It looks like you're using a DSA SSH key, and modern versions of OpenSSH don't support those by default because they're insecure. Here's why:
- DSA keys, as used in OpenSSH, are limited to 1024 bits. For the kind of algorithm they are, this is equivalent to an 80-bit level of security, whereas the lowest acceptable level of security these days is 128 bits.
- DSA keys can only use the insecure SHA-1 algorithm for signing. SHA-1 is known to be very weak and provide only 61 bits of security, so it should not be used any longer. It is so weak that any reasonably well-paid software developer can afford to attack SHA-1.
Your best bet is to generate a new key using something like the following:
$ ssh-keygen -t ed25519
Ed25519 keys are considered the best option these days and are recommended by Mozilla for all developers. It is possible to use RSA keys as well, but they should be at least 3072 bits in size and you must configure them not to use SHA-1 either by adding
PubkeyAcceptedTypes -ssh-rsa. Unless you are sure you know what you're doing or you have a compelling need otherwise, Ed25519 keys are the best option.
Note that Bitbucket does not offer any secure host key algorithms on their side (only SHA-1-based algorithms), so you're probably better off using HTTPS if you're using Bitbucket.