5 ms·

Newbie question here: how is a private key stored in a device I can easily lose more secure than a (long and sufficiently random) password that I've memorized a

by Rygian 5y ago

Newbie question here: how is a private key stored in a device I can easily lose more secure than a (long and sufficiently random) password that I've memorized and can type down only when intended?

hansel_der 5y ago
passwords are a symetric key, hence if the server is compromised, so is the password. with asymetric keys, a compromise of the public key is no problem. but you are right, key-files on a disk are more vulnerable to theft than secrets in your head. keyfiles with a password ontop are most secure but also most uncomfortable.
- zamfi 5y ago
  > passwords are a symetric key, hence if the server is compromised, so is the password Pretty sure that’s not how it works, iirc passwords are stored one-way encrypted. And if it were true, then anyone with root access to a box could comprise every other (Unix) user’s key, which seems like a potentially bigger problem…
  - hansel_der 5y ago
    pretty sure passwords are sent to the server in a way that reveals the actual password to the server. any chance you are thinking about key-agent?
  - atoav 5y ago
    Passwords are (or rather should be) indeed stored using crypt. However at login the provided password needs to be compared to the hashed one, which means the clear text password needs to be rehashed. I am not sure this happens on the client.
    - wang_li 5y ago
      It definitely doesn’t happen on the client. If it did then the server’s password file would effectively be plaintext.
      - zinekeller 5y ago
        > If it did then the server’s password file would effectively be plaintext. Send seed and hashing parameters to the client, then client does hashing, client sends hash, server compares hashes. It's vulnerable to replay attacks, but it's the same with client sending plaintext password to server (assuming that you're not using SSH or similar).
        dsr_ 5y ago
        You two are working your ways towards defining a zero-knowledge password proof: https://en.wikipedia.org/wiki/Zero-knowledge_password_proof https://en.wikipedia.org/wiki/Zero-knowledge_password_proof I think SRP is the most widely implemented version. https://en.wikipedia.org/wiki/Secure_Remote_Password_protocol https://en.wikipedia.org/wiki/Secure_Remote_Password_protoco...
        throw0101a 5y ago
        Note that SRP is an older protocol, and things have moved on from there. See various PAKE algorithms: * https://en.wikipedia.org/wiki/Password-authenticated_key_agreement https://en.wikipedia.org/wiki/Password-authenticated_key_agr...
  - oxapentane 5y ago
    Quick google led me to RFC4252[0], section 8 of which (as far as I understood) describes ssh auth sending password as UTF8 plaintext string (and the whole packet is encrypted at transport layer). While passwords in /etc/shadow are hashed, if someone got access to your server he can just put malicious listener that will catch this UTF8 string. I'm not a SSH guru, so if I'm mistaken please shout at me ;D [0] https://datatracker.ietf.org/doc/html/rfc4252#section-8 https://datatracker.ietf.org/doc/html/rfc4252#section-8
    - throw0101a 5y ago
