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SHA-1 is being phased out by a lot of major companies due to vulnerabilities. For example Chrome no longer considers it safe for public web PKI.

Is it insecure to use SHA-1 for PGP email signatures?

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  • Depends on what you are protecting in the e-mail. Depends if your e-mail must be compliant with specific encryption standards. Depends... need more info.
    – dtb_pen
    Mar 25, 2015 at 18:30

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There are no practical attacks on SHA-1 yet. Experts estimate that SHA-1 attacks will start to appear around 2018.

As you have correctly pointed out, companies like Google and Microsoft have already taken action and are replacing SHA-1 with newer and safer standards:

Microsoft is recommending that customers and CA’s stop using SHA-1 for cryptographic applications, including use in SSL/TLS and code signing. Microsoft Security Advisory 2880823 has been released along with the policy announcement that Microsoft will stop recognizing the validity of SHA-1 based certificates after 2016.

The recommendation is to move to SHA-2. So you'll have to reconfigure your PGP software to use another hash function, but the basic working principle of PGP will remain the same.

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    SHA-1 attacks rely on collisions. The first wave (~2018) will involve brute forcing two pieces of data such that they have the same hash. Finding a second piece of data to match an original (e.g. from a PGP signature) is significantly harder and likely won't be feasible until quite a while later. So from a PGP signing or encrypting context, there shouldn't be much concern.
    – Adam Katz
    Mar 26, 2015 at 1:18
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    @AdamKatz It can absolutely be an issue when signing: it's not that uncommon to use PGP to sign things received from others, which is a classic example of a collision vector.
    – cpast
    Mar 26, 2015 at 3:21
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    The experts were fairly close, the first published, actual SHA-1 attack published 2017-02-23: security.googleblog.com/2017/02/… Feb 23, 2017 at 14:20

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