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Description of problem:
SHA1 was published in 1993 and is still in wide use today. However there are several weaknesses and the algorithms is considered today cryptographically broken (less effort than brute force is required). It is highly likely we will have collision attacks in the near future; i.e., forged certificates. For that it is recommended for our tools which deal with digital signatures to no longer use the SHA1 as a hash function by default.

For maximum compatibility with old software we recommend the SHA2-256 algorithm to be used as the default replacement.

As such, it is recommended for all of the included in gnutls tools and shipped configuration files, to switch from SHA1 to SHA2-256 when generating certificates or digital signatures.

certtool --generate-privkey --outfile key
certtool --generate-self-signed --load-privkey key --outfile cert
certtool -i --infile cert|grep Signature 
Signature Algorithm: RSA-SHA256