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For my web application, users connect to a web server in HTTPS and need to have a client certificate installed on their computer/browser.

I understand that the sha1 deprecation is important for server certificates to prevent potential message decryption.

Is there the same problem for client certificates ? Is it safe to use sha1-signed client certificates ?

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I understand that the sha1 deprecation is important for server certificates to prevent potential message decryption.

The real problem is that a weak signature algorithm allows the creating of a new certificate with a valid signature by a trusted CA without having this CA involved in this. And this new certificate can then be used to impersonate an existing site, for example during a man in the middle attack.

The same impersonation is possible with client certificates. While this does not lead to a man in the middle attack it might lead to identity theft and unauthorized access in case the client certificate is used to authenticate the user.

  • Ok, I understand. Except that the server certificate is public while the client certificate is not. So the risk is not the exactly same. Am I wrong ? – JYL Sep 5 '17 at 11:14
  • @JYL: Why do you think the client certificate is not public? It is the same as with the server certificate: it contains no secret information, is sent in plain inside the TLS handshake (i.e. open for sniffing) and is not considered to be specifically protected. Only the matching private key must be protected but this is the same with server certificates too. – Steffen Ullrich Sep 5 '17 at 11:50
  • I though that the minimum required data to reproduce a client certificat is not fully present in the handshake and is not available directly from the client, at the opposite of the server. – JYL Sep 6 '17 at 12:40
  • @JYL: to create a certificate you need some public data which end up in the certificate and are also transmitted in clear in the handshake. You also need a private key but this is kept private both for client and server certificate. – Steffen Ullrich Sep 6 '17 at 13:23
  • I would like to know the answer to the question. We're suffering from a strange problem where IE11 and Edge on Windows 10 fail as soon as a smartcard with SHA-1 cert is attempted to be used for authentication. – Lee Nov 13 '19 at 15:22

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