrongCryptoSettings3:

• changes reach users

The plan is subject to change if it goes sideways somewhere along the way.

Owner

• Name: Alexander Sosedkin
• Email: asosedki@redhat.com

Current status

• Targeted release: [[Releases/<number> | Fedora Linux <number> ]]
• Last updated: 2022-04-27
• FESCo issue: <will be assigned by the Wrangler>
• Tracker bug: <will be assigned by the Wrangler>
• Release notes tracker: <will be assigned by the Wrangler>

Detailed Description

Secure defaults are an evermoving target. Fedora 28 had StrongCryptoSettings. Fedora 33 had StrongCryptoSettings2. Fedora 39 should have StrongCryptoSettings3.

By Fedora 39, the policies will be, in TLS perspective:

LEGACY
 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 (no DSA)
 Ciphers: all available > 112-bit key, >= 128-bit block (no RC4 or 3DES)
 Key exchange: ECDHE, RSA, DHE (no DHE-DSS)
 DH params size: >=2048
 RSA params size: >=2048
 TLS protocols: TLS >= 1.2

DEFAULT
 MACs: All HMAC with SHA1 or better + all modern MACs (Poly1305 etc.)
 Curves: all prime >= 255 bits (including Bernstein curves)
 Signature algorithms: with SHA-224 hash or better (not DSA)
 Ciphers: >= 128-bit key, >= 128-bit block (AES, ChaCha20, including AES-CBC)
 Key exchange: ECDHE, RSA, DHE (no DHE-DSS)
 DH params size: >= 2048
 RSA params size: >= 2048
 TLS protocols: TLS >= 1.2

FUTURE
 MACs: All HMAC with SHA256 or better + all modern MACs (Poly1305 etc.)
 Curves: all prime >= 255 bits (including Bernstein curves)
 Signature algorithms: SHA-256 hash or better (not DSA)
 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

The flagship change this time will be distrusting SHA-1 signatures on the cryptographic library level, affecting more than just TLS.

OpenSSL will start blocking signature creation and verification by default, with the fallout anticipated to be wide enough for us to roll out the change across multiple cycles with multiple forewarnings. In Fedora 36, 37 and 38 released distrusting SHA-1 signatures will be opt-in. In Fedora 38 rawhide and Fedora 39 distrusting SHA-1 signatures will happen by default.

Feedback

A discussion has been raised on fedora-devel, a summary is available on LWN.

A change has the potential to prove disruptive and controversial, with much effort being focused on stretching it out in time.

There seems to be a consensus that the change has to be done eventually, but the ideal means of implementing it are in no way clear. The decision to discover code reliant on SHA-1 signatures by blocking creation/verification has not gathered many fans, but not many alternative proposals have been raised in return. A notable one, making the library somehow log the offending operations, has been incorporated in the proposal, though the effectiveness of it is yet to be seen in practice. Another notable takeaway point is the need to call for testing, which would be done in form of writing four Fedora Changes and testing SHA-1 signature distrusting during Fedora 37 & 38 Test Days. The change owner doesn't see the plan as an ideal one and continues to be open for feedback.

Benefit to Fedora

Fedora 39 will ship with more secure defaults to better match the everchanging landscape of cryptographic practices. TLS 1.0 / 1.1 protocol version will be disabled as they're [deprecated https://datatracker.ietf.org/doc/rfc8996], minimum key sizes will be raised to keep up with the computational advances etc.

Distrusting SHA-1 signatures specifically is expected to trigger a topical distribution-wide crackdown on weak cryptography, raising the security of the distribution moving forward.

Scope

• Proposal owners: implement changes described in Summary and Dependencies sections

• Other developers:

Test your applications with FUTURE policy. Move away from trusting SHA-1 signatures; ideally in time for F38 branch-off, for F39 release at the latest.

Follow SHA1SignaturesGuidance: 1. move away from trusting SHA-1 signatures entirely, or 2. distrust them by default and require explicit user opt-in to use a workaround

• Release engineering: #Releng issue number

Not sure if mass-rebuild is required if we land the change right after f38 branch-off. Maybe a "preview" mass-rebuild can be done with a special build in the F37 timeframe to cut down on F38 FTBFS.

• Policies and guidelines: update needed in time for F38

CryptoPolicies section of the packaging guidelines will have to be updated to reflect that SHA-1 signatures must not be trusted by default and provide guidance for openssl and gnutls. Components using workaround APIs must not use them without explicit user opt-in and must be added to a list of applications using a workaround API.

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

• Alignment with Objectives: not with F36-era ones

Upgrade/compatibility impact

Nothing will change for Fedora 36 by default, the change is opt-in for now.

How To Test

On a Fedora 36 system, install crypto-policies-scripts package and switch to a more restrictive policy with either update-crypto-policies --set FUTURE or update-crypto-policies --set TEST-FEDORA39.

Proceed to use the system as usual, identify the workflows which are broken by this change.

Verify that the broken functionality works again if you the policy is relaxed back with update-crypto-policies --set FUTURE:SHA-1, file bug reports against the affected components if not filed already. Please start your ticket title with StrongCryptoSettings3: , mention this change page, the version of crypto-policies package and the policies under which your workflow does and does not work.

User Experience

Things will break. All kinds of things depending on SHA-1 signatures, openly and secretly.

• On Fedora 36 they'll break opt-in.
• On Fedora 37 they'll break opt-in, but it'll be possible to log them instead.
• On Fedora 38 rawhide they'll break by default.
• On Fedora 38 released they'll behave like in Fedora 37.
• On Fedora 39 they'll break by default again, including the released version.

Dependencies

No reverse dependencies of openssl have to react in time for Fedora 36. For now, test, file bugs and spark discussions. A small coordinated change with openssl is required.

Contingency Plan

• Contingency mechanism: not needed for F36
• Contingency deadline: not needed for F36
• Blocks release? no

Documentation

Workaround API should be added to SHA1SignaturesGuidance. Packaging guidelines should be modified accordingly.

Release Notes

Fedora 36-38: trusting SHA-1 signatures is deprecated, users are encouraged to test with restrictive policies and file bugs.

Fedora 39: SHA-1 signatures are no longer trusted by default.