package provisioner import ( "context" "encoding/base64" "strconv" "time" "github.com/pkg/errors" "github.com/smallstep/certificates/db" "github.com/smallstep/cli/jose" "golang.org/x/crypto/ssh" ) // sshPOPPayload extends jwt.Claims with step attributes. type sshPOPPayload struct { jose.Claims SANs []string `json:"sans,omitempty"` Step *stepPayload `json:"step,omitempty"` sshCert *ssh.Certificate } // SSHPOP is the default provisioner, an entity that can sign tokens necessary for // signature requests. type SSHPOP struct { *base Type string `json:"type"` Name string `json:"name"` Claims *Claims `json:"claims,omitempty"` db db.AuthDB claimer *Claimer audiences Audiences sshPubKeys *SSHKeys } // GetID returns the provisioner unique identifier. The name and credential id // should uniquely identify any SSH-POP provisioner. func (p *SSHPOP) GetID() string { return "sshpop/" + p.Name } // GetTokenID returns the identifier of the token. func (p *SSHPOP) GetTokenID(ott string) (string, error) { // Validate payload token, err := jose.ParseSigned(ott) if err != nil { return "", errors.Wrap(err, "error parsing token") } // Get claims w/out verification. We need to look up the provisioner // key in order to verify the claims and we need the issuer from the claims // before we can look up the provisioner. var claims jose.Claims if err = token.UnsafeClaimsWithoutVerification(&claims); err != nil { return "", errors.Wrap(err, "error verifying claims") } return claims.ID, nil } // GetName returns the name of the provisioner. func (p *SSHPOP) GetName() string { return p.Name } // GetType returns the type of provisioner. func (p *SSHPOP) GetType() Type { return TypeSSHPOP } // GetEncryptedKey returns the base provisioner encrypted key if it's defined. func (p *SSHPOP) GetEncryptedKey() (string, string, bool) { return "", "", false } // Init initializes and validates the fields of a SSHPOP type. func (p *SSHPOP) Init(config Config) error { switch { case p.Type == "": return errors.New("provisioner type cannot be empty") case p.Name == "": return errors.New("provisioner name cannot be empty") case config.SSHKeys == nil: return errors.New("provisioner public SSH validation keys cannot be empty") } // Update claims with global ones var err error if p.claimer, err = NewClaimer(p.Claims, config.Claims); err != nil { return err } p.audiences = config.Audiences.WithFragment(p.GetID()) p.db = config.DB p.sshPubKeys = config.SSHKeys return nil } // authorizeToken performs common jwt authorization actions and returns the // claims for case specific downstream parsing. // e.g. a Sign request will auth/validate different fields than a Revoke request. func (p *SSHPOP) authorizeToken(token string, audiences []string) (*sshPOPPayload, error) { sshCert, err := ExtractSSHPOPCert(token) if err != nil { return nil, errors.Wrap(err, "authorizeToken ssh-pop") } // Check for revocation. if isRevoked, err := p.db.IsSSHRevoked(strconv.FormatUint(sshCert.Serial, 10)); err != nil { return nil, errors.Wrap(err, "authorizeToken ssh-pop") } else if isRevoked { return nil, errors.New("authorizeToken ssh-pop: ssh certificate has been revoked") } jwt, err := jose.ParseSigned(token) if err != nil { return nil, errors.Wrapf(err, "error parsing token") } // Check validity period of the certificate. n := time.Now() if sshCert.ValidAfter != 0 && time.Unix(int64(sshCert.ValidAfter), 0).After(n) { return nil, errors.New("sshpop certificate validAfter is in the future") } if sshCert.ValidBefore != 0 && time.Unix(int64(sshCert.ValidBefore), 0).Before(n) { return nil, errors.New("sshpop certificate validBefore is in the past") } sshCryptoPubKey, ok := sshCert.Key.