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signing

import "github.com/TheManticoreProject/Manticore/network/smb/smb_v10/signing"

Package signing implements SMB 1.0 message signing.

The primitives are direction-agnostic, because signing is symmetric: each side signs what it sends and verifies what it receives with the same key and the same digest. Only the sequence numbers differ, and which of them a party uses is the one thing that distinguishes a client from a server here.

The MAC key

The key is the ExportedSessionKey derived from the authentication exchange, used as-is. [MS-CIFS] 3.1.5.1 also defines a MACKey of SessionKey || NTLMResponse for the paths that do not derive a session key, but neither side of this implementation takes such a path: extended session security is required precisely so a key exists to sign with.

Sequence numbers

Signing is bootstrapped by the authentication exchange and then advances in pairs. A request carries an even sequence number and the response to it carries the odd number above, so the AUTHENTICATE request is signed at 0, its response at 1, and the next request at 2.

A client therefore consumes a number when it sends and expects the next one back; a server verifies at the number it expects and signs its reply at the one above. ResponseSequenceNumber and NextRequestSequenceNumber name the two steps so both sides derive them the same way rather than each spelling out an increment.

Source: https://learn.microsoft.com/en-us/openspecs/windows_protocols/ms-cifs/59ec6b57-7e1b-4a0e-bbdf-1a4f4e0e1d75

Index

Constants

SecuritySignatureOffset is the byte offset, from the start of the SMB header, of the 8-byte SecurityFeatures field that carries the signature: Protocol(4) + Command(1) + Status(4) + Flags(1) + Flags2(2) + PIDHigh(2) = 14.

const SecuritySignatureOffset = 14

SecuritySignatureSize is the size, in bytes, of the SecuritySignature field.

const SecuritySignatureSize = 8

func NextRequestSequenceNumber

func NextRequestSequenceNumber(requestSequenceNumber uint32) uint32

NextRequestSequenceNumber returns the sequence number the following request must carry, given the number the current one was signed at. A request and its response consume one number each, so the next request skips both.

Parameters:

  • requestSequenceNumber: the number the current request was signed at

Returns:

  • The number the next request is signed at

func ResponseSequenceNumber

func ResponseSequenceNumber(requestSequenceNumber uint32) uint32

ResponseSequenceNumber returns the sequence number the response to a request must carry, given the number the request was signed at.

Parameters:

  • requestSequenceNumber: the number the request was signed at

Returns:

  • The number its response is signed at

func Sign

func Sign(macKey, message []byte, sequenceNumber uint32)

Sign computes the signature for a marshalled message and writes it into the message’s SecuritySignature field in place.

A message too short to carry a signature is left alone: there is no field to write into, and the caller’s own framing will have already rejected it.

Parameters:

  • macKey: the MAC key
  • message: the marshalled SMB message, modified in place
  • sequenceNumber: the sequence number to sign at

func Verify

func Verify(macKey, message []byte, sequenceNumber uint32) bool

Verify reports whether a marshalled message carries a valid signature for the given key and sequence number. The comparison is constant-time.

Parameters:

  • macKey: the MAC key
  • message: the marshalled SMB message as received
  • sequenceNumber: the sequence number the message is expected to be signed at

Returns:

  • true when the signature is valid

type Signature

Signature is an SMB 1.0 message signature.

type Signature = [SecuritySignatureSize]byte

func Compute

func Compute(macKey, message []byte, sequenceNumber uint32) Signature

Compute returns the signature for a fully marshalled SMB message.

The signature is the first 8 bytes of MD5(MACKey || message), where the message’s SecuritySignature field has first been replaced by the 32-bit sequence number in little-endian order followed by four zero bytes. The input slice is not modified.

Parameters:

  • macKey: the MAC key, which is the exported session key
  • message: the marshalled SMB message, header included
  • sequenceNumber: the sequence number this message is signed at

Returns:

  • The signature