In 93c8007, SRP signature errors were made non-fatal because many
users got the following error:
openpgp: unsupported feature: public key algorithm 22
This is because Protonmail started signing these messages with an
EDDSA key, an algorithm which the Go OpenPGP library does not
support. The switch to github.com/protonmail/crypto introduces this
algorithm, so messages that haven't been tampered with should pass
the verification.
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