def decryptKey(alg, enc, jwk, encryptedKey, header={}):
"""
Decrypt a JWE encrypted key.
@type alg: string
@param alg: The JWE "alg" value specifying the key management algorithm
@type enc: string
@param enc: The JWE "enc" value specifying the encryption algorithm
@type jwk: dict
@param jwk: The key to be used, as an unserialized JWK object
@type encryptedKey: bytes
@param encryptedKey: The key to decrypt
@type header: dict
@param header: A header object containing additional parameters
@rtype: bytes
@return: The decrypted key
"""
key = importKey(jwk, private=True)
if alg == "RSA1_5":
sentinel = "fnord"
cipher = PKCS1_v1_5.new(key)
CEK = cipher.decrypt(encryptedKey, sentinel)
if CEK == sentinel:
raise Exception("Unable to unwrap key")
return CEK
elif alg == "RSA-OAEP":
cipher = PKCS1_OAEP.new(key)
CEK = cipher.decrypt(encryptedKey)
return CEK
elif alg in ["A128KW", "A192KW", "A256KW"]:
return polyfills.aes_key_unwrap(key, encryptedKey)
elif alg == "dir":
if encryptedKey and len(encryptedKey) > 0:
raise Exception("Direct encryption with non-empty encrypted key")
return key
elif alg in ["ECDH-ES", "ECDH-ES+A128KW", "ECDH-ES+A192KW", "ECDH-ES+A256KW"]:
# Pull the input parameters
epk = importKey(getOrRaise(header, "epk"))
apu = b64dec(getOrRaise(header, "apu"))
apv = b64dec(getOrRaise(header, "apv"))
# Select the curve
curve = NISTEllipticCurve.byName(getOrRaise(header["epk"], "crv"))
# Derive the KEK and decrypt
if alg == "ECDH-ES":
dkLen = keyLength(enc)
return polyfills.ECDH_deriveKey(curve, key, epk, apu, apv, enc, dkLen)
elif alg == "ECDH-ES+A128KW":
kek = polyfills.ECDH_deriveKey(curve, key, epk, apu, apv, "A128KW", 128)
return polyfills.aes_key_unwrap(kek, encryptedKey)
elif alg == "ECDH-ES+A192KW":
kek = polyfills.ECDH_deriveKey(curve, key, epk, apu, apv, "A192KW", 192)
return polyfills.aes_key_unwrap(kek, encryptedKey)
elif alg == "ECDH-ES+A256KW":
kek = polyfills.ECDH_deriveKey(curve, key, epk, apu, apv, "A256KW", 256)
return polyfills.aes_key_unwrap(kek, encryptedKey)
elif alg in ["A128GCMKW", "A192GCMKW", "A256GCMKW"]:
iv = b64dec(getOrRaise(header, "iv"))
tag = b64dec(getOrRaise(header, "tag"))
gcm = AES_GCM(bytes_to_long(key))
return gcm.decrypt(bytes_to_long(iv), encryptedKey, bytes_to_long(tag), '')
elif alg == "PBES2-HS256+A128KW":
return polyfills.PBKDF_key_unwrap(key, encryptedKey, SHA256, 16, header)
elif alg == "PBES2-HS384+A192KW":
return polyfills.PBKDF_key_unwrap(key, encryptedKey, SHA384, 24, header)
elif alg == "PBES2-HS512+A256KW":
return polyfills.PBKDF_key_unwrap(key, encryptedKey, SHA512, 32, header)
else:
raise Exception("Unsupported key management algorithm " + alg)
def generateSenderParams(alg, enc, jwk, header={}, inCEK=None):
"""
Generate parameters for the sender of a JWE. This is essentially an
encryptKey method, except (1) in some cases, the key is specified directly
or derived ("dir", "ECDH"), and (2) other parameters besides the encrypted
key are generated (e.g., the IV).
