Resolving ssh-keychain.dylib Code Signature Errors in macOS

When using SSH with a CAC (Common Access Card) on macOS, you might encounter an error related to ssh-keychain.dylib during login. This issue often manifests as a long error message, but despite the error, you may still be able to log in. However, the constant appearance of the error message can be frustrating. The root cause is usually an invalid code signature on the ssh-keychain.dylib file.

In this post, we’ll walk through a simple solution to resolve this code signature issue, ensuring a smoother experience when using your CAC for SSH authentication!

The Problem
The Solution: Re-sign the ssh-keychain.dylib Library
Test Your SSH Connection:
Why Does This Fix Work?
Conclusion

The Problem

When attempting to SSH into a remote server, you might see a message like this:

ssh -A HOSTNAME dlopen /usr/local/lib/ssh-keychain.dylib failed: dlopen(/usr/local/lib/ssh-keychain.dylib, 0x0002): tried: '/usr/local/lib/ssh-keychain.dylib' (code signature invalid in <8978BDE8-8AE2-3EFE-833D-DC2C8781B6C4> '/usr/local/lib/ssh-keychain.dylib' (errno=1) sliceOffset=0x00018000, codeBlobOffset=0x0001DFC0, codeBlobSize=0x00004B40), '/System/Volumes/Preboot/Cryptexes/OS/usr/local/lib/ssh-keychain.dylib' (no such file), '/usr/local/lib/ssh-keychain.dylib' (code signature invalid in <8978BDE8-8AE2-3EFE-833D-DC2C8781B6C4> '/usr/local/lib/ssh-keychain.dylib' (errno=1) sliceOffset=0x00018000, codeBlobOffset=0x0001DFC0, codeBlobSize=0x00004B40)

This error occurs because macOS is trying to load the ssh-keychain.dylib library, but its code signature is invalid. This usually happens after system upgrades or changes that impact third-party libraries.

The Solution: Re-sign the `ssh-keychain.dylib` Library

The simplest and most effective way to fix this error is by re-signing the ssh-keychain.dylib library using macOS’s codesign tool. This will ensure the system can load the library without throwing an error.

Here’s the command to re-sign the library:

sudo codesign --force --deep --sign - /usr/local/lib/ssh-keychain.dylib

This command re-signs the ssh-keychain.dylib library, ensuring macOS no longer rejects the file due to an invalid or missing signature.

Test Your SSH Connection:

After re-signing the library, test your SSH connection again by using your CAC for authentication:

ssh -A username@remote_server

If the code signing process was successful, you should no longer see the long error message regarding the invalid ssh-keychain.dylib signature.

Why Does This Fix Work?

macOS requires all code libraries to have a valid code signature. When a library like ssh-keychain.dylib has an invalid or missing signature, macOS refuses to load it, resulting in an error. The codesign tool allows you to re-sign libraries, verifying their integrity and allowing macOS to load them without issues.

By using the --force, --deep, and --sign - options, you are:

Forcing the re-signing of the library (--force). Signing nested code if the library contains other bundles (--deep). Using a self-signed signature (--sign -). This ensures that the library will be accepted by macOS when SSH is trying to load it.

Conclusion

If you’re encountering the dlopen /usr/local/lib/ssh-keychain.dylib failed error when SSH-ing into remote servers with your CAC, re-signing the ssh-keychain.dylib library is a quick and effective solution. This method prevents macOS from rejecting the library due to an invalid code signature, ensuring a smoother login experience without lengthy error messages.

📝 For more information about Code Signatures, review this documentation archive from developer.apple.com.