openpgp/packet: reject too-short decrypted session key in EncryptedKey.Decrypt

[adilburaksen]

EncryptedKey.Decrypt (openpgp/packet/encrypted_key.go) decrypts a
public-key-encrypted session key and then reads it without any length
check:

e.CipherFunc = CipherFunction(b[0])
e.Key = b[1 : len(b)-2]
expectedChecksum := uint16(b[len(b)-2])<<8 | uint16(b[len(b)-1])

b is the result of RSA, ElGamal or ECDH decryption. A crafted
public-key-encrypted message whose ciphertext decrypts to fewer than
three octets makes these operations panic: len(b)==0 panics on b[0],
and len(b)<3 panics on b[1:len(b)-2] (e.g. b[1:0]). This is reachable
from ReadMessage when decrypting an attacker-supplied message with the
recipient's private key, a denial of service for any program that
decrypts untrusted OpenPGP messages.

The fix rejects len(b) < 3 with a StructuralError before indexing. A
valid session key is one cipher-algorithm octet plus key material plus
a two-octet checksum, so three octets is the minimum. This complements
PR #348 (which guards elgamal.Decrypt against an empty result): the
caller still needs at least the cipher octet plus the two checksum
octets, and the RSA and ECDH decryption paths were unguarded entirely.

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Patch Set 1:

(1 comment)

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This PR (HEAD: a9eefe7) has been imported to Gerrit for code review.

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