(ssh.CryptoPublicKey) if !ok { return nil, errors.New("ssh public key could not be cast to ssh CryptoPublicKey") } pubKey := sshCryptoPubKey.CryptoPublicKey() var ( found bool data = bytesForSigning(sshCert) keys []ssh.PublicKey ) if sshCert.CertType == ssh.UserCert { keys = p.sshPubKeys.UserKeys } else { keys = p.sshPubKeys.HostKeys } for _, k := range keys { if err = (&ssh.Certificate{Key: k}).Verify(data, sshCert.Signature); err == nil { found = true break } } if !found { return nil, errors.New("error: provisioner could could not verify the sshpop header certificate") } // Using the ssh certificates key to validate the claims accomplishes two // things: // 1. Asserts that the private key used to sign the token corresponds // to the public certificate in the `sshpop` header of the token. // 2. Asserts that the claims are valid - have not been tampered with. var claims sshPOPPayload if err = jwt.Claims(pubKey, &claims); err != nil { return nil, errors.Wrap(err, "error parsing claims") } // According to "rfc7519 JSON Web Token" acceptable skew should be no // more than a few minutes. if err = claims.ValidateWithLeeway(jose.Expected{ Issuer: p.Name, Time: time.Now().UTC(), }, time.Minute); err != nil { return nil, errors.Wrapf(err, "invalid token") } // validate audiences with the defaults if !matchesAudience(claims.Audience, audiences) { return nil, errors.New("invalid token: invalid audience claim (aud)") } if claims.Subject == "" { return nil, errors.New("token subject cannot be empty") } claims.sshCert = sshCert return &claims, nil } // AuthorizeSSHRevoke validates the authorization token and extracts/validates // the SSH certificate from the ssh-pop header. func (p *SSHPOP) AuthorizeSSHRevoke(ctx context.Context, token string) error { claims, err := p.authorizeToken(token, p.audiences.SSHRevoke) if err != nil { return err } if claims.Subject != strconv.FormatUint(claims.sshCert.Serial, 10) { return errors.New("token subject must be equivalent to certificate serial number") } return err } // AuthorizeSSHRenew validates the authorization token and extracts/validates // the SSH certificate from the ssh-pop header. func (p *SSHPOP) AuthorizeSSHRenew(ctx context.Context, token string) (*ssh.Certificate, error) { claims, err := p.authorizeToken(token, p.audiences.SSHRenew) if err != nil { return nil, err } if claims.sshCert.CertType != ssh.HostCert { return nil, errors.New("sshpop AuthorizeSSHRenew: sshpop certificate must be a host ssh certificate") } return claims.sshCert, nil } // AuthorizeSSHRekey validates the authorization token and extracts/validates // the SSH certificate from the ssh-pop header. func (p *SSHPOP) AuthorizeSSHRekey(ctx context.Context, token string) (*ssh.Certificate, []SignOption, error) { claims, err := p.authorizeToken(token, p.audiences.SSHRekey) if err != nil { return nil, nil, err } if claims.sshCert.CertType != ssh.HostCert { return nil, nil, errors.New("sshpop AuthorizeSSHRekey: sshpop certificate must be a host ssh certificate") } return claims.sshCert, []SignOption{ // Validate public key &sshDefaultPublicKeyValidator{}, // Validate the validity period. &sshCertificateValidityValidator{p.claimer}, // Require and validate all the default fields in the SSH certificate. &sshCertificateDefaultValidator{}, }, nil } // ExtractSSHPOPCert parses a JWT and extracts and loads the SSH Certificate // in the sshpop header. If the header is missing, an error is returned. func ExtractSSHPOPCert(token string) (*ssh.Certificate, error) { jwt, err := jose.ParseSigned(token) if err != nil { return nil, errors.Wrapf(err, "error parsing token") } encodedSSHCert, ok := jwt.Headers[0].ExtraHeaders["sshpop"] if !ok { return nil, errors.New("token missing sshpop header") } encodedSSHCertStr, ok := encodedSSHCert.(string) if !ok { return nil, errors.New("error unexpected type for sshpop header") } sshCertBytes, err := base64.StdEncoding.DecodeString(encodedSSHCertStr) if err != nil { return nil, errors.Wrap(err, "error decoding sshpop header") } sshPub, err := ssh.ParsePublicKey(sshCertBytes) if err != nil { return nil, errors.Wrap(err, "error parsing ssh public key") } sshCert, ok := sshPub.(*ssh.Certificate) if !ok { return nil, errors.New("error converting ssh public key to ssh certificate") } return sshCert, nil } func bytesForSigning(cert *ssh.Certificate) []byte { c2 := *cert c2.Signature = nil out := c2.Marshal() // Drop trailing signature length. return out[:len(out)-4] }