Fix SSH certificate authentication from agent #591
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This fixes SSH certificate authentication when certificates are provided by an SSH agent (e.g., OpenSSH agent, Pageant, or Vault-issued certificates). ## Problem The ssh-key crate's `parse_public_key()` function parses OpenSSH certificates as opaque keys containing only the certificate nonce (~32 bytes) rather than the full certificate blob (~600-700 bytes). When `.to_bytes()` is called on these parsed certificates, only the public key portion is returned, losing all certificate metadata including: - Serial number - Key ID - Valid principals - Validity period (not-before/not-after) - Critical options and extensions - CA signature This caused three specific failures during certificate authentication: 1. **Double certificate suffix bug**: The code called `algo.to_certificate_type()` on an algorithm string that was already a certificate type (`[email protected]`), resulting in an invalid algorithm name (`[email protected]`) that servers rejected. 2. **Incomplete certificate in auth probe**: Only 32 bytes of certificate data were sent to the server during the publickey probe phase instead of the full certificate blob. 3. **Incomplete certificate in signature request**: Only the nonce was sent to the SSH agent when requesting a signature, preventing the agent from matching the certificate to the correct private key. ## Solution The fix preserves the original certificate blobs throughout the authentication flow: 1. **Certificate detection and storage** (`keys/agent/client.rs`): When receiving keys from the agent, detect certificates by checking if the algorithm name contains `[email protected]`. Store the original certificate blob in a global HashMap keyed by the certificate's SHA256 fingerprint. 2. **Probe phase** (`client/encrypted.rs`): During publickey authentication probe, retrieve the stored certificate blob and send it to the server. Use the algorithm string as-is without calling `to_certificate_type()` since it's already the correct certificate type. 3. **Signature phase** (`client/encrypted.rs`): When constructing the data to be signed, use the stored certificate blob instead of the truncated key data. 4. **Agent signature request** (`keys/agent/client.rs`): When requesting a signature from the agent, send the full certificate blob instead of just the key data so the agent can match it to the correct private key. ## Testing Tested with Vault-issued ECDSA certificates on Windows using the built-in OpenSSH agent. Authentication now succeeds where it previously failed with "userauth_failure" errors. Verified server logs show successful authentication: ``` Accepted publickey for <user> from <ip> ssh2: ECDSA-CERT <fingerprint> ID <key-id> (serial <serial>) CA ECDSA <ca-fingerprint> ``` Regular SSH key authentication (non-certificate) remains unaffected. 🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude <[email protected]>
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Summary
russh/src/client/encrypted.rsto properly encode certificates during auth probes and signature creationrussh/src/keys/agent/client.rsto detect and store certificate blobs when listing keysTechnical Details
The issue was that SSH certificates from agents weren't being properly encoded during authentication. The ssh-key crate parses certificates but doesn't preserve the full blob needed for authentication. This PR:
CERT_BLOBS) to store complete certificate blobs keyed by fingerprintAgentClient::request_identities()to detect certificates and store their blobswrite_auth_request()to retrieve and use stored certificate blobs for proper encodingsign_request_with_hash_alg()to send full certificate blobs to the agentTest plan
🤖 Generated with Claude Code