Description:
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The ssh2 library cannot read OpenSSH RSA private keys stored in the default RFC4716 format. However, rather than reporting that it cannot understand the file, we simply get a misleading authentication failed error.

Converting the file to the older PEM format fixes the issue.

Test script:
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Convert an existing authorized RSA key to RFC4716 format:
ssh-keygen -p -m RFC4716 id_rsa

Test in PPP:
ssh2_auth_pubkey_file($conn, $user, "id_rsa.pub", "id_rsa");


Convert to PEM format and repeat:
ssh-keygen -p -m PEM id_rsa

ssh2_auth_pubkey_file($conn, $user, "id_rsa.pub", "id_rsa");