      Anything that is not PAKE (which SSH is not) generally sends the password to the server: * https://en.wikipedia.org/wiki/Password-authenticated_key_agreement https://en.wikipedia.org/wiki/Password-authenticated_key_agr... * https://blog.cryptographyengineering.com/2018/10/19/lets-talk-about-pake/ https://blog.cryptographyengineering.com/2018/10/19/lets-tal... A Password-Authenticated Key Exchange (PAKE) attempts to address this issue by constructing a cryptographic key exchange that does not result in the password, or password-derived data, being transmitted across an unsecured channel. * https://datatracker.ietf.org/doc/html/rfc8125 https://datatracker.ietf.org/doc/html/rfc8125 I'm sure there are other zero-knowledge protocols besides PAKE-like ones, but I'm not an expert here.
  - icedchai 5y ago
    I wouldn't worry about storage. Anyone with root access can modify the sshd daemon (along with imap, pop3, and whatever else) to log all the passwords received.
SahAssar 5y ago
For one a key is not transmitted over the network but the bigger reason is that most people don't use sufficiently long, random and unique passwords. If you are running a server where only you SSH in and you use a long random and unique password you are probably fine but for most people it's just easier to use keys at that point since it is not a lot easier to use long random and unique passwords than it is to use keys. One upside to keys is also that since the server does not have your private key you don't need to rotate it if that server is hacked so you can reuse the same key for multiple servers and services. If you reuse the same long random password it only takes one of those servers/services to be hacked for you to be compromised on all of them.
- Sebb767 5y ago
  Adding to that, some servers might have a secondary user with a weak password that was created by an installer or an admin for testing purposes. Disallowing password login prevents others from exploiting these accounts.
  - SahAssar 5y ago
    Agreed, that's why I put "If you are running a server where only you SSH in" but maybe I should have been more clear about it.
Aldipower 5y ago
If you plan to store your private key on a device you can loose the key itself should have a password too. So the attacker needs still a password to unlock the private key. This is actually a good idea in general. Securing the private key with a password.
- 3np 5y ago
  Nitpick: passphrase, not password
  - atoav 5y ago
    Nitpick: if you have a password manager it doesn't matter
    - 3np 5y ago
      Well it kind of does.. A password is validated, and if you lose it there is usually some recourse. Reset or whatnot. It may be a hassle but always possible somehow. If you lose a passphrase, no one can help you even if you hit HN front page and /r/all with a sob story. So backups and availability have a different cruciality. Also if you store a private key on the same medium as a password store with weak encryption or key that contains the passphrase, they key can't be considered as strong anymore. There are practical reasons to make a distinction and mistakes can be expensive.
  - Aldipower 5y ago
    Thanks, yes, passphrase is the correct word. :)
- makach 5y ago
  if you lose your key, make sure that local (not remote) login is the recovery method. locking yourself out is a real thing.
- jgalt212 5y ago
  passwords all the way down. /s
ahepp 5y ago
Use one key pair per device. If you lose the device, remove the associated public key from the server's authorized keys. Using a password on they key isn't a bad idea either.
nijave 5y ago
It's a good idea to encrypt devices you could potentially lose anyway. Besides private keys, they probably contain session tokens, API keys, and other things--especially those saved by the web browser (cookies, cache, local storage)

