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GP: LOAD/STORE DATA chunk size from SCP overhead

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SCP.overhead was so far set at construction time (SCP02: 8, SCP03:
s_mode), so the C-MAC length only.
Unfortunately sec lvl >= 3 pads the data field to the cipher block size
before encryption, so the real worst-case overhead is larger,
scc.max_cmd_len (255 - overhead) was too big, and ADF_SD.load()
used a hardcoded chunk_len=240.

Real world issue with a 286 byte CAP + SCP02 + sec lvl 3:
- 240-byte LOAD block is padded to 248,
- encrypted
- gets 8 byte C-MAC appended
-> Lc = 256
That dies with a weird "ValueError: bytes must be in range(0, 256)".
The only "fix" for that was to downgrade the seclevel.

STORE DATA has the same overflow with large max_cmd_len
(247 + padding + MAC = 256 as well).

Therefore the overhead must be properly calculated from the sec level.

While at it adjust the error in case I missed something to get a more
useful ValueError.

Change-Id: Ic208f3959a38896f64fb6ccefb24cc360a3ac3a2
This commit is contained in:
Eric Wild
2026-09-07 19:51:23 +02:00
parent 078ac2bf19
commit e03530f89a
3 changed files with 290 additions and 12 deletions
Download Patch File Download Diff File Expand all files Collapse all files
pySim/global_platform/__init__.py
+25 -6
View File
@@ -707,6 +707,14 @@ class ADF_SD(CardADF):
"""Perform the GlobalPlatform PUT KEY command in order to store a new key on the card.
See GlobalPlatform CardSpecification v2.3 Section 11.8 for details."""
key_data = self.build_put_key_data(kvn, keys, self._cmd.lchan.scc.scp)
# Lc of Table 11-64 is a single byte, while LOAD or STORE DATA splits we can't:
# 11.8.2.3.3 splits a key at component boundaries -> not helping here
max_cmd_len = self._cmd.lchan.scc.max_cmd_len
if len(key_data) > max_cmd_len:
raise ValueError('key data field of %u bytes exceeds the maximum command length of %u '
'(limited by the overhead of the current secure channel); use fewer '
'keys per command, a single key component that large needs STORE DATA' %
(len(key_data), max_cmd_len))
hdr = "80D8%02x%02x%02x" % (old_kvn, kid, len(key_data))
data, _sw = self._cmd.lchan.scc.send_apdu_checksw(hdr + b2h(key_data) + "00")
return data
@@ -886,23 +894,32 @@ class ADF_SD(CardADF):
load_parser_from_grp.add_argument('--from-hex', type=is_hexstr, help='load from hex string')
load_parser_from_grp.add_argument('--from-file', type=argparse.FileType('rb', 0), help='load from binary file')
load_parser_from_grp.add_argument('--from-cap-file', type=argparse.FileType('rb', 0), help='load from JAVA-card CAP file')
load_parser.add_argument('--chunk-len', type=auto_uint8, default=None,
help='Block size for the LOAD command; default: as large as the current secure channel overhead permits, at most 240')
@cmd2.with_argparser(load_parser)
def do_load(self, opts):
"""Perform a GlobalPlatform LOAD command. (We currently only support loading without DAP and
without ciphering.)"""
if opts.from_hex is not None:
self.load(h2b(opts.from_hex))
self.load(h2b(opts.from_hex), opts.chunk_len)
elif opts.from_file is not None:
self.load(opts.from_file.read())
self.load(opts.from_file.read(), opts.chunk_len)
elif opts.from_cap_file is not None:
cap = CapFile(opts.from_cap_file)
self.load(cap.get_loadfile())
self.load(cap.get_loadfile(), opts.chunk_len)
else:
raise ValueError('load source not specified!')
def load(self, contents:bytes, chunk_len:int = 240):
# TODO:tune chunk_len based on the overhead of the used SCP?
def load(self, contents:bytes, chunk_len:Optional[int] = None):
# scc.max_cmd_len knows the overhead the currently active SCP
# 240 is the old default, keep it for now.
max_chunk_len = self._cmd.lchan.scc.max_cmd_len
if chunk_len is None:
chunk_len = min(240, max_chunk_len)
elif not 1 <= chunk_len <= max_chunk_len:
raise ValueError('chunk_len must be in range 1..%u (limited by the overhead of the current secure channel)' %
max_chunk_len)
