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Clarify CMS_decrypt behaviour.

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(cherry picked from commit 5f8e9a477a)
This commit is contained in:
Dr. Stephen Henson 2014-04-15 18:17:12 +01:00
parent db34be4224
commit 9c1d63540f
1 changed files with 15 additions and 1 deletions
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16
doc/crypto/CMS_decrypt.pod
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@ -27,7 +27,21 @@ function or errors about unknown algorithms will occur.
Although the recipients certificate is not needed to decrypt the data it is
needed to locate the appropriate (of possible several) recipients in the CMS
structure. If B<cert> is set to NULL all possible recipients are tried.
structure.
If B<cert> is set to NULL all possible recipients are tried. This case however
is problematic. To thwart the MMA attack (Bleichenbacher's attack on
PKCS #1 v1.5 RSA padding) all recipients are tried whether they succeed or
not. If no recipient succeeds then a random symmetric key is used to decrypt
the content: this will typically output garbage and may (but is not guaranteed
to) ultimately return a padding error only. If CMS_decrypt() just returned an
error when all recipient encrypted keys failed to decrypt an attacker could
use this in a timing attack. If the special flag B<CMS_DEBUG_DECRYPT> is set
then the above behaviour is modified and an error B<is> returned if no
recipient encrypted key can be decrypted B<without> generating a random
content encryption key. Applications should use this flag with
B<extreme caution> especially in automated gateways as it can leave them
open to attack.
It is possible to determine the correct recipient key by other means (for
example looking them up in a database) and setting them in the CMS structure