by Rygian 5y ago

Newbie question here: how is a private key stored in a device I can easily lose more secure than a (long and sufficiently random) password that I've memorized and can type down only when intended?

hansel_der 5y ago

passwords are a symetric key, hence if the server is compromised, so is the password. with asymetric keys, a compromise of the public key is no problem. but you are right, key-files on a disk are more vulnerable to theft than secrets in your head. keyfiles with a password ontop are most secure but also most uncomfortable.

zamfi 5y ago
> passwords are a symetric key, hence if the server is compromised, so is the password Pretty sure that’s not how it works, iirc passwords are stored one-way encrypted. And if it were true, then anyone with root access to a box could comprise every other (Unix) user’s key, which seems like a potentially bigger problem…
- hansel_der 5y ago
  pretty sure passwords are sent to the server in a way that reveals the actual password to the server. any chance you are thinking about key-agent?
- atoav 5y ago
  Passwords are (or rather should be) indeed stored using crypt. However at login the provided password needs to be compared to the hashed one, which means the clear text password needs to be rehashed. I am not sure this happens on the client.
  - wang_li 5y ago
    It definitely doesn’t happen on the client. If it did then the server’s password file would effectively be plaintext.
    - zinekeller 5y ago
      > If it did then the server’s password file would effectively be plaintext. Send seed and hashing parameters to the client, then client does hashing, client sends hash, server compares hashes. It's vulnerable to replay attacks, but it's the same with client sending plaintext password to server (assuming that you're not using SSH or similar).
      - dsr_ 5y ago
        You two are working your ways towards defining a zero-knowledge password proof: https://en.wikipedia.org/wiki/Zero-knowledge_password_proof https://en.wikipedia.org/wiki/Zero-knowledge_password_proof I think SRP is the most widely implemented version. https://en.wikipedia.org/wiki/Secure_Remote_Password_protocol https://en.wikipedia.org/wiki/Secure_Remote_Password_protoco...
        throw0101a 5y ago
        Note that SRP is an older protocol, and things have moved on from there. See various PAKE algorithms: * https://en.wikipedia.org/wiki/Password-authenticated_key_agreement https://en.wikipedia.org/wiki/Password-authenticated_key_agr...
- oxapentane 5y ago
  Quick google led me to RFC4252[0], section 8 of which (as far as I understood) describes ssh auth sending password as UTF8 plaintext string (and the whole packet is encrypted at transport layer). While passwords in /etc/shadow are hashed, if someone got access to your server he can just put malicious listener that will catch this UTF8 string. I'm not a SSH guru, so if I'm mistaken please shout at me ;D [0] https://datatracker.ietf.org/doc/html/rfc4252#section-8 https://datatracker.ietf.org/doc/html/rfc4252#section-8
  - throw0101a 5y ago
    Anything that is not PAKE (which SSH is not) generally sends the password to the server: * https://en.wikipedia.org/wiki/Password-authenticated_key_agreement https://en.wikipedia.org/wiki/Password-authenticated_key_agr... * https://blog.cryptographyengineering.com/2018/10/19/lets-talk-about-pake/ https://blog.cryptographyengineering.com/2018/10/19/lets-tal... A Password-Authenticated Key Exchange (PAKE) attempts to address this issue by constructing a cryptographic key exchange that does not result in the password, or password-derived data, being transmitted across an unsecured channel. * https://datatracker.ietf.org/doc/html/rfc8125 https://datatracker.ietf.org/doc/html/rfc8125 I'm sure there are other zero-knowledge protocols besides PAKE-like ones, but I'm not an expert here.
- icedchai 5y ago
  I wouldn't worry about storage. Anyone with root access can modify the sshd daemon (along with imap, pop3, and whatever else) to log all the passwords received.

SahAssar 5y ago

For one a key is not transmitted over the network but the bigger reason is that most people don't use sufficiently long, random and unique passwords. If you are running a server where only you SSH in and you use a long random and unique password you are probably fine but for most people it's just easier to use keys at that point since it is not a lot easier to use long random and unique passwords than it is to use keys. One upside to keys is also that since the server does not have your private key you don't need to rotate it if that server is hacked so you can reuse the same key for multiple servers and services. If you reuse the same long random password it only takes one of those servers/services to be hacked for you to be compromised on all of them.

Sebb767 5y ago
Adding to that, some servers might have a secondary user with a weak password that was created by an installer or an admin for testing purposes. Disallowing password login prevents others from exploiting these accounts.
- SahAssar 5y ago
  Agreed, that's why I put "If you are running a server where only you SSH in" but maybe I should have been more clear about it.

Aldipower 5y ago

If you plan to store your private key on a device you can loose the key itself should have a password too. So the attacker needs still a password to unlock the private key. This is actually a good idea in general. Securing the private key with a password.

3np 5y ago
Nitpick: passphrase, not password
- atoav 5y ago
  Nitpick: if you have a password manager it doesn't matter
  - 3np 5y ago
    Well it kind of does.. A password is validated, and if you lose it there is usually some recourse. Reset or whatnot. It may be a hassle but always possible somehow. If you lose a passphrase, no one can help you even if you hit HN front page and /r/all with a sob story. So backups and availability have a different cruciality. Also if you store a private key on the same medium as a password store with weak encryption or key that contains the passphrase, they key can't be considered as strong anymore. There are practical reasons to make a distinction and mistakes can be expensive.
- Aldipower 5y ago
  Thanks, yes, passphrase is the correct word. :)
makach 5y ago
if you lose your key, make sure that local (not remote) login is the recovery method. locking yourself out is a real thing.
jgalt212 5y ago
passwords all the way down. /s

ahepp 5y ago

Use one key pair per device. If you lose the device, remove the associated public key from the server's authorized keys. Using a password on they key isn't a bad idea either.

nijave 5y ago

It's a good idea to encrypt devices you could potentially lose anyway. Besides private keys, they probably contain session tokens, API keys, and other things--especially those saved by the web browser (cookies, cache, local storage)