feat: support aes256-gcm@openssh.com encrypted private keys

x/crypto/ssh only supports aes256-ctr/cbc for private key encryption,
so keys exported from 1Password (aes256-gcm) could not be decrypted even
with the correct passphrase. Decrypt the openssh-key-v1 blob ourselves
(bcrypt_pbkdf ported from x/crypto's internal package, AES-256-GCM via
stdlib, auth tag stored as raw bytes after the encrypted block) and
re-marshal it as an unencrypted key for ssh.ParsePrivateKey.

internal/sshclient/bcryptpbkdf.go

@@ -0,0 +1,95 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found at https://go.dev/LICENSE.
+//
+// Adapted from golang.org/x/crypto/ssh/internal/bcrypt_pbkdf, which is not
+// importable (internal package). Implements bcrypt_pbkdf(3) from OpenBSD.
+//
+// See https://flak.tedunangst.com/post/bcrypt-pbkdf and
+// https://cvsweb.openbsd.org/cgi-bin/cvsweb/src/lib/libutil/bcrypt_pbkdf.c.
+package sshclient
+
+import (
+	"crypto/sha512"
+	"errors"
+
+	"golang.org/x/crypto/blowfish"
+)
+
+const bcryptBlockSize = 32
+
+// bcryptPBKDF derives a key from the password, salt and rounds count,
+// returning a []byte of length keyLen that can be used as cryptographic key.
+func bcryptPBKDF(password, salt []byte, rounds, keyLen int) ([]byte, error) {
+	if rounds < 1 {
+		return nil, errors.New("bcrypt_pbkdf: number of rounds is too small")
+	}
+	if len(password) == 0 {
+		return nil, errors.New("bcrypt_pbkdf: empty password")
+	}
+	if len(salt) == 0 || len(salt) > 1<<20 {
+		return nil, errors.New("bcrypt_pbkdf: bad salt length")
+	}
+	if keyLen > 1024 {
+		return nil, errors.New("bcrypt_pbkdf: keyLen is too large")
+	}
+
+	numBlocks := (keyLen + bcryptBlockSize - 1) / bcryptBlockSize
+	key := make([]byte, numBlocks*bcryptBlockSize)
+
+	h := sha512.New()
+	h.Write(password)
+	shapass := h.Sum(nil)
+
+	shasalt := make([]byte, 0, sha512.Size)
+	cnt, tmp := make([]byte, 4), make([]byte, bcryptBlockSize)
+	for block := 1; block <= numBlocks; block++ {
+		h.Reset()
+		h.Write(salt)
+		cnt[0] = byte(block >> 24)
+		cnt[1] = byte(block >> 16)
+		cnt[2] = byte(block >> 8)
+		cnt[3] = byte(block)
+		h.Write(cnt)
+		bcryptHash(tmp, shapass, h.Sum(shasalt))
+
+		out := make([]byte, bcryptBlockSize)
+		copy(out, tmp)
+		for i := 2; i <= rounds; i++ {
+			h.Reset()
+			h.Write(tmp)
+			bcryptHash(tmp, shapass, h.Sum(shasalt))
+			for j := 0; j < len(out); j++ {
+				out[j] ^= tmp[j]
+			}
+		}
+
+		for i, v := range out {
+			key[i*numBlocks+(block-1)] = v
+		}
+	}
+	return key[:keyLen], nil
+}
+
+var bcryptMagic = []byte("OxychromaticBlowfishSwatDynamite")
+
+func bcryptHash(out, shapass, shasalt []byte) {
+	c, err := blowfish.NewSaltedCipher(shapass, shasalt)
+	if err != nil {
+		panic(err)
+	}
+	for i := 0; i < 64; i++ {
+		blowfish.ExpandKey(shasalt, c)
+		blowfish.ExpandKey(shapass, c)
+	}
+	copy(out, bcryptMagic)
+	for i := 0; i < 32; i += 8 {
+		for j := 0; j < 64; j++ {
+			c.Encrypt(out[i:i+8], out[i:i+8])
+		}
+	}
+	// Swap bytes due to different endianness.
+	for i := 0; i < 32; i += 4 {
+		out[i+3], out[i+2], out[i+1], out[i] = out[i], out[i+1], out[i+2], out[i+3]
+	}
+}

internal/sshclient/client.go

@@ -9,7 +9,7 @@ import (
 	"path/filepath"
 	"strings"
 
-	"github.com/janvete/caddyview/internal/parser"
+	"git.janvete.com/seth/caddyview/internal/parser"
 	"golang.org/x/crypto/ssh"
 	"golang.org/x/crypto/ssh/agent"
 	"golang.org/x/term"
@@ -193,7 +193,7 @@ func unlockWithPassphrase(path string, data []byte) (ssh.Signer, error) {
 		if err != nil {
 			return nil, err
 		}
-		signer, err := ssh.ParsePrivateKeyWithPassphrase(data, pass)
+		signer, err := parsePrivateKeyWithPassphrase(data, pass)
 		if err == nil {
 			return signer, nil
 		}
@@ -202,6 +202,15 @@ func unlockWithPassphrase(path string, data []byte) (ssh.Signer, error) {
 	return nil, fmt.Errorf("could not decrypt key in 3 attempts")
 }
 