This method returns several things:
- A random CEK (can be overridden with the inCEK parameter)
- The encrypted CEK
- A random IV
- A dictionary of parameters generated within this function
The idea is that the parameters generated within this function (e.g.,
"epk", "p2s") should be added back to the JWE header
@type alg: string
@param alg: The JWE "alg" value specifying the key management algorithm
@type enc: string
@param enc: The JWE "enc" value specifying the encryption algorithm
@type jwk: dict
@param jwk: The key to be used, as an unserialized JWK object
@type header: dict
@param header: A header object with additional parameters
@type inCEK: bytes
@param inCEK: A fixed CEK (overrides random CEK generation)
@rtype: tuple
@return: (CEK, encryptedKey, IV, params), the first three as bytes,
params as dict.
"""
# Generate a random key/iv for enc
(CEK, IV) = generateKeyIV(enc)
if inCEK:
CEK = inCEK
encryptedKey = ""
params = {}
key = importKey(jwk, private=False)
# Encrypt key / generate params as defined by alg
if alg == "RSA1_5":
cipher = PKCS1_v1_5.new(key)
encryptedKey = cipher.encrypt(CEK)
elif alg == "RSA-OAEP":
(CEK, IV) = generateKeyIV(enc)
cipher = PKCS1_OAEP.new(key)
encryptedKey = cipher.encrypt(CEK)
elif alg in ["A128KW", "A192KW", "A256KW"]:
encryptedKey = polyfills.aes_key_wrap(key, CEK)
elif alg == "dir":
CEK = key
elif alg in ["ECDH-ES", "ECDH-ES+A128KW", "ECDH-ES+A192KW", "ECDH-ES+A256KW"]:
# Generate the input parameters
apu = b64dec(header["apu"]) if "apu" in header else Random.get_random_bytes(16)
apv = b64dec(header["apv"]) if "apv" in header else Random.get_random_bytes(16)
# Generate an ephemeral key pair
curve = NISTEllipticCurve.byName(getOrRaise(jwk, "crv"))
if "epk" in header:
epk = importKey(header["epk"], private=False)
eprivk = importKey(header["epk"], private=True)
else:
(eprivk, epk) = curve.keyPair()
# Derive the KEK and encrypt
params = {
"apu": b64enc(apu),
"apv": b64enc(apv),
"epk": exportKey(epk, "EC", curve)
}
if alg == "ECDH-ES":
dkLen = keyLength(enc)
CEK = polyfills.ECDH_deriveKey(curve, eprivk, key, apu, apv, enc, dkLen)
elif alg == "ECDH-ES+A128KW":
kek = polyfills.ECDH_deriveKey(curve, eprivk, key, apu, apv, "A128KW", 128)
encryptedKey = polyfills.aes_key_wrap(kek, CEK)
elif alg == "ECDH-ES+A192KW":
kek = polyfills.ECDH_deriveKey(curve, eprivk, key, apu, apv, "A192KW", 192)
encryptedKey = polyfills.aes_key_wrap(kek, CEK)
elif alg == "ECDH-ES+A256KW":
kek = polyfills.ECDH_deriveKey(curve, eprivk, key, apu, apv, "A256KW", 256)
encryptedKey = polyfills.aes_key_wrap(kek, CEK)
elif alg in ["A128GCMKW", "A192GCMKW", "A256GCMKW"]:
iv = Random.get_random_bytes(12)
gcm = AES_GCM(bytes_to_long(key))
encryptedKey, tag = gcm.encrypt(bytes_to_long(iv), CEK, '')
params = { "iv": b64enc(iv), "tag": b64enc(long_to_bytes(tag,16)) }
elif alg == "PBES2-HS256+A128KW":
(CEK, IV) = generateKeyIV(enc)
(encryptedKey, params) = \
polyfills.PBKDF_key_wrap( key, CEK, SHA256, 16, header )
elif alg == "PBES2-HS384+A192KW":
(CEK, IV) = generateKeyIV(enc)
(encryptedKey, params) = \
polyfills.PBKDF_key_wrap( key, CEK, SHA384, 24, header )
elif alg == "PBES2-HS512+A256KW":
(CEK, IV) = generateKeyIV(enc)
(encryptedKey, params) = \
polyfills.PBKDF_key_wrap( key, CEK, SHA512, 32, header )
else:
raise Exception("Unsupported key management algorithm " + alg)
return (CEK, encryptedKey, IV, params)
