gitea/models/asymkey/ssh_key_parse.go

// Copyright 2021 The Gitea Authors. All rights reserved.
// SPDX-License-Identifier: MIT

package asymkey

import (
	"crypto/rsa"
	"crypto/x509"
	"encoding/asn1"
	"encoding/base64"
	"encoding/binary"
	"encoding/pem"
	"errors"
	"fmt"
	"math/big"
	"strings"

	"code.gitea.io/gitea/modules/log"
	"code.gitea.io/gitea/modules/setting"
	"code.gitea.io/gitea/modules/util"

	"golang.org/x/crypto/ssh"
)

//  ____  __.             __________
// |    |/ _|____ ___.__. \______   \_____ _______  ______ ___________
// |      <_/ __ <   |  |  |     ___/\__  \\_  __ \/  ___// __ \_  __ \
// |    |  \  ___/\___  |  |    |     / __ \|  | \/\___ \\  ___/|  | \/
// |____|__ \___  > ____|  |____|    (____  /__|  /____  >\___  >__|
//         \/   \/\/                      \/           \/     \/
//
// This file contains functions for parsing ssh-keys
//
// TODO: Consider if these functions belong in models - no other models function call them or are called by them
// They may belong in a service or a module

const ssh2keyStart = "---- BEGIN SSH2 PUBLIC KEY ----"

func extractTypeFromBase64Key(key string) (string, error) {
	b, err := base64.StdEncoding.DecodeString(key)
	if err != nil || len(b) < 4 {
		return "", fmt.Errorf("invalid key format: %w", err)
	}

	keyLength := int(binary.BigEndian.Uint32(b))
	if len(b) < 4+keyLength {
		return "", fmt.Errorf("invalid key format: not enough length %d", keyLength)
	}

	return string(b[4 : 4+keyLength]), nil
}

// parseKeyString parses any key string in OpenSSH or SSH2 format to clean OpenSSH string (RFC4253).
func parseKeyString(content string) (string, error) {
	// remove whitespace at start and end
	content = strings.TrimSpace(content)

	var keyType, keyContent, keyComment string

	if strings.HasPrefix(content, ssh2keyStart) {
		// Parse SSH2 file format.

		// Transform all legal line endings to a single "\n".
		content = strings.NewReplacer("\r\n", "\n", "\r", "\n").Replace(content)

		lines := strings.Split(content, "\n")
		continuationLine := false

		for _, line := range lines {
			// Skip lines that:
			// 1) are a continuation of the previous line,
			// 2) contain ":" as that are comment lines
			// 3) contain "-" as that are begin and end tags
			if continuationLine || strings.ContainsAny(line, ":-") {
				continuationLine = strings.HasSuffix(line, "\\")
			} else {
				keyContent += line
			}
		}

		t, err := extractTypeFromBase64Key(keyContent)
		if err != nil {
			return "", fmt.Errorf("extractTypeFromBase64Key: %w", err)
		}
		keyType = t
	} else {
		if strings.Contains(content, "-----BEGIN") {
			// Convert PEM Keys to OpenSSH format
			// Transform all legal line endings to a single "\n".
			content = strings.NewReplacer("\r\n", "\n", "\r", "\n").Replace(content)

			block, _ := pem.Decode([]byte(content))
			if block == nil {
				return "", errors.New("failed to parse PEM block containing the public key")
			}
			if strings.Contains(block.Type, "PRIVATE") {
				return "", ErrKeyIsPrivate
			}

			pub, err := x509.ParsePKIXPublicKey(block.Bytes)
			if err != nil {
				var pk rsa.PublicKey
				_, err2 := asn1.Unmarshal(block.Bytes, &pk)
				if err2 != nil {
					return "", fmt.Errorf("failed to parse DER encoded public key as either PKIX or PEM RSA Key: %v %w", err, err2)
				}
				pub = &pk
			}

			sshKey, err := ssh.NewPublicKey(pub)
			if err != nil {
				return "", fmt.Errorf("unable to convert to ssh public key: %w", err)
			}
			content = string(ssh.MarshalAuthorizedKey(sshKey))
		}
		// Parse OpenSSH format.

