Mac cannot ssh to any EC2 instance, possible OpenSSL issue? - macos

Question

Using my current MacBook, at previous companies I've been able to ssh to EC2 instances. At the new company, which has EC2 instances running Centos 6, which is running an old version of OpenSSL, (an old version of libssl) I cannot seem to ssh. I always get:
Permission denied (publickey).
So, for instance, on one server, the devops engineer installed my ssh public key, and they white listed my home IP address.
They sent me this block of text, which I put in my ~/.ssh/config file.
Host never_dev
HostName neverstaydev.com
User neverst
IdentityFile ~/.ssh/id_ed25519
Host beta
HostName 12.18.125.53
User neverst
IdentityFile ~/.ssh/id_ed25519
The only thing I changed was the name of the key, which is correct. But if I try this:
ssh -v never_dev
I get:
OpenSSH_8.1p1, LibreSSL 2.7.3
debug1: Reading configuration data /Users/admin/.ssh/config
debug1: /Users/admin/.ssh/config line 1: Applying options for never_dev
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: /etc/ssh/ssh_config line 47: Applying options for *
debug1: Connecting to neverstaydev.com port 22.
debug1: Connection established.
debug1: identity file /Users/admin/.ssh/id_ed25519 type 3
debug1: identity file /Users/admin/.ssh/id_ed25519-cert type -1
debug1: Local version string SSH-2.0-OpenSSH_8.1
debug1: Remote protocol version 2.0, remote software version OpenSSH_5.3
debug1: match: OpenSSH_5.3 pat OpenSSH_5* compat 0x0c000002
debug1: Authenticating to neverstaydev.com:22 as 'neverst'
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug1: kex: algorithm: diffie-hellman-group-exchange-sha256
debug1: kex: host key algorithm: ssh-rsa
debug1: kex: server->client cipher: aes128-ctr MAC: umac-64#openssh.com compression: none
debug1: kex: client->server cipher: aes128-ctr MAC: umac-64#openssh.com compression: none
debug1: SSH2_MSG_KEX_DH_GEX_REQUEST(2048<3072<8192) sent
debug1: got SSH2_MSG_KEX_DH_GEX_GROUP
debug1: SSH2_MSG_KEX_DH_GEX_INIT sent
debug1: got SSH2_MSG_KEX_DH_GEX_REPLY
debug1: Server host key: ssh-rsa SHA256:UJ9EFg/QTnxCP+/9/P8b+jSpKozY1IbaN3M5jOoz8e0
debug1: Host 'neverstaydev.com' is known and matches the RSA host key.
debug1: Found key in /Users/admin/.ssh/known_hosts:24
debug1: rekey out after 4294967296 blocks
debug1: SSH2_MSG_NEWKEYS sent
debug1: expecting SSH2_MSG_NEWKEYS
debug1: SSH2_MSG_NEWKEYS received
debug1: rekey in after 4294967296 blocks
debug1: Will attempt key: /Users/admin/.ssh/id_ed25519 ED25519 SHA256:fBRwhBVpzb6ve5cDwmMS1R0UlhzSaVg9E62WW7riRZA explicit
debug1: SSH2_MSG_SERVICE_ACCEPT received
debug1: Authentications that can continue: publickey
debug1: Next authentication method: publickey
debug1: Offering public key: /Users/admin/.ssh/id_ed25519 ED25519 SHA256:fBRwhBVpzb6ve5cDwmMS1R0UlhzSaVg9E62WW7riRZA explicit
debug1: Authentications that can continue: publickey
debug1: No more authentication methods to try.
neverst#neverstaydev.com: Permission denied (publickey).
All of the developers are sharing one account on this EC2 instance, and everyone else can log in, except me. But my IP address has been whitelisted, since, as you can see above, my machine engages in dialogue with the EC2 instance.
I've tried different permissions on the private key, both 0400 and 0600. That has no effect.
Likewise, they have AMIs based on these machines, so I spun up a new EC2 instance based on the AMI, creating a new key pair. I then tried to ssh to the new EC2 instance, using the new key pair. But still the same problem:
Permission denied (publickey)
What might cause this?
The devops engineer suggested that I do:
ssh-keygen -t rsa -b 4096 -C "neverstay-key"
Now, and then I gave them my public key, which they installed on their side, and now when I try to ssh to the server, I get:
ssh-keygen -t rsa -b 4096 -C "lawarance-futurestay-key"