[crypto] ssh: verify declared key type matches decoded key in authorized_keys

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Gopher Robot (Gerrit)

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Jun 30, 2026, 1:24:37 PM (yesterday) Jun 30

to Nicola Murino, goph...@pubsubhelper.golang.org, golang-...@googlegroups.com, golang...@luci-project-accounts.iam.gserviceaccount.com, Junyang Shao, Neal Patel, Neal Patel, Filippo Valsorda, golang-co...@googlegroups.com

Gopher Robot submitted the change with unreviewed changes

Unreviewed changes

1 is the latest approved patch-set.
The change was submitted with unreviewed changes in the following files:

```
The name of the file: ssh/keys.go
Insertions: 3, Deletions: 3.

@@ -193,7 +193,7 @@
 			return "", nil, nil, "", nil, err
 		}
 		if pubKey.Type() != wantType {
-			return "", nil, nil, "", nil, fmt.Errorf("ssh: key type mismatch: found %q, want %q", pubKey.Type(), wantType)
+			return "", nil, nil, "", nil, fmt.Errorf("ssh: known hosts key type mismatch: human-readable type %q, encoded type %q", wantType, pubKey.Type())
 		}
 
 		return marker, strings.Split(hosts, ","), pubKey, comment, rest, nil
@@ -241,7 +241,7 @@
 			if string(in[:i]) == out.Type() {
 				return out, comment, options, rest, nil
 			}
-			err = fmt.Errorf("ssh: key type mismatch: found %q, want %q", out.Type(), in[:i])
+			err = fmt.Errorf("ssh: authorized keys key type mismatch: human-readable type %q, encoded type %q", in[:i], out.Type())
 		}
 		lastErr = err
 
@@ -289,7 +289,7 @@
 				options = candidateOptions
 				return out, comment, options, rest, nil
 			}
-			err = fmt.Errorf("ssh: key type mismatch: found %q, want %q", out.Type(), in[:i])
+			err = fmt.Errorf("ssh: authorized keys key type mismatch: human-readable type %q, encoded type %q", in[:i], out.Type())
 		}
 		lastErr = err
 
```

Change information

Commit message:

ssh: verify declared key type matches decoded key in authorized_keys

ParseAuthorizedKey and ParseKnownHosts previously ignored the key type
field (e.g. "ssh-rsa") in each entry, relying solely on the type
information embedded within the base64-encoded key blob.

For ParseAuthorizedKey this also caused a single-token option to be
silently dropped: a line such as "restrict <key>" (with the key type
omitted) was parsed as an unrestricted key, because the option token
landed in the key type position and was discarded together with its
meaning. The same happens for no-pty, no-port-forwarding and the other
single-word options. OpenSSH's sshkey_read rejects such lines outright,
as the option token cannot be read as a key type.

OpenSSH's sshkey_read also explicitly verifies that the key type
declared in the text matches the type of the parsed key, returning
SSH_ERR_KEY_TYPE_MISMATCH if they differ.

This change adds, in both functions, a check that the declared key type
matches the decoded key's type, returning an error for malformed lines
where they diverge. This mirrors the fix already applied to the
ssh/knownhosts package in CL 782427.

Change-Id: I9163108d964de4be8a415fe0efd68e6b71f77990

Reviewed-on: https://go-review.googlesource.com/c/crypto/+/792840

Reviewed-by: Filippo Valsorda <fil...@golang.org>

LUCI-TryBot-Result: golang...@luci-project-accounts.iam.gserviceaccount.com <golang...@luci-project-accounts.iam.gserviceaccount.com>

Auto-Submit: Nicola Murino <nicola...@gmail.com>

Reviewed-by: Neal Patel <neal...@google.com>

Reviewed-by: Junyang Shao <shaoj...@google.com>

Files:

M ssh/keys.go
M ssh/keys_test.go

Change size: M

Delta: 2 files changed, 92 insertions(+), 10 deletions(-)

Branch: refs/heads/master

Submit Requirements:

Code-Review: +2 by Filippo Valsorda, +1 by Neal Patel, +1 by Junyang Shao
TryBots-Pass: LUCI-TryBot-Result+1 by golang...@luci-project-accounts.iam.gserviceaccount.com

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