gpgsm: decrypting session key failed: Invalid session key

[Daniel Kahn Gillmor]

On Fri 2019-07-26 11:43:38 +0900, NIIBE Yutaka wrote:
> Henning Schild via Gnupg-devel <gnupg-devel at gnupg.org> wrote:
>> An increasing amount of x509 encrypted email i receive can not be
>> decrypted with gpgsm anymore.
>
> Is there any change of cipher used?
>
>> Details: (from latest git build)
>> $ /foo/gnupg/sm/gpgsm --debug-level guru --decrypt smime_bad.p7m
>> ...
>> gpgsm: DBG: chan_5 -> PKDECRYPT
>> gpgsm: DBG: chan_5 <- S INQUIRE_MAXLEN 4096
>> gpgsm: DBG: chan_5 <- INQUIRE CIPHERTEXT
>> gpgsm: DBG: chan_5 -> [ 44 20 28 37 3a 65 6e 63 2d 76 61 6c 28 33 3a 72 ...(273 byte(s) skipped) ]
>> gpgsm: DBG: chan_5 -> END
>> Vim: Reading from stdin...
>> gpgsm: DBG: chan_5 <- S PADDING 0
>> gpgsm: DBG: chan_5 <- [ 44 20 28 35 3a 76 61 6c 75 65 33 32 3a e5 ff cd ...(31 byte(s) skipped) ]
>> gpgsm: DBG: chan_5 <- OK
>> gpgsm: DBG: pkcs1 encoded session key: e5ffcd51107897682fc0d805173d85ce7088fddabda33ac74da73b0813c04593
>> gpgsm: decrypting session key failed: Invalid session key
>> gpgsm: message decryption failed: Invalid session key <GpgSM>
>
> The encoded session key is 32-byte, which looks like a key of AES-256.
>
> My guess is, this is the point where we need a fix:
>
> diff --git a/sm/decrypt.c b/sm/decrypt.c
> index ec9800840..af509fea1 100644
> --- a/sm/decrypt.c
> +++ b/sm/decrypt.c
> @@ -75,7 +75,7 @@ prepare_decryption (ctrl_t ctrl, const char *hexkeygrip, const char *desc,
>      log_printhex (seskey, seskeylen, "pkcs1 encoded session key:");
>  
>    n=0;
> -  if (seskeylen == 24 || seskeylen == 16)
> +  if (seskeylen == 32 || seskeylen == 24 || seskeylen == 16)
>      {
>        /* Smells like a 3-DES or AES-128 key.  This might happen
>         * because a SC has already done the unpacking.  A better


It looks like this change was adopted in GnuPG master as
ef2424144a070c9199e40424ec8d9b5a9919aa72.  Should this be backported to
the 2.2 stable branch?

    --dkg
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