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crypto/xml_encrypt.py
XML Encryption for SAML assertions: AES-256-GCM payload, RSA-OAEP key wrap.
Pure stdlib + cryptography library.
"""
import base64
import os
from xml.etree import ElementTree as ET
from cryptography.hazmat.primitives import hashes
from cryptography.hazmat.primitives.asymmetric import padding
from cryptography.hazmat.primitives.ciphers.aead import AESGCM
from cryptography.x509 import Certificate
XENC_NS = "http://www.w3.org/2001/04/xmlenc#"
DSIG_NS = "http://www.w3.org/2000/09/xmldsig#"
_XE = "{%s}" % XENC_NS
_DS = "{%s}" % DSIG_NS
AES256_GCM_ALG = "http://www.w3.org/2009/xmlenc11#aes256-gcm"
RSA_OAEP_ALG = "http://www.w3.org/2001/04/xmlenc#rsa-oaep-mgf1p"
class DecryptionError(Exception):
pass
def encrypt_assertion(assertion: ET.Element, sp_cert: Certificate) -> ET.Element:
"""
Wrap a saml:Assertion in an xenc:EncryptedAssertion.
Uses AES-256-GCM for the assertion, RSA-OAEP for the symmetric key.
"""
from crypto.xml_dsig import exclusive_c14n
plaintext = exclusive_c14n(assertion)
# Generate AES-256-GCM key + nonce
aes_key = os.urandom(32)
nonce = os.urandom(12)
aesgcm = AESGCM(aes_key)
ciphertext = aesgcm.encrypt(nonce, plaintext, None) # ciphertext includes 16-byte GCM tag
# RSA-OAEP encrypt the AES key
encrypted_key = sp_cert.public_key().encrypt(
aes_key,
padding.OAEP(mgf=padding.MGF1(hashes.SHA256()), algorithm=hashes.SHA256(), label=None),
)
# Build xenc:EncryptedAssertion tree
enc_assertion = ET.Element('{urn:oasis:names:tc:SAML:2.0:assertion}EncryptedAssertion')
enc_data = ET.SubElement(enc_assertion, _XE + 'EncryptedData')
enc_data.set('Type', 'http://www.w3.org/2001/04/xmlenc#Element')
enc_method = ET.SubElement(enc_data, _XE + 'EncryptionMethod')
enc_method.set('Algorithm', AES256_GCM_ALG)
key_info = ET.SubElement(enc_data, _DS + 'KeyInfo')
enc_key_el = ET.SubElement(key_info, _XE + 'EncryptedKey')
key_enc_method = ET.SubElement(enc_key_el, _XE + 'EncryptionMethod')
key_enc_method.set('Algorithm', RSA_OAEP_ALG)
key_cipher = ET.SubElement(enc_key_el, _XE + 'CipherData')
key_cipher_val = ET.SubElement(key_cipher, _XE + 'CipherValue')
key_cipher_val.text = base64.b64encode(encrypted_key).decode('ascii')
cipher_data = ET.SubElement(enc_data, _XE + 'CipherData')
cipher_value = ET.SubElement(cipher_data, _XE + 'CipherValue')
# Store nonce prepended to ciphertext
cipher_value.text = base64.b64encode(nonce + ciphertext).decode('ascii')
return enc_assertion
def decrypt_assertion(encrypted_assertion: ET.Element, sp_private_key) -> ET.Element:
"""
Decrypt an xenc:EncryptedAssertion back to saml:Assertion.
"""
enc_data = encrypted_assertion.find(_XE + 'EncryptedData')
if enc_data is None:
raise DecryptionError("No xenc:EncryptedData found")
# Extract encrypted AES key
key_cipher_val = encrypted_assertion.find(
f".//{_XE}EncryptedKey/{_XE}CipherData/{_XE}CipherValue"
)
if key_cipher_val is None:
raise DecryptionError("No encrypted key found")
encrypted_key = base64.b64decode(key_cipher_val.text.strip())
# Decrypt AES key
try:
aes_key = sp_private_key.decrypt(
encrypted_key,
padding.OAEP(mgf=padding.MGF1(hashes.SHA256()), algorithm=hashes.SHA256(), label=None),
)
except Exception as e:
raise DecryptionError(f"Key decryption failed: {e}")
# Extract ciphertext
cipher_val_el = enc_data.find(f".//{_XE}CipherData/{_XE}CipherValue")
if cipher_val_el is None:
raise DecryptionError("No CipherValue found")
raw = base64.b64decode(cipher_val_el.text.strip())
nonce = raw[:12]
ciphertext = raw[12:]
# Decrypt assertion
aesgcm = AESGCM(aes_key)
try:
plaintext = aesgcm.decrypt(nonce, ciphertext, None)
except Exception as e:
raise DecryptionError(f"AES-GCM decryption failed: {e}")
return ET.fromstring(plaintext.decode('utf-8'))
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