# build TLV according to GPC_SPE_034 section 11.6.2.3 / Table 11-58 for unencrypted case
remainder = b'\xC4' + bertlv_encode_len(len(contents)) + contents
# transfer this in various chunks to the card
@@ -941,6 +958,8 @@ class ADF_SD(CardADF):
install_cap_parser_inst_prm_grp.add_argument('--install-parameters-stk',
type=is_hexstr, default=None,
help='Load Parameters (ETSI TS 102 226, section 8.2.1.3.2.1)')
install_cap_parser.add_argument('--chunk-len', type=auto_uint8, default=None,
help='Block size for the LOAD command; default: as large as the current secure channel overhead permits, at most 240')
@cmd2.with_argparser(install_cap_parser)
def do_install_cap(self, opts):
@@ -979,7 +998,7 @@ class ADF_SD(CardADF):
self._cmd.poutput("step #1: install for load...")
self.do_install_for_load("--load-file-aid %s --security-domain-aid %s" % (load_file_aid, security_domain_aid))
self._cmd.poutput("step #2: load...")
self.load(load_file)
self.load(load_file, opts.chunk_len)
self._cmd.poutput("step #3: install_for_install (and make selectable)...")
self.do_install_for_install("--load-file-aid %s --module-aid %s --application-aid %s --install-parameters %s --make-selectable" %
(load_file_aid, module_aid, application_aid, install_parameters))
pySim/global_platform/scp.py
+37 -6
View File
@@ -182,6 +182,29 @@ class SCP(SecureChannel, abc.ABC):
"""Should we perform R-ENC?"""
return self.security_level & 0x20
@property
@abc.abstractmethod
def mac_len(self) -> int:
"""Length of the appended C-MAC, to be provided by derived class."""
@property
def overhead(self) -> int:
"""Worst-case len that wrapping a command APDU adds to its data field at the
current sec level is (255 - overhead), C-MAC + C-DECRYPTION encryption padding."""
if not self.do_cmac:
return 0
if not self.do_cenc:
return self.mac_len
# see Secure Channel Protocol '03' Card Specification v2.3 - Amendment D v1.1.2
# which defers to GPCS v2.3 Section B.2 which then defers to
# NIST SP 800-38B for encryption and points out that
# the padding is, as expected, just the usual padding from NIST SP 800-38A
# C-DECRYPTION pads with ('80'+['00'...] at least 1 byte) up to
# the cipher block size + C-MAC on top -> largest usable data field
# is one byte less than the largest block-size multiple within 255 - mac_len.
bs = self.sk.blocksize
return 255 - ((255 - self.mac_len) // bs * bs - 1)
def __str__(self) -> str:
return "%s[%02x]" % (self.__class__.__name__, self.security_level)
@@ -268,10 +291,8 @@ class SCP02(SCP):
# Key Version Number 0x70 is a non-spec special-case of sysmoISIM-SJA2/SJA5 and possibly more sysmocom products
# Key Version Number 0x01 is a non-spec special-case of sysmoUSIM-SJS1
kvn_ranges = [[0x01, 0x01], [0x20, 0x2f], [0x70, 0x70]]
def __init__(self, *args, **kwargs):
self.overhead = 8
super().__init__(*args, **kwargs)
# C-MAC (Single DES + final 3DES, B.1.2.2) is always one full DES block
mac_len = 8
def dek_encrypt(self, plaintext:bytes) -> bytes:
# See also GPC section B.1.1.2, E.4.7, and E.4.1
@@ -346,10 +367,16 @@ class SCP02(SCP):
# CMAC on modified APDU
mlc = lc + 8
clac = cla | CLA_SM
if mlc >= 256:
raise ValueError('Modified Lc (%u) would exceed maximum when appending 8 bytes of mac' % mlc)
mac = self.sk.calc_mac_1des(bytes([clac]) + apdu[1:4] + bytes([mlc]) + data)
if self.do_cenc:
padded_data = pad80(data, 8)
if len(padded_data) + 8 >= 256:
raise ValueError('Modified Lc (%u) would exceed maximum when appending padding and mac' %
(len(padded_data) + 8))
k = DES3.new(self.sk.enc, DES.MODE_CBC, b'\x00'*8)
data = k.encrypt(pad80(data, 8))
data = k.encrypt(padded_data)
lc = len(data)
lc += 8
@@ -485,9 +512,13 @@ class SCP03(SCP):
def __init__(self, *args, **kwargs):
self.s_mode = kwargs.pop('s_mode', 8)
self.overhead = self.s_mode
super().__init__(*args, **kwargs)
@property
def mac_len(self) -> int:
# C-MAC truncated to 8 in S8 or 16 bytes in S16 mode
return self.s_mode
def dek_encrypt(self, plaintext:bytes) -> bytes:
cipher = AES.new(self.card_keys.dek, AES.MODE_CBC, b'\x00'*16)
return cipher.encrypt(plaintext)