		// Remove all newlines
		content = strings.NewReplacer("\r\n", "", "\n", "").Replace(content)

		parts := strings.SplitN(content, " ", 3)
		switch len(parts) {
		case 0:
			return "", util.NewInvalidArgumentErrorf("empty key")
		case 1:
			keyContent = parts[0]
		case 2:
			keyType = parts[0]
			keyContent = parts[1]
		default:
			keyType = parts[0]
			keyContent = parts[1]
			keyComment = parts[2]
		}

		// If keyType is not given, extract it from content. If given, validate it.
		t, err := extractTypeFromBase64Key(keyContent)
		if err != nil {
			return "", fmt.Errorf("extractTypeFromBase64Key: %w", err)
		}
		if len(keyType) == 0 {
			keyType = t
		} else if keyType != t {
			return "", fmt.Errorf("key type and content does not match: %s - %s", keyType, t)
		}
	}
	// Finally we need to check whether we can actually read the proposed key:
	_, _, _, _, err := ssh.ParseAuthorizedKey([]byte(keyType + " " + keyContent + " " + keyComment))
	if err != nil {
		return "", fmt.Errorf("invalid ssh public key: %w", err)
	}
	return keyType + " " + keyContent + " " + keyComment, nil
}

// CheckPublicKeyString checks if the given public key string is recognized by SSH.
// It returns the actual public key line on success.
func CheckPublicKeyString(content string) (_ string, err error) {
	content, err = parseKeyString(content)
	if err != nil {
		return "", err
	}

	content = strings.TrimRight(content, "\n\r")
	if strings.ContainsAny(content, "\n\r") {
		return "", util.NewInvalidArgumentErrorf("only a single line with a single key please")
	}

	// remove any unnecessary whitespace now
	content = strings.TrimSpace(content)

	if !setting.SSH.MinimumKeySizeCheck {
		return content, nil
	}

	keyType, length, err := SSHNativeParsePublicKey(content)
	if err != nil {
		return "", fmt.Errorf("SSHNativeParsePublicKey: %w", err)
	}
	log.Trace("Key info [native: %v]: %s-%d", setting.SSH.StartBuiltinServer, keyType, length)

	if minLen, found := setting.SSH.MinimumKeySizes[keyType]; found && length >= minLen {
		return content, nil
	} else if found && length < minLen {
		return "", fmt.Errorf("key length is not enough: got %d, needs %d", length, minLen)
	}
	return "", fmt.Errorf("key type is not allowed: %s", keyType)
}

// SSHNativeParsePublicKey extracts the key type and length using the golang SSH library.
func SSHNativeParsePublicKey(keyLine string) (string, int, error) {
	fields := strings.Fields(keyLine)
	if len(fields) < 2 {
		return "", 0, fmt.Errorf("not enough fields in public key line: %s", keyLine)
	}

	raw, err := base64.StdEncoding.DecodeString(fields[1])
	if err != nil {
		return "", 0, err
	}

	pkey, err := ssh.ParsePublicKey(raw)
	if err != nil {
		if strings.Contains(err.Error(), "ssh: unknown key algorithm") {
			return "", 0, ErrKeyUnableVerify{err.Error()}
		}
		return "", 0, fmt.Errorf("ParsePublicKey: %w", err)
	}

	// The ssh library can parse the key, so next we find out what key exactly we have.
	switch pkey.Type() {
	case ssh.KeyAlgoDSA:
		rawPub := struct {
			Name       string
			P, Q, G, Y *big.Int
		}{}
		if err := ssh.Unmarshal(pkey.Marshal(), &rawPub); err != nil {
			return "", 0, err
		}
		// as per https://bugzilla.mindrot.org/show_bug.cgi?id=1647 we should never
		// see dsa keys != 1024 bit, but as it seems to work, we will not check here
		return "dsa", rawPub.P.BitLen(), nil // use P as per crypto/dsa/dsa.go (is L)
	case ssh.KeyAlgoRSA:
		rawPub := struct {
			Name string
			E    *big.Int
			N    *big.Int
		}{}
		if err := ssh.Unmarshal(pkey.Marshal(), &rawPub); err != nil {
			return "", 0, err
		}
		return "rsa", rawPub.N.BitLen(), nil // use N as per crypto/rsa/rsa.go (is bits)
	case ssh.KeyAlgoECDSA256:
		return "ecdsa", 256, nil
	case ssh.KeyAlgoECDSA384:
		return "ecdsa", 384, nil
	case ssh.KeyAlgoECDSA521:
		return "ecdsa", 521, nil
	case ssh.KeyAlgoED25519:
		return "ed25519", 256, nil
	case ssh.KeyAlgoSKECDSA256:
		return "ecdsa-sk", 256, nil
	case ssh.KeyAlgoSKED25519:
		return "ed25519-sk", 256, nil
	}
	return "", 0, fmt.Errorf("unsupported key length detection for type: %s", pkey.Type())
}