+// parsePrivateKeyWithPassphrase handles ciphers beyond what x/crypto/ssh
+// supports (aes256-gcm@openssh.com, e.g. 1Password key exports).
+func parsePrivateKeyWithPassphrase(data, pass []byte) (ssh.Signer, error) {
+	if isGCMEncryptedKey(data) {
+		return parseGCMKey(data, pass)
+	}
+	return ssh.ParsePrivateKeyWithPassphrase(data, pass)
+}
+
 // agentAuth returns an auth method backed by the SSH agent if SSH_AUTH_SOCK is set.
 func agentAuth() ssh.AuthMethod {
 	sock := os.Getenv("SSH_AUTH_SOCK")

internal/sshclient/gcmkey.go

@@ -0,0 +1,122 @@
+package sshclient
+
+import (
+	"crypto/aes"
+	"crypto/cipher"
+	"encoding/pem"
+	"errors"
+	"fmt"
+
+	"golang.org/x/crypto/ssh"
+)
+
+// aes256-gcm@openssh.com is a cipher OpenSSH (and 1Password key exports) may
+// use for private key files. golang.org/x/crypto/ssh only supports aes256-ctr
+// and aes256-cbc, so these keys are decrypted here and re-marshalled as
+// unencrypted openssh-key-v1 blobs that ssh.ParsePrivateKey accepts.
+const (
+	opensshKeyMagic  = "openssh-key-v1\x00"
+	opensshGCMCipher = "aes256-gcm@openssh.com"
+	gcmTagSize       = 16
+)
+
+type opensshPrivateKeyHeader struct {
+	CipherName   string
+	KdfName      string
+	KdfOpts      string
+	NumKeys      uint32
+	PubKey       []byte
+	PrivKeyBlock []byte
+	// For AEAD ciphers (aes256-gcm@openssh.com) OpenSSH stores the 16-byte
+	// auth tag as raw bytes after the encrypted block, not inside it.
+	Rest []byte `ssh:"rest"`
+}
+
+// isGCMEncryptedKey reports whether data is an OpenSSH private key encrypted
+// with aes256-gcm@openssh.com.
+func isGCMEncryptedKey(data []byte) bool {
+	block, _ := pem.Decode(data)
+	if block == nil || block.Type != "OPENSSH PRIVATE KEY" {
+		return false
+	}
+	if len(block.Bytes) < len(opensshKeyMagic) || string(block.Bytes[:len(opensshKeyMagic)]) != opensshKeyMagic {
+		return false
+	}
+	var header struct {
+		CipherName string
+		Rest       []byte `ssh:"rest"`
+	}
+	if err := ssh.Unmarshal(block.Bytes[len(opensshKeyMagic):], &header); err != nil {
+		return false
+	}
+	return header.CipherName == opensshGCMCipher
+}
+
+// parseGCMKey decrypts an aes256-gcm@openssh.com encrypted OpenSSH private
+// key with passphrase and returns the signer.
+func parseGCMKey(data, passphrase []byte) (ssh.Signer, error) {
+	block, _ := pem.Decode(data)
+	if block == nil || len(block.Bytes) < len(opensshKeyMagic) {
+		return nil, errors.New("invalid OpenSSH private key PEM")
+	}
+	var w opensshPrivateKeyHeader
+	if err := ssh.Unmarshal(block.Bytes[len(opensshKeyMagic):], &w); err != nil {
+		return nil, err
+	}
+	if w.NumKeys != 1 {
+		return nil, errors.New("multi-key files are not supported")
+	}
+	if w.KdfName != "bcrypt" {
+		return nil, fmt.Errorf("unknown KDF %q, only supports %q", w.KdfName, "bcrypt")
+	}
+	var kdfOpts struct {
+		Salt   string
+		Rounds uint32
+	}
+	if err := ssh.Unmarshal([]byte(w.KdfOpts), &kdfOpts); err != nil {
+		return nil, err
+	}
+	// Same cap as x/crypto/ssh so a hostile file cannot tie up the caller.
+	const maxRounds = 1 << 11
+	if kdfOpts.Rounds > maxRounds {
+		return nil, fmt.Errorf("bcrypt KDF rounds %d exceed maximum %d", kdfOpts.Rounds, maxRounds)
+	}
+
+	// AES-256-GCM: 32-byte key + 12-byte nonce derived by bcrypt_pbkdf.
+	// The 16-byte auth tag follows the encrypted block as raw bytes;
+	// gcm.Open expects it appended to the ciphertext.
+	if len(w.Rest) != gcmTagSize {
+		return nil, fmt.Errorf("unexpected GCM tag length %d", len(w.Rest))
+	}
+	k, err := bcryptPBKDF(passphrase, []byte(kdfOpts.Salt), int(kdfOpts.Rounds), 32+12)
+	if err != nil {
+		return nil, err
+	}
+	aesCipher, err := aes.NewCipher(k[:32])
+	if err != nil {
+		return nil, err
+	}
+	gcm, err := cipher.NewGCM(aesCipher)
+	if err != nil {
+		return nil, err
+	}
+	sealed := make([]byte, 0, len(w.PrivKeyBlock)+gcmTagSize)
+	sealed = append(sealed, w.PrivKeyBlock...)
+	sealed = append(sealed, w.Rest...)
+	plain, err := gcm.Open(nil, k[32:], sealed, nil)
+	if err != nil {
+		return nil, err
+	}
+
+	// Re-marshal as an unencrypted key (tag dropped). The plaintext keeps
+	// its original OpenSSH padding, which ssh.ParsePrivateKey validates.
+	w.CipherName = "none"
+	w.KdfName = "none"
+	w.KdfOpts = ""
+	w.PrivKeyBlock = plain
+	w.Rest = nil
+	raw := make([]byte, 0, len(opensshKeyMagic)+len(plain)+128)
+	raw = append(raw, opensshKeyMagic...)
+	raw = append(raw, ssh.Marshal(&w)...)
+	return ssh.ParsePrivateKey(pem.EncodeToMemory(&pem.Block{Type: "OPENSSH PRIVATE KEY", Bytes: raw}))
+}