cmd/age: reject malformed supported SSH recipients

Reported by Joe Doyle of Trail of Bits.

cmd/age/age_test.go

@@ -117,6 +117,52 @@ func TestParseFileSizeLimit(t *testing.T) {
 	}
 }
 
+func TestUnsupportedSSHKey(t *testing.T) {
+	tests := []struct {
+		name, key, want string
+		ok              bool
+	}{
+		{
+			"truncated Ed25519",
+			"ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIH9pO5pz22JZEasoS2LEWWvJiUYI9M6l1uZc31FG",
+			"", false,
+		},
+		{
+			"type-only RSA",
+			"ssh-rsa AAAAB3NzaC1yc2E=",
+			"", false,
+		},
+		{
+			"small RSA",
+			"ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAAAgQCxlfoYpG04TTnmjocggQZI5l0fjvaUMky1ZD5zwktDKAgBj441OAtFj/3m7Gujpxx/8w3jjJoUXVtU+NFbcC972ROpPI4aPq3OR5SmuuR5bCr6efeZlyAEMY/DmbkeUe1TrvdU3VMDGJAvEEzT1wzokKt9PiwbV2jNkdPjaG5LDQ==",
+			"ssh-rsa", true,
+		},
+		{
+			"invalid Ed25519 point",
+			"ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIAICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgIC",
+			"", false,
+		},
+		{
+			"unknown type",
+			"totally-not-a-key AAAAEXRvdGFsbHktbm90LWEta2V5",
+			"totally-not-a-key", true,
+		},
+		{
+			"ECDSA",
+			"ecdsa-sha2-nistp256 AAAAE2VjZHNhLXNoYTItbmlzdHAyNTYAAAAIbmlzdHAyNTYAAABBBOSh0Qo5zaBE33EiJnxwIBuZml9Nt9rCfvGi++FniWSk8X/10EO4E7KMgqGLlkCTDNIcbuKcsgfMw1/tifB8aTc=",
+			"ecdsa-sha2-nistp256", true,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got, ok := unsupportedSSHKey(tt.key)
+			if got != tt.want || ok != tt.ok {
+				t.Errorf("unsupportedSSHKey() = %q, %v; want %q, %v", got, ok, tt.want, tt.ok)
+			}
+		})
+	}
+}
+
 var buildExtraCommands = sync.OnceValue(func() error {
 	bindir := filepath.SplitList(os.Getenv("PATH"))[0]
 	// Build age-keygen and age-plugin-pq into the test binary directory.

cmd/age/parse.go

@@ -6,6 +6,7 @@ package main
 
 import (
 	"bufio"
+	"crypto/rsa"
 	"encoding/base64"
 	"fmt"
 	"io"
@@ -88,7 +89,7 @@ func parseRecipientsFile(name string) ([]age.Recipient, error) {
 		}
 		r, err := parseRecipient(line)
 		if err != nil {
-			if t, ok := sshKeyType(line); ok {
+			if t, unsupported := unsupportedSSHKey(line); unsupported {
 				// Skip unsupported but valid SSH public keys with a warning.
 				warningf("recipients file %q: ignoring unsupported SSH key of type %q at line %d", name, t, n)
 				continue
@@ -114,7 +115,7 @@ func parseRecipientsFile(name string) ([]age.Recipient, error) {
 	return recs, nil
 }
 
-func sshKeyType(s string) (string, bool) {
+func unsupportedSSHKey(s string) (string, bool) {
 	// TODO: also ignore options? And maybe support multiple spaces and tabs as
 	// field separators like OpenSSH?
 	fields := strings.Split(s, " ")
@@ -132,7 +133,26 @@ func sshKeyType(s string) (string, bool) {
 		return "", false
 	}
 	if t := fields[0]; t == string(typeBytes) {
-		return t, true
+		switch t {
+		case "ssh-ed25519":
+			return "", false
+		case "ssh-rsa":
+			out, _, _, _, err := ssh.ParseAuthorizedKey([]byte(s))
+			if err != nil {
+				return "", false
+			}
+			cryptoKey, ok := out.(ssh.CryptoPublicKey)
+			if !ok {
+				return "", false
+			}
+			rsaKey, ok := cryptoKey.CryptoPublicKey().(*rsa.PublicKey)
+			if !ok {
+				return "", false
+			}
+			return t, rsaKey.N.BitLen() < 2048
+		default:
+			return t, true
+		}
 	}
 	return "", false
 }