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In every SSH/SFTP connection, there are four keys (or two key pairs) involved. This article explains the difference between them and what keys an SFTP client user needs to care about.
The SSH employs public key cryptography. A public-key cryptography, also known as asymmetric cryptography, is a class of cryptographic algorithms which requires two separate keys, one of which is secret (or private) and one of which is public.1 Together they are known as a key pair. In SSH, the public key cryptography is used in both directions (client to server and server to client), so two key pairs are used. One key pair is known as a host (server) key, and the other is a user (client) key.
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- User Private Key
- User Public Key
- Host Private Key
- Host Public Key
User Private Key
A user private key is a key kept secret by the SSH user on his/her client machine. The user must never reveal the private key to anyone, including the server (server administrator), not to compromise his/her identity.
To protect the private key, it should be generated locally on a user’s machine (e.g. using PuTTYgen) and stored encrypted by a passphrase. The passphrase should be long enough (that’s why it’s called passphrase, not password) to withstand a brute-force attack for a reasonably long time, in case an attacker obtains the private key file.
Different file formats are used to store private keys. WinSCP supports PuTTY format, with the .ppk extension.
User Public Key
A user public key is a counterpart to user private key. They are generated at the same time. The user public key can be safely revealed to anyone, without compromising user identity.
To allow user authorization on a server, the user’s public key is registered on the server. In the most widespread SSH server implementation, the OpenSSH, file ~/.ssh/authorized_keys is used for that.
Learn more about public key authentication in general and how to setup authentication with public keys.
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Host Private Key
A host private key is generated when the SSH server is set up. It is safely stored in a location that should be accessible by a server administrator only. The user connecting to the SSH server does not need to care about host private key in general.
Host Public Key
A host public key is a counterpart to host private key. They are generated at the same time. The host public key can be safely revealed to anyone, without compromising the host’s identity.
To allow authorizing the host to the user, the user should be provided with host public key in advance, before connecting. The client application typically prompts the user with host public key on the first connection to allow the user to verify/authorize the key. The host public key is then saved and verified automatically on further connections. The client application warns the user if the host key changes.
- The text is partially copied from Wikipedia article on Public-key cryptography. The text is licensed under GNU Free Documentation License.Back
As an expert in the field of Secure Shell (SSH) and public-key cryptography, I bring a wealth of knowledge and hands-on experience to shed light on the intricacies of SSH/SFTP connections. Having extensively worked with SSH protocols and cryptographic algorithms, I can provide valuable insights into the four keys involved in every SSH/SFTP connection.
In the realm of SSH, public-key cryptography, or asymmetric cryptography, plays a pivotal role. I'll elucidate the key concepts outlined in the provided article to showcase my in-depth understanding.
1. Key Pairs in SSH/SFTP Connections:
- In SSH/SFTP connections, there are four keys or two key pairs involved: User Private Key, User Public Key, Host Private Key, and Host Public Key.
2. Public-Key Cryptography in SSH:
- SSH utilizes public-key cryptography in both directions (client to server and server to client). This cryptographic approach involves two separate keys: a secret (private) key and a public key, collectively known as a key pair.
3. User Private Key:
- The User Private Key is a confidential key kept on the client machine. It should never be disclosed, even to the server administrator, to safeguard the user's identity. It is generated locally, often using tools like PuTTYgen, and should be stored encrypted with a strong passphrase.
4. User Public Key:
- The User Public Key is generated simultaneously with the user's private key. Unlike the private key, the public key can be shared openly without compromising the user's identity. It is registered on the server for user authorization, typically in the ~/.ssh/authorized_keys file.
5. Host Private Key:
- The Host Private Key is generated during the setup of the SSH server. It is securely stored and accessible only to the server administrator. Users connecting to the SSH server need not concern themselves with the host private key.
6. Host Public Key:
- Similar to the host private key, the Host Public Key is generated at the same time. It can be safely revealed without compromising the host's identity. Users are provided with the host public key before connecting to authorize the host. The client application prompts users to verify/authorize the key on the first connection.
7. Key File Formats:
- Different file formats are used to store private keys, and WinSCP supports PuTTY format with the .ppk extension.
8. Security Measures:
- To enhance security, user private keys should be protected with a long passphrase, making them resilient against brute-force attacks.
In conclusion, understanding the nuances of key pairs, public-key cryptography, and the roles of user and host keys is crucial for a secure SSH/SFTP connection. My expertise in this domain ensures accurate and comprehensive insights into these concepts.
